A steady-state population inversion in the ground state of Mn2+ in ZnO was detected by application of continuous microwave and circularly polarized optical pumping in the temperature range of 3-6 K. Multiple spin-flip processes occur in view of a simultaneous saturation in the harmonically related transitions of Mn2+ spins. It is found that an additional relaxation channel arises at 2.7 K due to dynamic polarization of the 55Mn nuclei through the saturation of the first order electron-nuclear forbidden transitions. The transient populations are created between 55Mn nuclear sublevels. © 2022 Author(s).

We show that in cubic crystals with anisotropic impurity centers the sum of squares of the magnetic resonance [electron paramagnetic resonance (EPR)] frequencies is invariant with respect to the magnetic field direction. The connection between such an invariant and the g-tensor components of the impurity is derived for different types of centers. The established regularity is confirmed experimentally for the spin-noise spectra of a cubic CaF2-Nd3+ crystal. We show how this property of the EPR spectra can be efficiently used for the assignment of paramagnetic centers in cubic crystals. © 2022 American Physical Society.

The outstanding optical quality of lead halide perovskites inspires studies of their potential for the optical control of carrier spins as pursued in other materials. Entering largely uncharted territory, time-resolved pump–probe Kerr rotation is used to explore the coherent spin dynamics of electrons and holes in bulk formamidinium caesium lead iodine bromide (FA0.9Cs0.1PbI2.8Br0.2) and to determine key parameters characterizing interactions of their spins, such as the g-factors and relaxation times. The demonstrated long spin dynamics and narrow g-factor distribution prove the perovskites as promising competitors for conventional semiconductors in spintronics. The dynamic nuclear polarization via spin-oriented holes is realized and the identification of the lead (207Pb) isotope in optically detected nuclear magnetic resonance proves that the hole–nuclei interaction is dominated by the lead ions. A detailed theoretical analysis accounting for the specifics of the lead halide perovskite materials allows the evaluation of the underlying hyperfine interaction constants, both for electrons and holes. Recombination and spin dynamics evidence that at low temperatures, photogenerated electrons and holes are localized at different regions of the perovskite crystal, resulting in their long lifetimes up to 44 μs. The findings form the base for the tailored development of spin-optoelectronic applications for the large family of lead halide perovskites and their nanostructures. © 2021 The Authors. Advanced Materials published by Wiley-VCH GmbH

Lead halide perovskites show remarkable performance when used in photovoltaic and optoelectronic devices. However, the peculiarities of light-matter interactions in these materials in general are far from being fully explored experimentally and theoretically. Herein, we specifically address the energy level order of optical transitions and demonstrate photon echoes in a methylammonium lead triiodide single crystal, thereby determining the optical coherence times (T2) for excitons and biexcitons at cryogenic temperature to be 0.79 and 0.67 ps, respectively. Most importantly, we have developed an experimental photon-echo polarimetry method that not only identifies the contributions from exciton and biexciton complexes but also allows accurate determination of the biexciton binding energy of 2.4 meV, even though the period of quantum beats between excitons and biexcitons is much longer than the coherence times of the resonances. Our experimental and theoretical analysis methods contribute to the understanding of the complex mechanism of quasiparticle interactions at moderate pump density and show that even in high-quality perovskite crystals and at very low temperatures, inhomogeneous broadening of excitonic transitions due to local crystal potential fluctuations is a source of optical dephasing. ©

For low-density plasmas, the classical limit described by the Debye-Hückel theory is still considered as an appropriate description even though a clear experimental proof of this paradigm is lacking due to the problems in determining the plasma-induced shift of single-particle energies in atomic systems. We show that Rydberg excitons in states with a high principal quantum number are highly sensitive probes for their surrounding making it possible to unravel accurately the basic properties of low-density nondegenerate electron-hole plasmas. To this end, we accurately measure the parameters of Rydberg excitons such as energies and linewidths in absorption spectra of bulk cuprous oxide crystals in which a tailored electron-hole plasma has been generated optically. Since from the absorption spectra exciton energies, as well as the shift of the single-particle energies given by the band edge, can be directly derived, the measurements allow us to determine the plasma density and temperature independently, which has been a notoriously hard problem in semiconductor physics. Our analysis shows unambiguously that the impact of the plasma cannot be described by the classical Debye model, but requires a quantum many-body theory, not only for the semiconductor plasma investigated here, but in general. Furthermore, it reveals an exciton scattering mechanism with coupled plasmon-phonon modes becoming important even at very low plasma densities. © 2022 American Physical Society.

We study experimentally and theoretically the temperature dependence of transverse magnetic routing of light emission from hybrid plasmonic-semiconductor quantum well structures where the exciton emission from the quantum well is routed into surface plasmon polaritons propagating along a nearby semiconductor-metal interface. In II-VI and III-V direct-band semiconductors the magnitude of routing is governed by the circular polarization of exciton optical transitions, that is induced by a magnetic field. For structures comprising a (Cd,Mn)Te/(Cd,Mg)Te diluted magnetic semiconductor quantum well we observe a strong directionality of the emission up to 15% at low temperature of 20K and magnetic field of 485mT due to giant Zeeman splitting of holes mediated via the strong exchange interaction with Mn2+ ions. For increasing temperatures towards room temperature the magnetic susceptibility decreases and the directionality strongly drops to 4% at about 65 K. We also propose an alternative design based on a nonmagnetic (In,Ga)As/(In,Al)As quantum well structure, suitable for higher temperatures. According to our calculations, such structure can demonstrate emission directionality up to 5% for temperatures below 200 K and moderate magnetic fields of 1 T. © 2022 authors. Published by the American Physical Society.

The optical spectra of antiferromagnetic copper metaborate CuB2O4 are characterized by an exceptionally rich structure of narrow absorption lines due to electronic transitions within the magnetic Cu2+ ions, but their unambiguous identification and behavior in magnetic fields have remained far from being fully understood. We study the polarized magnetoabsorption spectra of this tetragonal antiferromagnet with high spectral resolution across the energy range of 1.4055-1.4065 eV in magnetic fields up to 9.5 T for temperatures from 1.6 up to the Néel temperature TN=20 K. We observe a set of eight absorption lines at T=1.6 K in magnetic fields exceeding 1.4 T, which we identify as arising from Frenkel excitons related to the ground and first excited states of the Cu2+ ions. The number of these excitons is defined by the presence of the four Cu2+ ions with doubly degenerate spin state S=1/2 at the 4b positions in the crystallographic unit cell. The energies of these excitons are determined by the exchange interaction of 0.5 meV of the Cu2+ ions in the excited state with the surrounding ions and by the Davydov splitting of 0.12 meV. In large magnetic field the observed Zeeman splitting is controlled by the anisotropic g-factors of both the ground and excited states. We develop a theoretical model of Frenkel excitons in the magnetic field that accounts for specific features of the spin structure and exchange interactions in CuB2O4. The model is used for fitting the experimental data and evaluation of the Frenkel exciton parameters, such as the Davydov splitting, the molecular exchange energy, and the g-factors of the ground and excited states of the Cu2+ ions. © 2022 American Physical Society.

The exciton states in cuprous oxide show a pronounced fine structure splitting associated with the crystal environment and the resulting electronic band structure. High-resolution spectroscopy reveals an especially pronounced splitting of the yellow D excitons with one state pushed above any other state with the same principal quantum number. This large splitting offset is related to a strong mixing of these D states with the 1S exciton of the green series, as suggested by previously published calculations. Here, a detailed comparison of this theory with experimental data is given, which leads to a complete reassignment of the experimentally observed D exciton lines. The origin of different amounts of green admixture to D-envelope states is deduced by analyzing the different terms of the Hamiltonian. The yellow–green mixing leads to level repulsion and induces an exchange interaction splitting to D-envelope states, from which one of them becomes the highest state within each multiplet. Furthermore, the assignment of D exciton states according to their total angular momentum F is given and corrects an earlier description given in a former study. © 2021 The Authors. physica status solidi (RRL) Rapid Research Letters published by Wiley-VCH GmbH

The ability to generate and control strong long-range interactions via highly excited electronic states has been the foundation for recent breakthroughs in a host of areas, from atomic and molecular physics to quantum optics and technology. Rydberg excitons provide a promising solid-state realization of such highly excited states, for which record-breaking orbital sizes of up to a micrometer have indeed been observed in cuprous oxide semiconductors. Here, we demonstrate the generation and control of strong exciton interactions in this material by optically producing two distinct quantum states of Rydberg excitons. This is made possible by two-color pump-probe experiments that allow for a detailed probing of the interactions. Our experiments reveal the emergence of strong spatial correlations and an inter-state Rydberg blockade that extends over remarkably large distances of several micrometers. The generated many-body states of semiconductor excitons exhibit universal properties that only depend on the shape of the interaction potential and yield clear evidence for its vastly extended-range and power-law character. © 2021, The Author(s).

In a Fe/(Cd,Mg)Te/CdTe quantum well hybrid structure, short-range and long-range ferromagnetic proximity effects are found to coexist. The former is observed for conduction band electrons, while the latter is observed for holes bound to shallow acceptors in the CdTe quantum well. These effects arise from the interaction of charge carriers confined in the quantum well with different ferromagnets, where electrons interact with the Fe film and holes with an interfacial ferromagnet at the Fe/(Cd,Mg)Te interface. The two proximity effects originate from fundamentally different physical mechanisms. The short-range proximity effect for electrons is determined by the overlap of their wave functions with d-electrons of the Fe film. On the contrary, the long-range effect for holes bound to acceptors is not associated with overlapping wave functions and can be mediated by elliptically polarized phonons. The coexistence of the two ferromagnetic proximity effects reveals the presence of a nontrivial spin texture within the same heterostructure. © 2021 American Chemical Society. All rights reserved.

The spin dynamics in CsPbBr3 lead halide perovskite nanocrystals are studied by picosecond pump-probe Faraday rotation in an external magnetic field. Coherent Larmor precession of electrons and holes with spin dephasing times of ∼600 ps is detected in a transversal magnetic field. The longitudinal spin relaxation time in weak magnetic fields reaches 80 ns at a temperature of 5 K. In this regime, the carrier spin dynamics is governed by nuclear spin fluctuations characterized by an effective hyperfine field strength of 25 mT. The Landé factors determining the carrier Zeeman splittings are ge = +1.73 for electrons and gh = +0.83 for holes. A comparison with a CsPbBr3 polycrystalline film and bulk single crystals evidences that the spatial confinement of electrons and holes in the nanocrystals only slightly affects their g factors and spin dynamics. © 2021 American Chemical Society.

In this paper, we analyze the transmission spectrum of a thin plate of cuprous oxide in the range of the absorption of the yellow exciton states with the coherent transfer matrix method. We demonstrate that, in contrast to the usual analysis using the Bouguer-Lambert law, a consistent quantitative description over the whole spectral range under consideration is possible. This leads to more accurate parameters not only for the Rydberg exciton states but also for the strengths of indirect transitions. Furthermore, the results have consequences for the interaction of Rydberg excitons with other systems, e.g., Rydberg states themselves or for the determination of the density of electron-hole pairs after optical excitation. © 2021 American Physical Society

Following the ultrafast optical excitation of an inhomogeneously broadened ensemble, the macroscopic optical polarization decays rapidly due to dephasing. This destructive interference is, however, reversible in photon echo experiments. Here, we propose a concept in which a control pulse slows down either the dephasing or the rephasing of the exciton ensemble during its presence. We analyze and visualize this optical freezing process by showing and discussing results for different single and multiple sequences of control pulses using a simple model of inhomogeneously broadened two-level systems. This idea has been realized in experiments performed on self-assembled (In,Ga)As quantum dots where it was possible to retard or advance the photon echo emission time by several picoseconds. The measurements are in very good agreement with numerical simulations for a more realistic model which, in particular, takes the spatial shape of the laser pulses into account. © 2021 SPIE.

A novel spin orientation mechanism - dynamic electron spin polarization - has been recently suggested in Phys. Rev. Lett. 125, 156801 (2020)PRLTAO0031-900710.1103/PhysRevLett.125.156801. It takes place for unpolarized optical excitation in weak magnetic fields of the order of a few millitesla. In this paper we demonstrate experimentally and theoretically that the dynamic electron spin polarization degree changes sign as a function of time, strength of the applied magnetic field, and its direction. The studies are performed on indirect band-gap (In,Al)As/AlAs quantum dots and their results are explained in the framework of a theoretical model developed for our experimental setting. © 2021 American Physical Society.

We experimentally and numerically investigate the exchange interaction of the yellow excitons in cuprous oxide. By varying the material parameters in the numerical calculations, we can interpret experimental findings and understand their origin in the complex band structure and central-cell corrections. In particular, we experimentally observe the reversal of the ortho- and paraexciton for the 2S yellow exciton and explain this phenomenon by an avoided crossing with the green 1S orthoexciton in a detailed numerical analysis. Furthermore, we discuss the exchange splitting as a function of the principal quantum number n and its deviation from the n-3 behavior expected from a hydrogenlike model. We also explain why the observed exchange splitting of the green 1S exciton is more than twice the splitting of the yellow 1S state. © 2021 American Physical Society.

Colloidal semiconductor nanoplatelets exhibit strong quantum confinement for electrons and holes as well as excitons in one dimension, while their in-plane motion is free. Because of the large dielectric contrast between the semiconductor and its ligand environment, the Coulomb interaction between electrons and holes is strongly enhanced. By means of one- and two-photon photoluminescence excitation spectroscopy, we measure the energies of the 1S and 1P exciton states in CdSe nanoplatelets with thicknesses varied from 3 up to 7 monolayers. By comparison with calculations, performed in the effective mass approximation with account of the dielectric enhancement, we evaluate exciton binding energies of 195–315 meV, which is about 20 times greater than that in bulk CdSe. Our calculations of the effective Coulomb potential for very thin nanoplatelets are close to the Rytova-Keldysh model, and the exciton binding energies are comparable with the values reported for monolayer-thick transition metal dichalcogenides. © 2021 American Chemical Society

The exciton recombination and spin dynamics are studied in monolayer-thick (In,Al)As/AlAs quantum wells characterized by an indirect band gap and a type-I band alignment. The exciton recombination time and the photoluminescence intensity are strongly dependent on strength and orientation of an applied magnetic field. In contrast to no effect of an in-plane field, at a temperature of 1.8 K a magnetic field applied parallel to the growth axis drastically slows down the recombination and reduces the intensity of photoluminescence. The magnetic-field-induced circular polarization of photoluminescence is studied as a function of the magnetic field strength and direction, as well as sample temperature. The observed nonmonotonic behavior of these functions is provided by the interplay of bright and dark exciton states contributing to the emission. Taking into account the magnetic-field-induced redistribution of the indirect excitons between their bright and dark states, we evaluate the heavy-hole longitudinal factor of 3.6, the radiative recombination time for the bright excitons of 0.13 ms, and the nonradiative recombination time of the bright and dark excitons of 0.43 ms, as well as the spin relaxation times of electron of and heavy hole of , bound in the exciton. ©2021 American Physical Society

Qubits based on crystal defect centers have been shown to exhibit long spin coherence times, up to seconds at room temperature. However, they are typically characterized by a comparatively slow initialization timescale. Here, fluorine implantation into ZnSe epilayers is used to induce defect states that are identified as zinc vacancies. We study the carrier spin relaxation in these samples using various pump-probe measurement methods, assessing phenomena such as resonant spin amplification, polarization recovery, and spin inertia in transverse or longitudinal magnetic field. The spin dynamics in isotopically natural ZnSe show a significant influence of the nuclear spin bath. Removing this source of relaxation by using isotopic purification, we isolate the anisotropic exchange interaction as the main spin dephasing mechanism and find spin coherence times of 100 ns at room temperature, with the possibility of fast optical access on the picosecond time scales through excitonic transitions of ZnSe. © 2021, The Author(s).

We reveal the existence of a large in-plane heavy-hole g factor in symmetric self-Assembled (001) (In,Ga)As/GaAs quantum dots due to the warping of valence-band states. This warping dominates over the well-established mechanism associated with a reduced symmetry of the quantum dots and the corresponding mixing of heavy-hole and light-hole states. The effect of band warping is manifested in a unique angular dependence of the trion photon echo signal on the direction of the external magnetic field with respect to the sample axes. It results in a uniform magnetic-field-induced optical anisotropy for the entire quantum dot ensemble which is a prerequisite for the realization of spin quantum memories and spin-photon entanglement in the ensemble. © 2021 American Physical Society.

Hypersonic phononic bandgap structures confine acoustic vibrations whose wavelength is commensurate with that of light, and have been studied using either time- or frequency-domain optical spectroscopy. Pulsed pump-probe lasers are the preferred instruments for characterizing periodic multilayer stacks from common vacuum deposition techniques, but the detection mechanism requires the injected sound wave to maintain coherence during propagation. Beyond acoustic Bragg mirrors, frequency-domain studies using a tandem Fabry–Perot interferometer (TFPI) find dispersions of two- and three-dimensional phononic crystals (PnCs) even for highly disordered samples, but with the caveat that PnCs must be transparent. Here, we demonstrate a hybrid technique for overcoming the limitations that time- and frequency-domain approaches exhibit separately. Accordingly, we inject coherent phonons into a non-transparent PnC using a pulsed laser and acquire the acoustic transmission spectrum on a TFPI, where pumped appear alongside spontaneously excited (i.e. incoherent) phonons. Choosing a metallic Bragg mirror for illustration, we determine the bandgap and compare with conventional time-domain spectroscopy, finding resolution of the hybrid approach to match that of a state-of-the-art asynchronous optical sampling setup. Thus, the hybrid pump–probe technique retains key performance features of the established one and going forward will likely be preferred for disordered samples. © 2021, The Author(s).

The effective g factor of holes is measured in modulation-doped ZnSe/(Zn,Mg)(S,Se) quantum wells and from surface-state p-doped CdTe/(Cd,Mg)Te quantum wells by time-resolved pump-probe Kerr rotation. The measurements are performed at a temperature of 1.7 K and in magnetic fields up to 5 T applied in the Voigt geometry with orientation perpendicular to the quantum-well growth axis. The absolute value of the in-plane hole g factor increases with growing magnetic field in both studied heterostructures. A theoretical model is developed that considers the influence of magnetic field and interface mixing of heavy-hole and light-hole states on the g factor. The model results are in good agreement with the experimental data. © 2021 American Physical Society.

The spin-noise spectroscopy (SNS) method implies high efficiency of conversion of the spin-system magnetization to the Faraday rotation angle. Generally, this efficiency cannot be estimated using the characteristics of the regular magnetooptical activity of a paramagnet. However, it may be drastically enhanced in systems with strong inhomogeneous broadening of the optical transitions. This enhancement leads to the giant spin-noise gain effect and previously allowed one to apply the SNS to rare-earth activated crystals. We show that the nonlinear resonant Faraday effect can be used to measure the homogeneous width of the inhomogeneously broadened transition and, thus, to estimate the applicability of the SNS to this type of paramagnet. We present the theoretical description of the effect and perform measurements on intraconfigurational (4f-4f) transitions of the trivalent rare-earth ions of neodymium and ytterbium in fluorite-based crystals. The proposed experimental approach establishes new links between the effects of nonlinear optics and spin-noise characteristics of crystals with paramagnetic impurities and offers new methods of research in the physics of impurity crystals. © 2021 American Physical Society.

Optically inactive or dark excitons play an important role in exciton and polariton devices. On one hand, they supply excitons to the light cone and feed the photoluminescence signal. On the other hand, they repel radiatively active excitons due to the exchange interaction and contribute to the formation of lateral potentials for exciton and polariton condensates. On top of this, they play an important role in scattering and energy relaxation dynamics of quasiparticles in semiconductors. So far, because of optical inaccessibility, studies were focused typically on one experimental technique, giving information about one quantity of dark excitons. Here we present a comprehensive study of the dark exciton reservoir in a high-quality 14-nm GaAs/AlGaAs quantum well using several experimental techniques. We develop a new method of nonradiative broadening spectroscopy of exciton resonances and combine it with nondegenerate pump-probe spectroscopy. The exciton and carrier dynamics in the reservoir is monitored via dynamic broadening of exciton resonances induced by exciton-exciton and exciton-carrier scattering. The dynamics is found to be strongly dependent on the optical excitation conditions. Based on the experimental results, we develop a model of dynamics in a reservoir of excitons and free carriers. The model allows us to describe the experimentally measured photoluminescence kinetics with no fitting parameters. We also demonstrate the optical control of the dark exciton density by means of an additional excitation that creates imbalance of free carriers depleting the reservoir. These results shed light onto the dynamics of the excitonic "dark matter"and pave the way to the high-precision engineering of optically induced potentials in exciton-polariton and integrated photonic devices. We expect that the observed results can be transferred also to other semiconductors so that the current quantum well serves as a high-quality model system. © 2021 American Physical Society.

The energy level structure as well as the exciton recombination and spin dynamics are studied in a dense ensemble of (In,Al,Ga)As/(Al,Ga)As quantum dots (QDs). The band alignment in the QDs is shown to have type-I, indirect character with the lowest electron state at the X valleys of the conduction band and the top hole state in the Γ point of the valence band, so that indirect excitons are formed in the QDs. Time-resolved photoluminescence and magnetic-field-induced circular polarization allow us to distinguish electron states belonging to the QDs and the wetting layer. Suppression of the exciton migration within the QD ensemble and along the wetting layer in the magnetic field is found. A pronounced effect of applied microwave radiation on the recombination and spin polarization of the indirect excitons is observed in longitudinal magnetic fields. Optically detected magnetic resonance (ODMR) is detected in both the intensity and the circular polarization degree of the QD emission. The ODMR resonance corresponds to the g factor of 1.97, associated with X-valley electrons. The spin relaxation time of the X-valley electrons is measured to be 600±25 ns. © 2021 American Physical Society.

Optical spin control is the basis for ultrafast spintronics: circularly polarized light in combination with spin-orbit coupling enables spin manipulation of electronic states in condensed matter. However, the conventional approach is limited to longitudinal spin initialization along one particular axis that is dictated by the direction of light propagation. Here, plasmonics opens new possibilities, allowing one to tailor light polarization at the nanoscale. We demonstrate ultrafast optical excitation of electron spin on femtosecond timescales via plasmon-to-exciton spin conversion. By time resolving the THz spin dynamics in a hybrid (Cd,Mn)Te quantum-well structure covered with a metallic grating, we unambiguously determine the orientation of the photoexcited electron spins which is locked to the propagation direction of the optically excited surface plasmon polaritons. Using the spin of the incident photons as an additional degree of freedom, one can adjust not only the longitudinal, but also the transverse electron spin components normal to the light propagation at will. © 2021 American Physical Society.

The recombination dynamics and spin polarization of excitons in CdSe nanocrystals synthesized in a glass matrix are investigated using polarized photoluminescence in high magnetic fields up to 30 Tesla. The dynamics are accelerated by increasing temperature and magnetic field, confirming the dark exciton nature of low-temperature photoluminescence (PL). The circularly polarized PL in magnetic fields reveals several unusual appearances: (i) a spectral dependence of the polarization degree, (ii) its low saturation value, and (iii) a stronger intensity of the Zeeman component which is higher in energy. The latter feature is the most surprising being in contradiction with the thermal population of the exciton spin sublevels. The same contradiction was previously observed in the ensemble of wet-chemically synthesized CdSe nanocrystals but was not understood. We present a theory which explains all the observed features and shows that the inverted ordering of the circularly polarized PL maxima from the ensemble of nanocrystals is a result of competition between the zero phonon (ZPL) and one optical phonon-assisted (1PL) emission of the dark excitons. The essential aspects of the theoretical model are different polarization properties of the dark exciton emission via ZPL and 1PL recombination channels and the inhomogeneous broadening of the PL spectrum from the ensemble of nanocrystals exceeding the optical phonon energy. This journal is © The Royal Society of Chemistry.

In nanoscale communications, high-frequency surface acoustic waves are becoming effective data carriers and encoders. On-chip communications require acoustic wave propagation along nanocorrugated surfaces which strongly scatter traditional Rayleigh waves. Here, we propose the delivery of information using subsurface acoustic waves with hypersound frequencies of ∼20 GHz, which is a nanoscale analogue of subsurface sound waves in the ocean. A bunch of subsurface hypersound modes are generated by pulsed optical excitation in a multilayer semiconductor structure with a metallic nanograting on top. The guided hypersound modes propagate coherently beneath the nanograting, retaining the surface imprinted information, at a distance of more than 50 μm which essentially exceeds the propagation length of Rayleigh waves. The concept is suitable for interfacing single photon emitters, such as buried quantum dots, carrying coherent spin excitations in magnonic devices and encoding the signals for optical communications at the nanoscale. ©

We demonstrate the realization of the resonant spin amplification (RSA) effect in Faraday geometry where a magnetic field is applied parallel to the optically induced spin polarization so that no RSA is expected. However, model considerations predict that it can be realized for a central spin interacting with a fluctuating spin environment. As a demonstrator, we choose an ensemble of singly-charged (In,Ga)As/GaAs quantum dots, where the resident electron spins interact with the surrounding nuclear spins. The observation of RSA in Faraday geometry requires intense pump pulses with a high repetition rate and can be enhanced by means of the spin-inertia effect. Potentially, it provides the most direct and reliable tool to measure the longitudinal g factor of the charge carriers. © 2021 American Physical Society.

Optical second harmonic generation (SHG) on excitons is studied in polar ZnO/(Zn,Mg)O quantum wells grown on ZnO and sapphire substrates. We observe SHG signal on exciton resonances in a symmetry-forbidden geometry for light propagation along the c axis of the structure. A symmetry analysis allows us to suggest that the signals are induced by a symmetry reduction with respect to ZnO bulk crystals by the well-barrier interfaces in combination with built-in electric fields due to spontaneous and piezoelectric polarization as well as structural defects. © 2021 American Physical Society.

Nonlinear optical studies of the yellow and green exciton series in Cu2O have been reported for more than 40 years. Because of the band structure (the two highest even-parity valence and lowest conduction bands), the S excitons of the two lowest exciton series are dipole-forbidden for one-photon absorption and thus dipole-allowed for two-photon absorption. There is an odd-parity higher conduction band that leads with the two even-parity valence bands to the blue and violet exciton series. We report on second-harmonic generation (SHG) of the blue exciton series. The odd-parity S-exciton SHG is due to a dipole-quadrupole excitation and a dipole emission process. Because of their high oscillator strength density, polariton effects have to be taken into account, since resonances might be shifted to higher energies by up to 10meV compared to the transverse exciton energies. The polariton dispersion for the blue excitons up to n=4 is calculated and compared to the experimental results. In magnetic fields up to 10T applied in a Voigt configuration (B⊥k), SHG of S excitons by a dipole-dipole excitation is observed, which is due to the admixture of dipole-dipole excited P excitons by the effective electric field from the magneto-Stark effect (MSE). From the analysis of the diamagnetic shift and the MSE interaction of the three-level system of 1S, 2S, and 2P excitons, we derive experimental results for the ratio (rn,l2)/μX between the average of radius squared for the three states and the reduced exciton mass. For higher principal quantum number states, we observe magnetoexcitons up to n=8. We analyze their magnetic field dependence and derive the electron effective-mass values for the crystalline orientations [111], [11¯0], and [001]. © 2021 American Physical Society.

In quantum-confined semiconductor nanostructures, electrons exhibit distinctive behavior compared with that in bulk solids. This enables the design of materials with tunable chemical, physical, electrical, and optical properties. Zero-dimensional semiconductor quantum dots (QDs) offer strong light absorption and bright narrowband emission across the visible and infrared wavelengths and have been engineered to exhibit optical gain and lasing. These properties are of interest for imaging, solar energy harvesting, displays, and communications. Here, we offer an overview of advances in the synthesis and understanding of QD nanomaterials, with a focus on colloidal QDs, and discuss their prospects in technologies such as displays and lighting, lasers, sensing, electronics, solar energy conversion, photocatalysis, and quantum information. © 2021 American Association for the Advancement of Science. All rights reserved.

In quantum-confined semiconductor nanostructures, electrons exhibit distinctive behavior compared with that in bulk solids. This enables the design of materials with tunable chemical, physical, electrical, and optical properties. Zero-dimensional semiconductor quantum dots (QDs) offer strong light absorption and bright narrowband emission across the visible and infrared wavelengths and have been engineered to exhibit optical gain and lasing. These properties are of interest for imaging, solar energy harvesting, displays, and communications. Here, we offer an overview of advances in the synthesis and understanding of QD nanomaterials, with a focus on colloidal QDs, and discuss their prospects in technologies such as displays and lighting, lasers, sensing, electronics, solar energy conversion, photocatalysis, and quantum information. Copyright © 2021, American Association for the Advancement of Science.

The dynamics of the coupled electron-nuclear spin system is studied in an ensemble of singly charged (In,Ga)As/GaAs quantum dots (QDs) using periodic optical excitation at 1 GHz repetition rate. In combination with the electron-nuclei interaction, the highly repetitive excitation allows us to lock the electron spins into magnetic resonance in a transverse external magnetic field. Sweeping the field to higher values, the locking leads to an effective "diamagnetic"response of significant strength due to dynamic nuclear polarization, which shields the QD electrons at least partly from the external field and can even keep the internal magnetic field constant up to 1.3 T field variation. We model the effect through a magnetic field-dependent polarization rate of the nuclei, from which we suggest a strategy for adjusting the nuclear polarization through the detuning between optical excitation and electronic transition, in addition to tuning the magnetic field. © 2021 American Physical Society.

The coherent electron spin dynamics of an ensemble of singly charged (In,Ga)As/GaAs quantum dots in a transverse magnetic field is driven by periodic optical excitation at 1 GHz repetition frequency. Despite the strong inhomogeneity of the electron g factor, the spectral spread of optical transitions, and the broad distribution of nuclear spin fluctuations, we are able to push the whole ensemble of excited spins into a single Larmor precession mode that is commensurate with the laser repetition frequency. Furthermore, we demonstrate that an optical detuning of the pump pulses from the probed optical transitions induces a directed dynamic nuclear polarization and leads to a discretization of the total magnetic field acting on the electron ensemble. Finally, we show that the highly periodic optical excitation can be used as universal tool for strongly reducing the nuclear spin fluctuations and preparation of a robust nuclear environment for subsequent manipulation of the electron spins, also at varying operation frequencies. © 2021, The Author(s).

We demonstrate mechanisms of reciprocity breaking in nonlinear optics driven by the toroidal dipole moment which characterizes nontrivial spatial distributions of spins in solids. Using high-resolution femtosecond spectroscopy at electronic resonances in the magnetoelectric antiferromagnet CuB2O4, we show that nonreciprocity reaches 100% for opposite magnetic fields due to the interference of nonlinear coherent sources of second harmonic generation originating from the toroidal dipole moment, applied magnetic field, and noncentrosymmetric crystal structure. The experimental results are corroborated by theoretical analysis based on the crystal and magnetic symmetry of CuB2O4. Our findings open degrees of freedom in nonlinear optics and pave the way for future nonreciprocal spin-optronic devices operating on the femtosecond timescale. © 2021 American Physical Society.

The physics of interacting nuclear spins in solids is well interpreted within the nuclear spin temperature concept. A common approach to cooling the nuclear spin system is adiabatic demagnetization of the initial, optically created, nuclear spin polarization. Here, the selective cooling of 75As spins by optical pumping followed by adiabatic demagnetization in the rotating frame is realized in a nominally undoped GaAs/(Al,Ga)As quantum well. The lowest nuclear spin temperature achieved is 0.54 μK. The rotation of 6 kG strong Overhauser field at the 75As Larmor frequency of 5.5 MHz is evidenced by the dynamic Hanle effect. Despite the presence of the quadrupole induced nuclear spin splitting, it is shown that the rotating 75As magnetization is uniquely determined by the spin temperature of coupled spin-spin and quadrupole reservoirs. The dependence of heat capacity of these reservoirs on the external magnetic field direction with respect to crystal and structure axes is investigated. © 2021, The Author(s).

Semiconducting monolayers of transition-metal dichalcogenides are outstanding platforms to study both electronic and phononic interactions as well as intra- and intervalley excitons and trions. These excitonic complexes are optically either active (bright) or inactive (dark) due to selection rules from spin or momentum conservation. Exploring ways of brightening dark excitons and trions has strongly been pursued in semiconductor physics. Here, we report on a mechanism in which a dark intervalley exciton upconverts light into a bright intravalley exciton in hBN-encapsulated WSe2 monolayers. Excitation spectra of upconverted photoluminescence reveals resonances at energies 34.5 and 46.0 meV below the neutral exciton in the nominal WSe2 transparency range. The required energy gains are theoretically explained by cooling of resident electrons or by exciton scattering with D- or K-valley phonons. Accordingly, an elevated temperature and a moderate concentration of resident electrons are necessary for observing the upconversion resonances. The interaction process observed between the inter- and intravalley excitons elucidates the importance of dark excitons for the optics of two-dimensional materials. ©

Semiconductor quantum dots are excellent candidates for ultrafast coherent manipulation of qubits by laser pulses on picosecond timescales or even faster. In inhomogeneous ensembles a macroscopic optical polarization decays rapidly due to dephasing, which, however, is reversible in photon echoes carrying complete information about the coherent ensemble dynamics. Control of the echo emission time is mandatory for applications. Here, we propose a concept to reach this goal. In a two-pulse photon echo sequence, we apply an additional resonant control pulse with multiple of 2π area. Depending on its arrival time, the control slows down dephasing or rephasing of the exciton ensemble during its action. We demonstrate for self-assembled (In,Ga)As quantum dots that the photon echo emission time can be retarded or advanced by up to 5 ps relative to its nominal appearance time without control. This versatile protocol may be used to obtain significantly longer temporal shifts for suitably tailored control pulses. © 2020, The Author(s).

We study the spin dynamics in a high-mobility two-dimensional electron gas confined in a GaAs/AlGaAs quantum well. An unusual magnetic field dependence of the spin relaxation is found: As the magnetic field becomes stronger, the spin relaxation time first increases quadratically but then changes to a linear dependence before it eventually becomes oscillatory, whereby the longitudinal and transverse times reach maximal values at even and odd filling Landau level factors, respectively. We show that the suppression of spin relaxation is due to the effect of electron gyration on the spin-orbit field, while the oscillations correspond to oscillations of the density of states appearing at low temperatures and high magnetic fields. The transition from quadratic to linear dependence can be related to a transition from classical to Bohm diffusion and reflects an anomalous behavior of the two-dimensional electron gas analogous to that observed in magnetized plasmas. © 2020 American Physical Society.

In Zn1-xMnxSe/(Zn,Be)Se quantum wells with x<0.035, nonthermalized Mn2+ ions demonstrate in spin-flip scattering spectra multiple Stokes and anti-Stokes transitions whose absolute energies deviate by up to 20% from each other. This asymmetry is tuned significantly by the optical power density, magnetic field direction, and Mn ion concentration. The nonequidistant Mn2+ spin transitions are modeled by the Zeeman splitting and quadrupolar crystal-field components taking values of up to 7 GHz. We suggest that nonequilibrium carriers dynamically polarize the Mn-ion spins so that they occupy levels with positive and negative spin projection numbers giving rise to asymmetric spin transitions. © 2020 American Physical Society.

We investigate the charge separation dynamics provided by carrier surface trapping in CdSe/CdS core/shell nanoplatelets by means of a three-laser-beam pump-orientation-probe technique, detecting the electron spin coherence at room temperature. Signals with two Larmor precession frequencies are found, which strongly differ in their dynamical characteristics and dependencies on pump power and shell thickness. The electron trapping process occurs on a time scale of about 10 ns, and the charge separation induced thereby has a long lifetime of up to hundreds of microseconds. On the other hand, the hole trapping requires times from subpicoseconds to hundreds of picoseconds, and the induced charge separation has a lifetime of a few nanoseconds. With increasing CdS shell thickness the hole trapping vanishes, while the electron trapping is still detectable. These findings have important implications for understanding the photophysical processes of nanoplatelets and other colloidal nanostructures. Copyright © 2020 American Chemical Society.

Experimental and theoretical investigations of excitons in cuprous oxide have revealed a significant fine-structure splitting of the excitonic Rydberg states caused by a strong impact of the valence band structure. We provide a semiclassical interpretation of that splitting by investigating the classical dynamics of the excitonic electron-hole pair beyond the hydrogenlike model. Considering the slow motion of Rydberg excitons in coordinate space compared to the fast dynamics of quasispin and hole spin we use an adiabatic approach and energy surfaces in momentum space for the computation of the exciton dynamics. We observe quasiperiodic motion on near-integrable tori. Semiclassical torus quantization yields the energy regions of the fine-structure splitting of n manifolds in agreement with quantum-mechanical computations. © 2020 American Physical Society.

Fundamental properties of the spin-noise signal formation in a quantum-dot microcavity are studied by measuring the angular characteristics of the scattered light intensity. A distributed Bragg reflector microcavity was used to enhance the light-matter interaction with an ensemble of n-doped (In,Ga)As/GaAs quantum dots, which allowed us to study subtle effects of coherent scattering at the quantum dot ensemble. Detecting the scattered light outside of the aperture of the transmitted light, we measured the basic electron spin properties, such as g factor and spin dephasing time. Further, we investigated the influence of the microcavity on the scattering distribution and possibilities of signal amplification by additional resonant excitation. © 2020 American Physical Society.

We suggest a new spin orientation mechanism for localized electrons: Dynamic electron spin polarization provided by nuclear spin fluctuations. The detrimental effect of nuclear spin fluctuations can be harnessed and employed to provide angular momentum for the electrons via the hyperfine interaction in a weak magnetic field. For this, the sample is illuminated by an unpolarized light, which directly polarizes neither the electrons nor the nuclei. We predict that, for the electrons bound in localized excitons, 100% spin polarization can be reached in longitudinal magnetic fields of a few millitesla. The proof of principle experiment is performed on momentum-indirect excitons in (In,Al)As/AlAs quantum dots, where in a magnetic field of 17 mT the electron spin polarization of 30% is measured. © 2020 American Physical Society.

The effect of a lateral electric current on the photoluminescence H band of an AlGaAs/GaAs heterostructure is investigated. The photoluminescence intensity and optical orientation of electrons contributing to the H band are studied by means of continuous-wave and time-resolved photoluminescence spectroscopy and time-resolved Kerr rotation. It is shown that the H band is due to recombination of the heavy holes localized at the heterointerface with photoexcited electrons attracted to the heterointerface from the GaAs layer. Two lines with significantly different decay times constitute the H band: a short-lived high-energy one and a long-lived low-energy one. The high-energy line originates from recombination of electrons freely moving along the structure plane, while the low-energy one is due to recombination of donor-bound electrons near the interface. Application of a lateral electric field of ∼100-200 V/cm results in a quenching of both lines. This quenching is due to a decrease of electron concentration near the heterointerface as a result of a photocurrent-induced heating of electrons in the GaAs layer. On the contrary, electrons near the heterointerface are effectively cooled, so the donors near the interface are not completely empty up to ∼100 V/cm, which is in stark contrast with the case of bulk materials. The optical spin polarization of the donor-bound electrons near the heterointerface weakly depends on the electric field. Their polarization kinetics is determined by the spin dephasing in the hyperfine fields of the lattice nuclei. The long spin memory time (>40 ns) can be associated with suppression of the Bir-Aronov-Pikus mechanism of spin relaxation for electrons. © 2020 American Physical Society.

The electron-nuclei hyperfine interaction of electrons in indirect band gap (In,Al)As/AlAs quantum dots with type-I band alignment has been experimentally studied by measuring the polarization degree of the photoluminescence in a transverse magnetic field (Hanle effect) and the polarization recovery in a longitudinal magnetic field. The different symmetries of the X valley electron Bloch amplitudes at the As, In, and Al nuclei strongly affect the hyperfine interaction. The hyperfine constants corresponding to these nuclei have been determined. © 2020 American Physical Society.

Copper metaborate CuB2O4 crystallizes in a unique noncentrosymmetric structure, becomes antiferromagnetically ordered below TN1=20 K, and exhibits a great diversity in magnetic, optical, and magneto-optical properties. In particular, it shows strong photoluminescence rarely observed before in other magnetically ordered copper oxides in which magnetic properties are defined by magnetic Cu2+ (3d9, S=1/2) ions. Here we report on the detailed spectroscopic study of the photoluminescence originating from the Cu2+ ions. Our investigations are focused on understanding the energy-level scheme of the multiple excitations below the energetically lowest, crystal-field-split d-d electronic transition at 1.405 eV. We identify multiple emission lines, and among them we distinguish three sets of lines, each composed of an exciton line and a satellite attributed to magnon-assisted exciton recombination. The emission intensity of the three sets changes strongly in the temperature range 1.7-40 K, showing pronounced correlations with the magnetic phase transitions between the commensurate and incommensurate phases. Photoluminescence excitation spectra and time-resolved emission dynamics give closer insight into the energy relaxation channels populating the exciton-magnon sets. © 2020 American Physical Society.

So far, the observation of the yellow P exciton series in cuprous oxide has been limited to principal quantum numbers up to nmax=25 at a crystal temperature of T=1.35K. Here, we address the origin of this limitation and whether an extension to higher quantum numbers is possible. To that end, absorption experiments are performed to study the variation of nmax with the spot position on a particular sample and from sample to sample. In addition the temperature is varied. By reducing T to below 1 K, we can extend nmax to 28, not limited by the thermal energy exceeding the exciton binding energy. The data rather suggest that the ultimate limit is provided by residual charged impurities, despite their low density in the order of less than 109cm-2, which exert an electric field on the highly excited excitons causing their ionization. © 2020 American Physical Society.

Excitons in diluted magnetic semiconductors represent excellent probes for studying the magnetic properties of these materials. Various magneto-optical effects, which depend sensitively on the exchange interaction of the excitons with the localized spins of the magnetic ions can be used for probing. Here, we study core/shell CdSe/(Cd,Mn)S colloidal nanoplatelets hosting diluted magnetic semiconductor layers. The inclusion of the magnetic Mn2+ ions is evidenced by three magneto-optical techniques using high magnetic fields up to 15 T: polarized photoluminescence, optically detected magnetic resonance, and spin-flip Raman scattering. We show that the holes in the excitons play the dominant role in exchange interaction with magnetic ions. We suggest and test an approach for evaluation of the Mn2+ concentration based on the spin-lattice relaxation dynamics of the Mn2+ spin system. Copyright © 2020 American Chemical Society.

We report on the experimental and theoretical investigation of magnetic-field-induced second harmonic generation (SHG) and two-photon absorption of excited exciton states (n≥3) of the yellow series in the cuprous oxide Cu2O. In this centrosymmetric material, SHG can occur due to constructive interplay of electric dipole and electric quadrupole/magnetic dipole transitions for light propagating along the low-symmetry directions [111] or [112]. By application of a magnetic field in Voigt configuration, SHG gets also allowed for excitation along the [110] axis and even the high-symmetry cubic direction [001]. Combining a symmetry analysis and a microscopic theory, we uncover the two key contributions to the magnetic-field-induced SHG: the Zeeman effect and the magneto-Stark effect. We demonstrate systematic dependencies of the SHG intensity on the linear polarization angles of the ingoing fundamental laser and the outgoing SHG beam, complementary to the paper by Rommel et al. [Phys. Rev. B 101, 115202 (2020)PRLTAO0031-900710.1103/PhysRevB.101.115202]. In general, the resulting contour plots in combination with a symmetry analysis allow one to determine uniquely the character of involved transitions. Moreover, we can separate in magnetic field the Zeeman and the magneto-Stark effect through appropriate choice of the experimental geometry and polarization configuration. We present a microscopic theory of the second harmonic generation of excitons in a centrosymmetric cubic semiconductor taking into account the symmetry and the band structure of cuprous oxide. Based on the developed microscopic theory, we identify the main contributions to the second-order nonlinear susceptibility of S, P, and D excitons. We analyze the redistribution of SHG intensities between the excitonic states both in the absence and presence of the magnetic field and show good agreement with the experimental data. With increasing exciton principal quantum number, the magneto-Stark effect overpowers the influence of the Zeeman effect. © 2020 American Physical Society.

Strong coupling between two quanta of different excitations leads to the formation of a hybridized state that paves a way for exploiting new degrees of freedom to control phenomena with high efficiency and precision. A magnon polaron is the hybridized state of a phonon and a magnon, the elementary quanta of lattice vibrations and spin waves in a magnetically ordered material. A magnon polaron can be formed at the intersection of the magnon and phonon dispersions, where their frequencies coincide. The observation of magnon polarons in the time domain has remained extremely challenging because the weak interaction of magnons and phonons and their short lifetimes jeopardize the strong coupling required for the formation of a hybridized state. Here, we overcome these limitations by spatial matching of magnons and phonons in a metallic ferromagnet with a nanoscale periodic surface pattern. The spatial overlap of the selected phonon and magnon modes formed in the periodic ferromagnetic structure results in a high coupling strength which, in combination with their long lifetimes, allows us to find clear evidence of an optically excited magnon polaron. We show that the symmetries of the localized magnon and phonon states play a crucial role in the magnon polaron formation and its manifestation in the optically excited magnetic transients. ©2020 American Physical Society.

The low-Temperature emission spectrum of CdSe colloidal nanoplatelets (NPLs) consists of two narrow lines. The high-energy line stems from the recombination of neutral excitons. The origin of the low-energy line is currently debated. We experimentally study the spectral shift, emission dynamics, and spin polarization of both lines at low temperatures down to 1.5 K and in high magnetic fields up to 60 T and show that the low-energy line originates from the recombination of negatively charged excitons (trions). This assignment is confirmed by the NPL photocharging dynamics and associated variations in the spectrum. We show that the negatively charged excitons are considerably less sensitive to the presence of surface spins than the neutral excitons. The trion binding energy in three-monolayer-Thick NPLs is as large as 30 meV, which is 4 times larger than its value in the two-dimensional limit of a conventional CdSe quantum well confined between semiconductor barriers. A considerable part of this enhancement is gained by the dielectric enhancement effect, which is due to the small dielectric constant of the environment surrounding the NPLs. Copyright © 2020 American Chemical Society.

We develop a simple method for measuring the electron spin relaxation times T1, T2 and T2∗ in semiconductors and demonstrate its exemplary application to n-type GaAs. Using an abrupt variation of the magnetic field acting on electron spins, we detect the spin evolution by measuring the Faraday rotation of a short laser pulse. Depending on the magnetic field orientation, this allows us to measure either the longitudinal spin relaxation time T1 or the inhomogeneous transverse spin dephasing time T2∗. In order to determine the homogeneous spin coherence time T2, we apply a pulse of an oscillating radiofrequency (rf) field resonant with the Larmor frequency and detect the subsequent decay of the spin precession. The amplitude of the rf-driven spin precession is significantly enhanced upon additional optical pumping along the magnetic field. © 2020, The Author(s).

We study optical harmonic generation on the 1S exciton-polariton in the semiconductor ZnSe. Intense and spectrally narrow exciton resonances are found in optical second (SHG), third (THG), and fourth (FHG) harmonic generation spectra. The resonances are shifted to higher energy by 3.2 meV from the exciton energy in the linear reflectivity spectrum. Additional resonances are observed in the THG and FHG spectra and assigned to combinations of incident and backscattered photons in the crystal. Rotational anisotropy diagrams are measured and further information on the origin of the optical harmonic generation and the involved exciton states is obtained by a symmetry analysis using group theory in combination with a microscopic consideration. © 2020 American Physical Society.

Optical harmonic generation on excitons is found in ZnSe/BeTe quantum wells with type-II band alignment. Two experimental approaches with spectrally broad femtosecond laser pulses and spectrally narrow picosecond pulses are used for spectroscopic studies by means of second and third harmonic generation (SHG and THG). The SHG signal is symmetry-forbidden in the electric-dipole approximation for light propagation along the structure's growth axis, which is the [001] crystal axis, but it can be induced by an external magnetic field. The THG signal is detected at zero magnetic field and its intensity is field-independent. A group theory analysis of SHG and THG rotational anisotropy diagrams allows us to identify the involved excitation mechanisms. © 2020 American Physical Society.

Core/shell CdSe/(Cd,Mn)S colloidal nanoplatelets containing magnetic Mn2+ ions are investigated by the optically detected magnetic resonance technique, combining 60 GHz microwave excitation and photoluminescence detection. Resonant heating of the Mn spin system is observed. We identify two mechanisms of optical detection, via variation of either the photoluminescence polarization or its intensity in an external magnetic field. The spin-lattice relaxation dynamics of the Mn spin system is measured and used for evaluation of the Mn concentration. In CdSe/(Cd,Zn,Mn)S nanoplatelets the addition of Zn in the shells significantly broadens the magnetic resonance, evidencing local strain. © The Royal Society of Chemistry.

There is a great desire to extend ultrasonic techniques to the imaging and characterization of nanoobjects. This can be achieved by picosecond ultrasonics, where by using ultrafast lasers it is possible to generate and detect acoustic waves with frequencies up to terahertz and wavelengths down to nanometers. In our work we present a picosecond ultrasonics setup based on miniaturized mode-locked semiconductor lasers, whose performance allows us to obtain the necessary power, pulse duration and repetition rate. Using such a laser, we measure the ultrasonic echo signal with picosecond resolution in a 112 nm thick Al film deposited on a semiconductor substrate. We show that the obtained signal is as good as the signal obtained with a standard bulky mode-locked Ti-Sa laser. The experiments pave the way for designing integrated portable picosecond ultrasonic setups on the basis of miniaturized semiconductor lasers. © 2020 Elsevier B.V.

We study the quantum beats in the polarization of the two-pulse photon echo from donor-bound exciton ensembles in semiconductor quantum wells. To induce these quantum beats, a sequence composed of a circularly polarized and a linearly polarized picosecond laser pulse in combination with an external transverse magnetic field is used. This results in an oscillatory behavior of the photon echo amplitude, detected in the σ+ and σ-circular polarizations, occurring with opposite phases relative to each other. The beating frequency is the sum of the Larmor frequencies of the resident electron and the heavy hole when the second pulse is polarized along the magnetic field. The beating frequency is, on the other hand, the difference of these Larmor frequencies when the second pulse is polarized orthogonal to the magnetic field. The measurement of both beating frequencies serves as a method to determine precisely the in-plane hole g factor, including its sign. We apply this technique to observe the quantum beats in the polarization of the photon echo from the donor-bound excitons in a 20-nm-thick CdTe/Cd0.76Mg0.24Te quantum well. From these quantum beats we obtain the in-plane heavy-hole g factor gh=-0.143±0.005. © 2020 American Physical Society.

In this chapter, we discuss quantum optical effects that may be observed on ensembles of quantum dots. Photon statistics are an essential tool to study them. We will discuss how they may be used both as a tool to identify quantum optical effects by studying the photon statistics of light fields emitted from quantum dot ensembles as well as a direct spectroscopic tool by varying the photon statistics of light fields used to excite quantum dot ensembles. Finally, we will discuss superradiance in quantum dot lasers as a genuine quantum optical ensemble effect. © 2020 Elsevier Inc.

The dynamics of exciton recombination and spin relaxation in thin (Ga,Al)(Sb,As)/AlAs quantum wells (QWs) with indirect band gap are studied. The band alignment in these QWs is identified as type I. The exciton recombination time exceeds hundreds of microseconds, while the spin relaxation times of the electron and the heavy hole in an exciton do not exceed hundreds of nanoseconds. The heavy-hole longitudinal g factor is determined to be +2.5. Despite the long exciton lifetimes, the photoluminescence circular polarization degree induced by a magnetic field is unexpectedly small and does not exceed 25%. © 2020 American Physical Society.

The effective electron g factor, geff, is measured in a two-dimensional electron gas (2DEG) in modulation-doped ZnSe- and CdTe-based quantum wells by means of time-resolved pump-probe Kerr rotation. The measurements are performed in magnetic fields applied in the Voigt geometry, i.e., normal to the optical axis parallel to the quantum well plane, in the field range 0.05-6 T at temperatures 1.8-50K. The geff absolute value considerably increases with increasing electron density ne. geff changes in the ZnSe-based QWs from +1.1 to +1.9 in the ne range 3×1010-1.4×1012cm-2 and in the CdTe-based QWs from -1.55 down to -1.76 in the ne range 5×109-3×1011cm-2. The modification of geff reduces with increasing magnetic field, increasing temperature of lattice and 2DEG, the latter achieved by a higher photoexcitation density. A theoretical model is developed that considers the renormalization of the spin-orbit coupling constant of the two-dimensional electrons by the electron-electron interaction and takes into account corrections to the electron-electron interaction in the Hubbard form. The model results are in good agreement with experimental data. © 2020 American Physical Society.

Energy harvesting is a concept which makes dissipated heat useful by transferring thermal energy to other excitations. Most of the existing principles are realized in systems which are heated continuously. We present the concept of high-frequency energy harvesting where the dissipated heat in a sample excites resonant magnons in a thin ferromagnetic metal layer. The sample is excited by femtosecond laser pulses with a repetition rate of 10 GHz, which results in temperature modulation at the same frequency with amplitude ~0.1 K. The alternating temperature excites magnons in the ferromagnetic nanolayer which are detected by measuring the net magnetization precession. When the magnon frequency is brought onto resonance with the optical excitation, a 12-fold increase of the amplitude of precession indicates efficient resonant heat transfer from the lattice to coherent magnons. The demonstrated principle may be used for energy harvesting in various nanodevices operating at GHz and sub-THz frequency ranges. © 2020, The Author(s).

We demonstrate the Rydberg series of dark excitons, known as paraexcitons, up to the principal quantum number n=6 for the yellow exciton series in Cu2O, using second harmonic generation. Each of these states is optically inactive to all orders, but their observation becomes possible by application of a magnetic field which leads to mixing with the quadrupole-allowed bright excitons, called orthoexcitons, of the same n. The dark parastates are generally located below the bright orthostates, whose energies are increased by the electron-hole exchange interaction, except for n=2, where this order is reversed. This inversion occurs due to band mixing, namely, of the 2Sy,o orthoexciton of the yellow series with the 1Sg,o orthoexciton of the green exciton series. © 2020 American Physical Society.

Recently, second harmonic generation (SHG) for the yellow exciton series in cuprous oxide was demonstrated [Phys. Rev. B 98, 085203 (2018)2469-995010.1103/PhysRevB.98.085203]. Assuming perfect Oh symmetry, SHG is forbidden along certain high-symmetry axes. Perturbations can break this symmetry and forbidden transitions may become allowed. We investigate theoretically the effect of external magnetic fields on the yellow exciton lines of cuprous oxide. We identify two mechanisms by which an applied magnetic field can induce a second harmonic signal in a forbidden direction. First of all, a magnetic field by itself generally lifts the selection rules. In the Voigt configuration, an additional magneto-Stark electric field appears. This also induces certain SHG processes differing from those induced by the magnetic field alone. Complementary to the paper by Farenbruch et al. [Phys. Rev. B 101, 115201 (2020)10.1103/PhysRevB.101.115201], we perform a full numerical diagonalization of the exciton Hamiltonian including the complex valence-band structure. Numerical results are compared with experimental data. © 2020 American Physical Society.

The coherent spin dynamics of electrons in tunnel-coupled CdTe and (Cd,Mn)Te quantum wells (QWs) is studied by time-resolved pump-probe Kerr rotation. The coupled QWs have different thicknesses; the narrow one is doped by Mn2+ magnetic ions. A short range proximity effect between them is observed: the Zeeman splitting of electrons in the wide QW is given in addition to the intrinsic electron g factor by the exchange interaction with the Mn2+ ions mediated by electron tunneling into the narrow QW. The exchange interaction strength scales with the Cd0.88Mg0.12Te barrier thickness separating the QWs. The Kerr rotation signal measured on the wide QW shows two close frequencies of electron spin Larmor precession in a transverse magnetic field. These components have very different spin dephasing times, 50 ps and 1 ns. The two frequencies originate from electrons in the wide QW being either part of an exciton or being resident. The proximity effect of the exciton electron is smaller due to the binding by Coulomb interaction, which decreases the tunneling to the narrow well. The experimental data are in good agreement with model calculations. © 2020 American Physical Society.

CdSe colloidal nanoplatelets are studied by spin-flip Raman scattering in magnetic fields up to 5 T. We find pronounced Raman lines shifted from the excitation laser energy by an electron Zeeman splitting. Their polarization selection rules correspond to those expected for scattering mediated by excitons interacting with resident electrons. Surprisingly, Raman signals shifted by twice the electron Zeeman splitting are also observed. The theoretical analysis and experimental dependences show that the mechanism responsible for the double flip involves two resident electrons interacting with a photoexcited exciton. Effects related to various orientations of the nanoplatelets in the ensemble and different orientations of the magnetic field are analyzed. Copyright © 2019 American Chemical Society.

Abstract: The dynamics of the photoluminescence negative circular polarization of an ensemble of InP/(In,Ga)P quantum dots is studied. It was found that, the time-resolved polarization has no oscillations in the magnetic field in the Voigt geometry. At the same time, with increasing field, the polarization decreases to zero. This effect is explained by the specificities of the spin dynamics of a negatively charged exciton; in particular, by the fact that, in the ground state, its spin dynamics is determined by a heavy hole. It is shown that, in order that the photoluminescence be depolarized by a magnetic field, it is necessary to overcome the field of dynamically polarized nuclear spins acting on the electron spins. © 2020, Pleiades Publishing, Ltd.

The electron spin echo detected inversion recovery technique at the Q-band frequency was used to characterize spin diffusion effects in spin-lattice relaxation of compensating Fe3+ impurities in n-type doped GaN crystals. It was found that the selective saturation can be achieved in the GaN:Fe3+ system due to magnetization transfer based on the spin flip-flop cross-relaxation processes. The temperature dependence of 1/T1 can be explained by direct spin-phonon processes (∼ T) below 25 K and by Raman two-phonon processes (∼ T 9) at higher temperatures. Spin diffusion in this system is characterized by an additional cross-relaxation rate which is weakly temperature-dependent below 25 K. The transferred hyperfine interactions of Fe3+ centers with gallium and nitrogen neighbor nuclei were resolved using pulsed-electron nuclear double resonance. A comparative analysis of quadrupole interactions indicates the essential increase in the electric field gradients on the nearest nitrogen and gallium shells. © 2020 Author(s).

We report on theoretical and experimental study of the spin polarization recovery and Hanle effect for the charge carriers interacting with the fluctuating nuclear spins in the semiconductor structures. We start the theoretical description from the simplest model of static and isotropic nuclear spin fluctuations. Then we describe the modification of the polarization recovery and Hanle curves due to the anisotropy of the hyperfine interaction, finite nuclear spin correlation time, and the strong pulsed spin excitation. For the latter case, we predict the appearance of the resonant spin amplification in the Faraday geometry and of the quantum Zeno effect. The set of the experimental results for various structures and experimental conditions is chosen to highlight the specific effects predicted theoretically. We show that the joint analysis of the spin polarization recovery and the Hanle effect is a very valuable tool for addressing carrier spin dynamics in semiconductors and their nanostructures. © 2020 American Physical Society.

The origin of the steplike shoulder on the high-energy side of the low-temperature photoluminescence spectrum of heavily p-doped GaAs is studied experimentally. It is shown that it is controlled by both the Fermi-Dirac distribution of the holes and the energy distribution of the photoexcited electrons exhibiting a sharp steplike dependence. The latter results from abrupt changes in the energy relaxation rate at the percolation threshold separating localized from delocalized electron states. A comprehensive set of optical techniques based on spin orientation of electrons, namely, the Hanle effect, time- and polarization-resolved photoluminescence, as well as transient pump-probe Faraday rotation, are used for these studies. Two different electron ensembles with substantially different lifetimes of 20 and 280 ps are identified. Their spin relaxation times are longer than 2 ns, so that the spin lifetime is limited by the electron lifetime. The relative contribution of short- and long-lived photoexcited electrons to the emission spectrum changes abruptly at the step in the high-energy photoluminescence tail. For energies above the percolation threshold, the electron states are empty due to fast energy relaxation, while for lower energies the relaxation is suppressed and the majority of photoelectrons populates the states located there. © 2020 American Physical Society.

The surface of nominally diamagnetic colloidal CdSe nanoplatelets can demonstrate paramagnetic behaviour owing to the uncompensated spins of dangling bonds, as we reveal here by optical spectroscopy in high magnetic fields up to 15 T using the exciton spin as a probe of the surface magnetism. The strongly nonlinear magnetic field dependence of the circular polarization of the exciton emission is determined by the magnetization of the dangling-bond spins (DBSs), the exciton spin polarization as well as the spin-dependent recombination of dark excitons. The sign of the exciton–DBS exchange interaction depends on the nanoplatelet growth conditions. © 2020, The Author(s), under exclusive licence to Springer Nature Limited.

We report on two-photon absorption (TPA) and second harmonic generation (SHG) spectroscopy of para- and orthoexcitons in Cu2O subject to a strong magnetic field up to 10T. The magnetic field splits the orthoexciton into its three quasispin components M=0,±1 and activates the symmetry and spin forbidden paraexciton by an admixture from the M=0 component of the orthoexciton. For the excitation of the paraexciton we suggest an alternative mechanism of TPA without an external perturbation. It involves instead of the electric dipole-electric dipole the electric quadrupole-magnetic dipole excitation process. By application of group theory we derive for both mechanisms of TPA and SHG polarization selection rules for each of the four resonances (one para- and three orthoexcitons) and present experimental results for different crystalline orientations which agree perfectly with the derivations from group theory. High spectral resolution of the used SHG technique allows us to refine the exchange splitting between the para- and orthoexciton of ɛ=12.120meV and the g values of the upmost valence band gv=-0.72±0.03 and the lowest conduction band gc=2.38±0.08. © 2020 American Physical Society.

Nonequilibrium transitions are investigated in a plane exciton-polariton system with complex acousto-optical excitation: stationary resonant optical excitation of polaritons along the normal to the surface of the resonator and picosecond strain pulses causing reversible exciton energy perturbations. It is shown that acoustic pulses can be used for ultrafast switching of the optical response of a bistable polariton system. Switching is experimentally implemented in a high-Q GaAs microcavity. © 2020 IOP Publishing Ltd.

Transverse magneto-optical Kerr effect (TMOKE) is known to be an effective tool for external magnetic field control of optical properties of magnetoplasmonic crystals. In some applications there is a demand for the pronounced TMOKE in the wide wavelength range. In this work we experimentally and theoretically demonstrate that a magnetite based magnetoplasmnic crystal exhibit a multiple wide band enhancement of TMOKE response in transmission compared to a plain magnetite film without metal. Our RCWA calculations are in good agreement with experimental results. © 2020 IOP Publishing Ltd.

The anisotropic exchange splitting of the Γ exciton δ1 is measured in (In,Al)As/AlAs quantum dots with a type-I band alignment by means of two photoluminescence techniques: The macroscopic technique exploits the competition between the anisotropic exchange interaction and the Zeeman splitting, whereas with the microscopic technique the energy splitting of the exciton fine-structure in a single quantum dot is measured directly. We find that in the spectral region of the ΓX mixing the anisotropic exchange splitting decreases strongly. © 2019 American Physical Society.

An overview is given of two distinct classes of semiconductor quantum dots, epitaxial and colloidal structures that have been studied intensely for more than 30 years by now, however, without large interconnection between the two involved research communities. The largely parallel and independent evolution of the two structure classes may be partly related to the origin of colloidal systems from chemistry, while epitaxial quantum dots have been addressed mostly by the physics community. These independent evolutions are somewhat surprising because the interest in optics-related applications is shared by both communities. Here, a short summary of the development of the two structure classes, the present status of activities, and some perspectives for future developments are presented. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The lead halide perovskites demonstrate huge potential for optoelectronic applications, high energy radiation detectors, light emitting devices and solar energy harvesting. Those materials exhibit strong spin-orbit coupling enabling efficient optical orientation of carrier spins in perovskite-based devices with performance controlled by a magnetic field. Here we show that elaborated time-resolved spectroscopy involving strong magnetic fields can be successfully used for perovskites. We perform a comprehensive study of high-quality lead halide perovskite CsPbBr3 crystals by measuring the exciton and charge carrier g-factors, spin relaxation times and hyperfine interaction of carrier and nuclear spins by means of coherent spin dynamics. Owing to their ‘inverted’ band structure, perovskites represent appealing model systems for semiconductor spintronics exploiting the valence band hole spins, while in conventional semiconductors the conduction band electrons are considered for spin functionality. © 2019, The Author(s).

Colloidal CdSe nanocrystals (NCs) overcoated with an ultrathick CdS shell, also known as dot-in-bulk (DiB) structures, can support two types of excitons, one of which is core-localized and the other, shell-localized. In the case of weak "sub-single-exciton" pumping, emission alternates between the core-and shell-related channels, which leads to two-color light. This property makes these structures uniquely suited for a variety of photonic applications as well as ideal model systems for realizing complex excitonic quasi-particles that do not occur in conventional core/shell NCs. Here, we show that the DiB design can enable an unusual regime in which the same long-lived resident electron can endow trionlike characteristics to either of the two excitons of the DiB NC (core-or shell-based). These two spectrally distinct trion states are apparent in the measured photoluminescence (PL) and spin dynamics of core and shell excitons conducted over a wide range of temperatures and applied magnetic fields. Low-Temperature PL measurements indicate that core-and shell-based trions are characterized by a nearly ideal (â 100%) emission quantum yield, suggesting the strong suppression of Auger recombination for both types of excitations. Polarization-resolved PL experiments in magnetic fields of up to 60 T reveal that the core-and the shell-localized trions exhibit remarkably similar spin dynamics, which in both cases are controlled by spin-flip processes involving a heavy hole. © 2019 American Chemical Society.

The spin dynamics of positively (X+) and negatively (X-) charged excitons in InP/In0.48Ga0.52P quantum dots subject to magnetic field is studied. We find that a characteristic feature of the system under study is the presence of nuclear quadrupole interaction, which leads to stabilization of the nuclear and electron spins in a quantum dot in zero external magnetic field. In detail, the nuclear quadrupole interaction leads to pinning of the Overhauser field along the quadrupole axis, which is close to the growth axis of the heterostructure. The nuclear effects are observed only when resident electrons are confined in the quantum dots, i.e., for X-trion photoexcitation. The presence of X-and X+ trion contributions to the photoluminescence together with the quadrupole interaction significantly affects the dynamics of optical orientation in Voigt magnetic field. In the absence of dynamic nuclear spin polarization the time evolution of the photoluminescence polarization is fitted by a form which describes the electron spin relaxation in "frozen" nuclear field fluctuations. In relatively large external magnetic fields exceeding 60 mT good agreement between theory and experiment is achieved. © 2019 American Physical Society.

In tunnel-coupled quantum wells (QWs) CdTe (20 nm wide) and CdMnTe (8 nm wide) separated by Cd0.88Mg0.12Te barrier with a thickness of L B = 5, 7, 9, and 11 monolayers (MLs), the dependence of electron g-factor on the barrier thickness and temperature is investi-gated by means of the pump-probe Kerr rotation technique. The renormalization of the electron g-factor occurs due to the s-d exchange interaction of electrons with manganese ions in a magnetic QW. The most change of the electron g-factor value Δg = 0.25 is registered at the least barrier width of 5 MLs and temperature T = 5 K. In this case, the penetration of the electron wave function from the nonmagnetic QW into the magnetic one is estimated to be 0.6%. © Published under licence by IOP Publishing Ltd.

Unique structural and optical properties of atomically thin two-dimensional semiconducting transition metal dichalcogenides enable in principle their efficient coupling to photonic cavities having the optical mode volume close to or below the diffraction limit. Recently, it has become possible to make all-dielectric nano-cavities with reduced mode volumes and negligible non-radiative losses. Here, we realise low-loss high-refractive-index dielectric gallium phosphide (GaP) nano-antennas with small mode volumes coupled to atomic mono- and bilayers of WSe2. We observe a photoluminescence enhancement exceeding 104 compared with WSe2 placed on planar GaP, and trace its origin to a combination of enhancement of the spontaneous emission rate, favourable modification of the photoluminescence directionality and enhanced optical excitation efficiency. A further effect of the coupling is observed in the photoluminescence polarisation dependence and in the Raman scattering signal enhancement exceeding 103. Our findings reveal dielectric nano-antennas as a promising platform for engineering light-matter coupling in two-dimensional semiconductors. © 2019, The Author(s).

Abstract: Quantum dots of indium gallium arsenide buried in a thin layer of aluminum gallium arsenide were grown by means of molecular-beam epitaxy. The influence of silver nanoparticles grown on the surface of the semiconductor structure by vacuum thermal evaporation on photoluminescence of quantum dots was investigated. Photoluminescence spectra of quantum dots were obtained under stationary and pulsed excitation. The influence of silver nanoparticles exhibiting plasmon resonances on spectral distribution and kinetics of luminescence of the epitaxial quantum dots was studied. © 2019, Pleiades Publishing, Ltd.

Optical spectroscopy of resonant reflection has been used for both experimental and theoretical studies of the exciton-light interaction in wide, high-quality quantum-well structures with the widths exceeding the exciton Bohr radius by an order of magnitude or more. The light mixes the low-lying confined exciton states which are captured by the generalized model developed by M. M. Voronov et al. [Phys. Solid State 49, 1792 (2007)PSOSED1063-783410.1134/S1063783407090302]. We demonstrate that the high-energy confined states in the wide QWs can still be described by the standard model in which the amplitude reflection coefficient from the QW is a sum of individual size-quantized exciton resonances. The excitonic parameters extracted from fitting the experimental spectrum to the standard model agree with those obtained by the numerical solution of the two-particle Schrödinger equation in a rectangular quantum well. The measured and microscopically calculated spectra are compared with those found with the widely used model of the center-of-mass exciton quantization and the polaritonic model. The comparison shows that the two approximate models considerably underestimate the interaction of confined excitons with light because they ignore the strong modification of the exciton wave function near the QW interfaces. © 2019 American Physical Society.

Self-organized disk-shaped quantum dots of CdSe embedded in diluted magnetic ZnMnSe barrier were studied by means of pump-probe time-resolved Kerr rotation (TRKR) technique at low temperature T = 7 K. In absence of the external magnetic field TRKR signal exhibits long-living spin dynamics with the decay time exceeding the period between laser pulses. Such spin dynamics is not typical for diluted magnetic semiconductors and nano-structures based on them and could be a trace of a bound magnetic polaron. Resonant spin amplification measured in transversal magnetic field up to 1 T shows the only one peak near B = 0. In B = 1 T the long-living non-precessing signal practically vanishes, while the precessing one appears with the Larmor frequency corresponding to the Mn2+ ions' net spin precession around the magnetic field. It was found that the signal consists of three components with slightly different precession frequencies that could be due to the fine structure of the manganese spin sublevels occurring because of a stress in quantum dots. © Published under licence by IOP Publishing Ltd.

Colloidal InP core nanocrystals are taking over CdSe-based nanocrystals, notably in optoelectronic applications. Despite their use in commercial devices, such as display screens, the optical properties of InP nanocrystals and especially their relation to the exciton fine structures remain poorly understood. In this work, we show that the ensemble magneto-optical properties of InP-based core/shell nanocrystals investigated in strong magnetic fields up to 30 T are strikingly different from other colloidal nanostructures. Notably, the mixing of the lowest spin-forbidden dark exciton state with the nearest spin-allowed bright state does not occur up to the highest magnetic fields applied. This lack of mixing in an ensemble of nanocrystals suggests an anisotropy tolerance of InP nanocrystals. This striking property allowed us to unveil the slow spin dynamics between Zeeman sublevels (up to 400 ns at 15 T). Furthermore, we show that the unexpected magnetic-field-induced lengthening of the dark exciton lifetime results from the hyperfine interaction between the spin of the electron in the dark exciton with the nuclear magnetic moments. Our results demonstrate the richness of the spin physics in InP quantum dots and stress the large potential of InP nanostructures for spin-based applications. © 2019 American Chemical Society.

The Design of Experiments is a promising method to investigate the cause-effect relation between the mid-frequency magnetron sputtering parameters on the structural and mechanical properties of amorphous carbon (a-C) films. Based on the Central Composite Design, the cathode power of two graphite targets, bias voltage, and mid-frequency were simultaneously varied from 1500 to 4000 W, −100 to −200 V, and 20 to 50 kHz, respectively. The chemical bonding state was characterized using UV and visible Raman spectroscopy with excitation wavelengths of 266 and 532 nm. Corresponding measurements were performed by X-ray photoelectron spectroscopy (XPS) using synchrotron radiation. Additionally, hardness and elastic modulus of the sputtered a-C films were determined in nanoindentation tests. Multi-wavelength Raman spectroscopy identified an sp3 content below 20%, with most a-C films having an sp3 value in the range of 12 to 18%. The formation of sp3 bonded atoms is negatively influenced by a high cathode power and bias voltage, whereas the highest sp3 content is obtained with a-C films sputtered with a cathode power and bias voltage of 2750 W and −150 V. However, higher values of the cathode power and bias voltage result in a film delamination and decrease of the sp3 concentration. The bonding state affects the mechanical properties, as high hardness and elastic modulus result from a high sp3 content. Therefore, a targeted adjustment of cathode power and bias voltage is necessary to obtain a-C films with a high hardness. In contrast, the mid-frequency does not have a significant impact on the mechanical properties. In conclusion, the Central Composite Design has proven to be a suitable method to investigate the cause-effects of the sputtering parameters on the properties of the a-C film. © 2019

The exciton dynamics in transverse magnetic field is investigated both experimentally and theoretically in two-monolayer-thick GaAs/AlAs quantum wells with an indirect band gap and a type-II band alignment. The observed linear polarization of the quantum well photoluminescence has two contributions, one of which arises from the crystalline structure of the quantum well. It does not depend on temperature and demonstrates a strong spectral dependence across the emission band. The other one is induced by a transverse magnetic field. It strongly decreases with increasing temperature, has no spectral dependence, and demonstrates an unexpectedly long-time dynamics. The experimental findings can be explained in the framework of the developed theoretical model which accounts for the quantum well anisotropy, the Zeeman effect of electrons and holes in the transverse magnetic field, and the redistribution of excitons over the spin sublevels. It provides quantitative agreement with the experiment and allows us to evaluate, for the studied structure, the heavy-hole in-plane g-factor tensor, which turns out to be extremely anisotropic with principal values of opposite signs and the same magnitude of 0.25. © 2019 American Physical Society.

Photoinduced charging in CdSe colloidal quantum dots (QDs) is investigated by time-resolved pump-probe spectroscopy that is sensitive to electron spin polarization. This technique monitors the coherent spin dynamics of optically oriented electrons precessing around an external magnetic field. By addition of 1-octanethiol to the CdSe QD solution in toluene, an extremely long-lived negative photocharging is detected that lives up to 1 month in an N2 atmosphere and hours in an air atmosphere at room temperature. 1-Octanethiol not only acts as a hole acceptor but also results in a reduction of the oxygen-induced photo-oxidation in CdSe QDs, allowing air-stable negative photocharging. Two types of negative photocharging states with different spin precession frequencies and very different lifetimes are identified. These findings have important implications for understanding the photophysical processes in colloidal nanostructures. Copyright © 2019 American Chemical Society.

Voltage control of ferromagnetism on the nanometer scale is highly appealing for the development of novel electronic devices with low power consumption, high operation speed, reliable reversibility and compatibility with semiconductor technology. Hybrid structures based on the assembly of ferromagnetic and semiconducting building blocks are expected to show magnetic order as a ferromagnet and to be electrically tunable as a semiconductor. Here, we demonstrate the electrical control of the exchange coupling in a hybrid consisting of a ferromagnetic Co layer and a semiconductor CdTe quantum well, separated by a thin non-magnetic (Cd,Mg)Te barrier. The electric field controls the phononic ac Stark effect—the indirect exchange mechanism that is mediated by elliptically polarized phonons emitted from the ferromagnet. The effective magnetic field of the exchange interaction reaches up to 2.5 Tesla and can be turned on and off by application of 1V bias across the heterostructure. © 2019, The Author(s).

Spin-dependent photon echoes in combination with pump-probe Kerr rotation are used to study the microscopic electron spin transport in a CdTe/(Cd,Mg)Te quantum well in the hopping regime. We demonstrate that, independent of the particular spin relaxation mechanism, the hopping of resident electrons leads to a shortening of the photon echo decay time, while the transverse spin relaxation time evaluated from pump-probe transients increases due to motional narrowing of spin dynamics in the fluctuating effective magnetic field of the lattice nuclei. © 2019 American Physical Society.

Abstract: The melt method is used for synthesizing monodispersed spherical silica nanoparticles Gd x -Si y O z :Eu 3+ . The particle diameter is 450 nm, and the standard deviation does not exceed 5%. The nanoparticles have a line luminescence spectrum with a dominant band at 614 nm. The effect of a constant magnetic field up to 15 T on the intensity and shape of the luminescence spectra of Eu 3+ ions is studied. It is shown that the obtained material has a stable photoluminescence, the intensity of which does not depend on the magnetic field in the entire studied range. The synthesized nanoparticles Gd x Si y O z : Eu 3+ are promising for use as a contrast agent for magnetic resonance tomography and luminescent marker. © 2019, Pleiades Publishing, Ltd.

We report on the experimental evidence for a nanosecond timescale spin memory based on nonradiative excitons with large in-plane wave vector. The effect manifests itself in magnetic-field-induced oscillations of the energy of the optically active (radiative) excitons. The oscillations detected by a spectrally resolved pump-probe technique applied to a GaAs/AlGaAs quantum well structure in a transverse magnetic field persist over a timescale, which is orders of magnitude longer than the characteristic decoherence time in the system. The effect is attributed to the spin-dependent electron-electron exchange interaction of the optically active and inactive excitons. The spin relaxation time of the electrons belonging to nonradiative excitons appears to be much longer than the hole spin relaxation time. © 2019 American Physical Society.

Nuclear spin coherence and relaxation dynamics of all constituent isotopes of an n-doped CdTe/(Cd,Mg)Te quantum well structure are studied employing optically detected nuclear magnetic resonance. Using time-resolved pump-probe Faraday ellipticity, we generate and detect the coherent spin dynamics of the resident electrons. The photogenerated electron spin polarization is transferred into the nuclear spin system, which becomes polarized and acts back on the electron spins as the Overhauser field. Under the influence of resonant radio frequency pulses, we trace the coherent spin dynamics of the nuclear isotopes Cd111, Cd113, and Te125. We measure nuclear Rabi oscillations, the inhomogeneous dephasing time T2∗, the spin coherence time T2, and the longitudinal relaxation time T1. Furthermore, we investigate the influence of the laser excitation and the corresponding electron spin polarization on the nuclear spin relaxation time and find a weak extension of this time induced by interaction with the electron spins. © 2019 American Physical Society.

We demonstrate a variety of precessional responses of the magnetization to ultrafast optical excitation in nanolayers of galfenol (Fe,Ga), which is a ferromagnetic material with large saturation magnetization and enhanced magnetostriction. The particular properties of galfenol, including cubic magnetic anisotropy and weak damping, allow us to detect up to six magnon modes in a 120-nm layer, and a single mode with effective damping αeff=0.005 and frequency up to 100 GHz in a 4-nm layer. This is the highest frequency observed to date in time-resolved experiments with metallic ferromagnets. We predict that detection of magnetization precession approaching THz frequencies should be possible with galfenol nanolayers. © 2019 American Physical Society.

The spin structure and spin dynamics of excitons in an ensemble of (In,Al)As/AlAs quantum dots (QDs) with type-I band alignment, containing both direct and indirect band gap dots, are studied. Time-resolved and spectral selective techniques are used to distinguish between the direct and indirect QDs. The exciton fine structure is studied by means of optical alignment and optical orientation techniques in magnetic fields applied in the Faraday or Voigt geometries. A drastic difference in emission polarization is found for the excitons in the direct QDs involving a Γ-valley electron and the excitons in the indirect QDs contributed by an X-valley electron. We show that in the direct QDs the exciton spin dynamics is controlled by the anisotropic exchange splitting, while in the indirect QDs it is determined by the hyperfine interaction with nuclear field fluctuations. The anisotropic exchange splitting is determined for the direct QD excitons and compared with model calculations. © 2019 American Physical Society.

The optically induced long-lived spin polarization in the bulk diluted magnetic semiconductor (Cd,Mn)Te with small manganese concentration is studied by picosecond pump-probe Kerr rotation. At temperatures below 6 K and in transversal magnetic field the Kerr rotation signal contains three components: two oscillating components, corresponding to the Larmor precession of manganese spins and spins of photoexcited electrons, and a long-lived (up to 15 ns) nonoscillating component. The latter one is provided by optical orientation of equilibrium hole magnetic polarons involving holes bound to acceptors. The origin of the anisotropy controlling the orientation and the spin dynamics of the acceptor-bound hole magnetic polaron in bulk (Cd,Mn)Te is discussed. © 2019 American Physical Society.

Coherent spin dynamics in colloidal CdSe quantum dots (QDs) typically show two spin components with different Larmor frequencies, whose origin is an open question. We exploit the photocharging approach to identify their origin and find that surface states play a key role in the appearance of the spin signals. By controlling the photocharging with electron or hole acceptors, we show that the specific spin component can be enhanced by the choice of acceptor type. In core/shell CdSe/ZnS QDs, the spin signals are significantly weaker. Our results exclude the neutral exciton as the spin origin and suggest that both Larmor frequencies are related to the coherent spin precession of electrons in photocharged QDs. The lower frequency is due to the electron confined in the middle of the QD, and the higher frequency to the electron additionally localized in the vicinity of the surface. © 2019 American Chemical Society.

Coherent optical spectroscopy such as four-wave mixing and photon echo generation deliver rich information on the energy levels involved in optical transitions through the analysis of polarization of the coherent response. In semiconductors, it can be applied to distinguish between different exciton complexes, which is a highly non-trivial problem in optical spectroscopy. We develop a simple approach based on photon echo polarimetry, in which polar plots of the photon echo amplitude are measured as function of the angle φ between the linear polarizations of the two exciting pulses. The rosette-like polar plots reveal a distinct difference between the neutral and charged exciton (trion) optical transitions in semiconductor nanostructures. We demonstrate this experimentally by photon echo polarimetry of a CdTe/(Cd, Mg)Te quantum well. The echoes of the trion and donor-bound exciton are linearly polarized at the angle 2φ with respect to the first pulse polarization and their amplitudes are weakly dependent on φ. While on the exciton the photon echo is co-polarized with the second exciting pulse and its amplitude scales as cosφ. © 2019, The Author(s).

We suggest a pump-probe method for studying semiconductor spin dynamics based on pumping of carrier spins by a pulse of oscillating radiofrequency (rf) magnetic field and probing by measuring the Faraday rotation of a short laser pulse. We demonstrate this technique on n-GaAs and observe the onset and decay of coherent spin precession during and after the course of rf pulse excitation. We show that the rf field resonantly addresses the electron spins with Larmor frequencies close to that of the rf field. This opens the opportunity to determine the homogeneous spin coherence time T2, that is inaccessible directly in standard all-optical pump-probe experiments. © 2019 American Physical Society.

We observe optical second harmonic generation (SHG) on the 1S exciton resonance of the yellow exciton series in Cu2O in four crystal directions ([001], [1̄10],[112̄], and [111]). For light with the k-vector parallel [001] and [1̄10], SHG is symmetry forbidden in an ideal crystal according to band structure. We explain the observation of SHG signals in the forbidden geometries by local strain inducing splitting and mixing of the 1S components. Experimental data and a microscopic theory confirm that this effect observed on the 1S exciton requires a relatively long exciton lifetime. The SHG signals of excitons with principal quantum number n≥2 are compliant with selection rules derived by group theory from the symmetry of an unstrained crystal. © 2019 American Physical Society.

We report on ultrafast time-resolved pump-probe studies in a CdZnTe/CdMgTe planar guiding structure covered with a metallic grating. The one-dimensional periodic gold structure allows for efficient coupling into the guiding layer for p-polarized 30 fs optical pulses with a large spectral bandwidth of about 60 nm. The resulting spectral width of optical pulses propagating inside the guiding layer corresponds to 20-30 nm. We demonstrate that the excitation of exciton-polariton modes in the guiding layer leads to a modulation of the optical response in the vicinity of the excitonic resonance. Spatially resolved pump-probe measurements show an asymmetric behavior in the optical response when the relative position of the pump and probe spots is varied on the scale of ten micrometers perpendicular to the metal ridges. This is attributed to the excitation of resonant and off-resonant exciton-polariton modes which propagate in opposite directions inside the guiding layer in accordance with their dispersion relations. Two main mechanisms are considered and evaluated, namely, Pauli blocking and excitation-induced dephasing, which are shown to be responsible for the pump-induced changes in the exciton absorption spectrum. While both of these processes lead to the generation of photoexcited carriers in the guiding layer, their impact on the optical properties (transmission and reflection) are different which leads to the asymmetric behavior of the spatially resolved transients. © 2019 American Physical Society.

The coherent spin dynamics of resident electrons and holes in an isotopically purified ZnSe/(Zn,Mg)Se single quantum well is investigated in different regimes, requiring corresponding adaption of the applied time-resolved pump-probe Kerr rotation technique. The purification of the Zn and Se atom species in the crystal to the isotopes with zero nuclear spin is expected to lead to an extension of the spin dephasing times of resident carriers, due to the suppression of their interaction with the nuclear spins. Indeed, we find no indication of carrier-nuclear interaction in this sample and link the observed carrier spin relaxation to the spin-orbit interaction. Theoretical considerations support the experimental results. © 2019 American Physical Society.

Demands for miniaturization, increasing the operation speed and energy efficiency of electronic devices led to the emergence and rapid development of spin electronics, or spintronics. Several areas of experimental and theoretical research are considered, in which the Ioffe Institute is actively involved. We discuss current progress in developing semiconductor and hybrid structures that exhibit specified magnetic properties, the development of methods for manipulating individual spins, a theoretical description of switching of metallic heterostructures magnetization by an electric field, and ultrafast control of magnetization via manipulating the magnetic anisotropy by femtosecond laser pulses. © 2019 Uspekhi Fizicheskikh Nauk, Russian Academy of Sciences and IOP Publishing.

In spin noise spectroscopy, the magnetic susceptibility spectrum is known to be provided by the spin-system untouched by any external perturbation, or, better to say, disturbed only by its thermal bath. We propose a new version of spin noise spectroscopy, with the detected magnetization (Faraday-rotation) noise being "stimulated" by an external fluctuating magnetic field with a quasiwhite spectrum. An experimental study of the stimulated spin noise performed on a BaF 2: U 3 + crystal in a longitudinal magnetic field has revealed specific features of this approach and allowed us to identify the Van-Vleck and population-related contributions to the AC susceptibility of the system and to discover unusual magnetic-field dependence of the longitudinal spin relaxation rate in low magnetic fields. It is shown that spectra of the stimulated and spontaneous spin noise, being both closely related to the spin-system magnetic susceptibility, are still essentially different. Distinctions between the two types of the spin-noise spectra and two approaches to spin noise spectroscopy are discussed. © 2019 Author(s).

We use time-resolved detection of the Hanle effect and polarized photoluminescence with dark intervals to investigate the buildup and decay of the spin polarization of nuclei interacting with donor-bound electrons in n-doped GaAs. Strong hyperfine coupling defines the millisecond time scale of the spin dynamics of these nuclei, as distinct from the nuclei far from impurity centers, characterized by a thousand times longer spin-relaxation time. The dynamics of spin polarization and relaxation attributed to the nuclei inside the donor orbit is observed on the time scale from 200 to 425 ms. © 2019 American Physical Society.

This work is devoted to a theoretical analysis of the effect of nuclear-induced field (Overhauser field) on the Larmor frequencies of electron spins under the periodic pulsed excitation. To describe the dynamical nuclear spin polarization, we use the model where the optically induced Stark field determines the magnitude and direction of the Overhauser field. The Stark field strongly depends on the detuning between the photon energy of excitation and the optical transition energy in the quantum system. Detailed calculations which show that the precession frequencies of fluorine donor-bound electron spins in ZnSe deviate from the linear dependence of the Larmor frequencies on the external magnetic field have been performed. A similar effect is observed for the (In,Ga)As/GaAs quantum dots, where it has been shown that the Overhauser field strongly changes the spectrum of the electron spin precession frequencies. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Dark excitons are of fundamental importance for a wide variety of processes in semiconductors but are difficult to investigate using optical techniques due to their weak interaction with light fields. We reveal and characterize dark excitons nonresonantly injected into a semiconductor microcavity structure containing InGaAs/GaAs quantum wells by a gated train of eight 100 fs pulses separated by 13 ns by monitoring their interactions with the bright lower polariton mode. We find a surprisingly long dark exciton lifetime of more than 20 ns, which is longer than the time delay between two consecutive pulses. This creates a memory effect that we clearly observe through the variation of the time-resolved transmission signal. We propose a rate equation model that provides a quantitative agreement with the experimental data. © 2019 American Physical Society.

Magneto-optical spectroscopy based on the transverse magneto-optical Kerr effect (TMOKE) is a sensitive method for investigating magnetically-ordered media. Previous studies were limited to the weak coupling regime where the spectral width of optical transitions considerably exceeded the Zeeman splitting in magnetic field. Here, we investigate experimentally and theoretically the transverse Kerr effect in the vicinity of comparatively narrow optical resonances in confined quantum systems. For experimental demonstration we studied the ground-state exciton resonance in a (Cd,Mn)Te diluted magnetic semiconductor quantum well, for which the strong exchange interaction with magnetic ions leads to giant Zeeman splitting of exciton spin states. For low magnetic fields in the weak coupling regime, the Kerr effect magnitude grows linearly with increasing Zeeman splitting showing a dispersive S-shaped spectrum, which remains almost unchanged in this range. For large magnetic fields in the strong coupling regime, the magnitude saturates, whereas the spectrum becomes strongly modified by the appearance of two separate peaks. TMOKE is sensitive not only to the sample surface but can also be used to probe in detail the confined electronic states in buried nanostructures if their capping layer is sufficiently transparent. © 2019 Olga V. Borovkova, Ilya A. Akimov et al.

In this study transverse magneto-optical Kerr effect in transmission for the magnetite thin film with gold grating on top was examined theoretically and experimentally. The magnetite films with thickness of 200 nm were fabricated by the laser electrodispersion technique. Arrays of gold stripes with 600 nm period and 350 nm and 500 nm stripe widths were created by lift-off e-beam lithography. It was shown experimentally that the TMOKE enhancement in structures with 350 nm width is attributed to SPP excitation on gold-magnetite surface. TMOKE value δ reaches magnitude of 5•10-3 at the wavelengths 790 nm and 860 nm at light incidence angles of 8-10° and 12-13°, respectively. In case of 500 nm stripe width, SPP excitation is poor, however δ reaches magnitude of 6.6•10-3 at angles of incidence more than 20°, that can be associated with the quasiguided mode resonance in such structure. © Published under licence by IOP Publishing Ltd.

Semiconductor nanocrystalline heterostructures can be produced by the immersion of semiconductor substrates into an aqueous precursor solution, but this approach usually leads to a high density of interfacial traps. In this work, we study the effect of a chemical passivation of the substrate prior to the nanocrystalline growth. PbS nanoplatelets grown on sulfur-treated InP (001) surfaces at temperatures as low as 95 °C exhibit abrupt crystalline interfaces that allow a direct and reproducible electron transfer to the InP substrate through the nanometer-thick nanoplatelets with scanning tunnelling spectroscopy. It is in sharp contrast with the less defined interface and the hysteresis of the current-voltage characteristics found without the passivation step. Based on a tunnelling effect occurring at energies below the bandgap of PbS, we show the formation of a type II, trap-free, epitaxial heterointerface, with a quality comparable to that grown on a nonreactive InP (110) substrate by molecular beam epitaxy. Our scheme offers an attractive alternative to the fabrication of semiconductor heterostructures in the gas phase. Copyright © 2019 American Chemical Society.

In the reported experiment, a picosecond strain pulse induces a sharp transition between the steady states in a bistable cavity-polariton system. The strain pulse of 10-ps duration, generated in the GaAs substrate and injected into a high-Q GaAs/AlAs microcavity, modulates the exciton resonance energies of the embedded quantum wells and correspondingly of the polariton resonances. When the microcavity is pumped by a laser with the photon energy slightly above the lower-polariton resonance, the strain-induced energy shift triggers the irreversible switching of the bistable polariton system from the lower to the upper state. This transition is accompanied by an instant increase of the optical emission from the microcavity by more than an order of magnitude. © 2019 American Physical Society.

The transverse magneto-optical Kerr effect (TMOKE) in magnetite-based magnetoplasmonic crystals is studied experimentally and theoretically. We analyze angle-resolved TMOKE spectra from two types of structures where noble metallic stripes are incorporated inside a thin magnetite film or located on top of a homogeneous film. A multiple-wide-band enhancement of the TMOKE signal in transmission is demonstrated. The complex dielectric permittivity and gyration are experimentally determined using the ellipsometry technique as well as Faraday rotation and ellipticity measurements. The obtained parameters are used in rigorous coupled-wave analysis (RCWA) calculations for studying the optical resonances. Our RCWA calculations of transmittance and TMOKE are in good agreement with the experimental data. The role of guiding and plasmonic modes in the TMOKE enhancement is revealed. We demonstrate that the TMOKE provides rich information about the studied optical resonances. © 2019 American Physical Society.

We study the band-edge exciton fine structure and in particular its bright-dark splitting in colloidal semiconductor nanocrystals by four different optical methods based on fluorescence line narrowing and time-resolved measurements at various temperatures down to 2 K. We demonstrate that all these methods provide consistent splitting values and discuss their advances and limitations. Colloidal CdSe nanoplatelets with thicknesses of 3, 4 and 5 monolayers are chosen for experimental demonstrations. The bright-dark splitting of excitons varies from 3.2 to 6.0 meV and is inversely proportional to the nanoplatelet thickness. Good agreement between experimental and theoretically calculated size dependence of the bright-dark exciton splitting is achieved. The recombination rates of the bright and dark excitons and the bright to dark relaxation rate are measured by time-resolved techniques. © The Royal Society of Chemistry 2018.

We study the linear polarization properties of the photoluminescence of ensembles of neutral and negatively charged nitrogen vacancies and neutral vacancies in diamond crystals as a function of their symmetry and their response to strong external magnetic fields. The linear polarization degree, which exceeds 10% at room temperature, and rotation of the polarization plane of their zero-phonon lines significantly depend on the crystal rotation around specific axes demonstrating anisotropic angular evolutions. The sign of the polarization plane rotation is changed periodically through the crystal rotation, which indicates a switching between electron excited states of orthogonal linear polarizations. At external magnetic fields of up to 10 T, the angular dependencies of the linear polarization degree experience a remarkable phase shift. Moreover, the rotation of the linear polarization plane increases linearly with rising magnetic field at 6 K and room temperature, for the negatively charged nitrogen vacancies, which is attributed to magneto-optical Faraday rotation. © 2018 American Physical Society.

We describe the major requirements to experimentally perform and observe resonant spin-flip Raman scattering on excitonic resonances in low-dimensional semiconductors. We characterize in detail the properties of this resonant light scattering technique and evaluate the criteria, which must be fulfilled by the experimental setup and the semiconductor sample studied to be able to observe a spin-flip scattering process. We also demonstrate the influence of additional unpolarized laser illumination with energies, which exceed considerably the band gap energy of the semiconductor nanostructure under study, on the resonantly excited electron spin-flip scattering in InAs-based quantum dot ensembles as well as on the paramagnetic Mn-ion spin-flip in (Zn,Mn)Se/(Zn,Be)Se quantum wells. © 2018, Pleiades Publishing, Ltd.

We study the circularly polarized photoluminescence of negatively charged (NV-) and neutral (NV0) nitrogen-vacancy ensembles and neutral vacancies (V0) in diamond crystals exposed to magnetic fields of up to 10 T. We determine the orbital and spin Zeeman splitting as well as the energetic ordering of their ground and first-excited states. The spin-triplet and -singlet states of the NV- are described by an orbital Zeeman splitting of about 9 μeV/T, which corresponds to a positive orbital g-factor of gL=0.164 under application of the magnetic field along the (001) and (111) crystallographic directions, respectively. The zero-phonon line (ZPL) of the NV- singlet is defined as a transition from the 1E′ states, which are split by gLμBB, to the 1A1 state. The energies of the zero-phonon triplet transitions show a quadratic dependence on intermediate magnetic field strengths, which we attribute to a mixing of excited states with nonzero orbital angular momentum. Moreover, we identify slightly different spin Zeeman splittings in the ground (gs) and excited (es) triplet states, which can be expressed by a deviation between their spin g-factors: gS,es=gS,gs+Δg with values of Δg=0.014 and 0.029 in the (001) and (111) geometries, respectively. The degree of circular polarization of the NV- ZPLs depends significantly on the temperature, which is explained by an efficient spin-orbit coupling of the excited states mediated through acoustic phonons. We further demonstrate that the sign of the circular polarization degree is switched under rotation of the diamond crystal. A weak Zeeman splitting similar to ΔgμBB measured for the NV- ZPLs is also obtained for the NV0 zero-phonon lines, from which we conclude that the ground state is composed of two optically active states with compensated orbital contributions and opposite spin-1/2 momentum projections. The zero-phonon lines of the V0 show Zeeman splittings and degrees of the circular polarization with opposite signs. The magnetophotoluminescence data indicate that the electron transition from the T21 states to the A1 ground state defines the zero-phonon emission at 1.674 eV, while the T21→E1 transition is responsible for the zero-phonon line at 1.666 eV. The T21 (E1) states are characterized by an orbital Zeeman splitting with gL=0.071 (0.128). © 2018 American Physical Society.

We study optically the coherent evolution of trions and excitons in a δ-doped 3.5-nm-thick ZnO/Zn0.91Mg0.09O multiple quantum well by means of time-resolved four-wave mixing at a temperature of 1.5 K. Employing spectrally narrow picosecond laser pulses in the χ(3) regime allows us to address differently localized trion and exciton states, thereby avoiding many-body interactions and excitation-induced dephasing. The signal in the form of photon echoes from the negatively charged A excitons (TA, trions) decays with coherence times varying from 8 up to 60 ps, depending on the trion energy: more strongly localized trions reveal longer coherence dynamics. The localized neutral excitons decay on the picosecond time scale with coherence times up to T2=4.5 ps. The coherent dynamics of the XB exciton and TB trion are very short (T2<1 ps), which is attributed to the fast energy relaxation from the trion and exciton B states to the respective A states. The trion population dynamics is characterized by the decay time T1, rising from 30 to 100 ps with decreasing trion energy. © 2018 American Physical Society.

We present results on photon echo spectroscopy for resonant excitation of localized charged exciton complexes (trions) in CdTe/CdMgTe semiconductor quantum wells. We demonstrate that the Zeeman splitting of resident electron spin levels in transverse magnetic field leads to quantum beats in the photon echoes with the Larmor precession frequency. This allows us to perform a coherent transfer of optical excitation into a spin ensemble and to observe long-lived photon echoes. Our approach can be used as a tool for remarkably high resolution spectroscopy of the ground state levels: We are able to resolve splittings between the spin levels with sub-μeV precision and to distinguish between different types of electrons in the ensemble, namely electrons either bound to donors or localized on quantum well potential fluctuations. To that end we show that stimulated step-like Raman processes in the two-pulse excitation scheme allow us to probe the electron spin ensemble with high selectivity and precision even for systems with broad optical transitions. Next, Rabi oscillations for exciton complexes with different degree of localization are detected by photon echo spectroscopy. We observe that an increase of the area of either the first or the second pulse leads to a significant decrease of the photon echo signal, which is strongest for the neutral excitons and less pronounced for the donor-bound exciton complex. © 2018 COPYRIGHT SPIE. Downloading of the abstract is permitted for personal use only.

We study the modification of the exciton absorption in cuprous oxide by the presence of free carriers excited through above band gap excitation. Without this pumping, the absorption spectrum below the band gap consists of the yellow exciton series with principal quantum numbers up to more than n = 20, depending on the temperature, changing over to an about constant, only slowly varying absorption above the gap. Careful injection of free carriers, starting from densities well below 1 μm–3, leads to a reduction of the band gap through correlation effects. The excitons in the Rydberg regime above n = 10 remain unaffected until the band gap approaches them. Then they lose oscillator strength and ultimately vanish upon crossing with the band gap. © 2018, Pleiades Publishing, Ltd.

The periodic optical orientation of electron spins in (In,Ga)As/GaAs quantum dots leads to the formation of electron spin precession modes about an external magnetic field which are resonant with the pumping periodicity. As the electron spin is localized within a nuclear spin bath, its polarization imprints onto the spin polarization of the bath. The latter acts back on the electron spin polarization. We implement a pulse protocol where a train of laser pulses is followed by a long, dark gap. It allows us to obtain a high-resolution precession mode spectrum from the free evolution of the electron spin polarization. Additionally, we vary the number of pump pulses in a train to investigate the buildup of the precession modes. To separate out nuclear effects, we suppress the nuclear polarization by using a radio-frequency field. We find that a long-living nuclear spin polarization imprinted by the periodic excitation significantly speeds up the buildup of the electron spin polarization and induces the formation of additional electron spin precession modes. To interpret these findings, we extend an established dynamical nuclear polarization model to take into account optically detuned quantum dots for which nuclear spins activate additional electron spin precession modes. © 2018 American Physical Society.

We report on detecting continuous 60-GHz microwave radiation with powers in the nanowatt range by the photoluminescence of an ensemble of negatively charged nitrogen vacancy (NV-) centers in diamond at room temperature. The high contrast of the optically detected magnetic resonance and the efficient photon collection yield a magnetic field sensitivity of 86 nT / Hz for continuous-wave laser excitation with a photon energy of 2.33 eV and a power density of 93 W/cm2. The efficiency of the microwave-power-to-magnetic-field conversion amounts to 0.54 mT / W. The microwave excitation also enhances the degree of the linear polarization of NV- photoluminescence at magnetic resonance conditions, and for linearly co-polarized NV- photoluminescence and laser light, the magnetic field sensitivity is improved by about 7%. © 2018 Author(s).

We study the coherent evolution of an InAs self-assembled quantum dot (QD) ensemble in the ultrafast regime. The evolution of the entire frequency distribution is revealed by performing prepulse two-dimensional (2D) coherent spectroscopy. Charged and neutral QDs display distinct nonlinear responses arising from two-level trion and four-level exciton-biexciton systems, respectively, and each signal is clearly separated in 2D spectra. Whereas the signals for charged QDs are symmetric with respect to the detuning, those for neutral QDs are asymmetric due to the asymmetric four-level energy structure. Experimental results for charged and neutral QDs are well reproduced by solving the optical Bloch equations, including detuning and excitation-induced dephasing (EID) effects. The temperature dependence suggests that wetting-layer carriers play an important role in EID. © 2018 American Physical Society.

The coherent spin dynamics of fluorine donor-bound electrons in ZnSe induced by pulsed optical excitation is studied in a perpendicular applied magnetic field. The Larmor precession frequency serves as a measure for the total magnetic field exerted onto the electron spins and, surprisingly, does not increase linearly with the applied field, but shows a step-like behavior with pronounced plateaus, given by multiples of the laser repetition rate. This discretization occurs by a feedback mechanism in which the electron spins polarize the nuclear spins, which in turn generate a local Overhauser field adjusting the total magnetic field accordingly. Varying the optical excitation power, we can control the plateaus, in agreement with our theoretical model. From this model, we trace the observed discretization to the optically induced Stark field, which causes the dynamic nuclear polarization. © 2018 The Author(s).

The electric field-induced dissociation is studied for excited states of the yellow exciton series of Cu2O. With increasing principal quantum number n, corresponding to rising exciton energy, the field strength for dissociation decreases as expected. Surprisingly, within a manifold belonging to a particular n this trend is reversed as the required dissociation field increases with rising energy. In agreement with calculations we attribute this finding to the distribution of the exciton wave functions in the potential landscape. While the low energy states in the multiplet are shifted towards the side where the potential is lowered by the electric field, thereby facilitating dissociation, the high energy states are moved to the other side stabilizing them up to higher fields. © 2018 American Physical Society.

We address spin properties and spin dynamics of carriers and charged excitons in CdSe/CdS colloidal nanoplatelets with thick shells. Magneto-optical studies are performed by time-resolved and polarization-resolved photoluminescence, spin-flip Raman scattering and picosecond pump-probe Faraday rotation in magnetic fields up to 30 T. We show that at low temperatures the nanoplatelets are negatively charged so that their photoluminescence is dominated by radiative recombination of negatively charged excitons (trions). Electron g-factor of 1.68 is measured, and heavy-hole g-factor varying with increasing magnetic field from -0.4 to -0.7 is evaluated. Hole g-factors for two-dimensional structures are calculated for various hole confining potentials for cubic- and wurtzite lattice in CdSe core. These calculations are extended for various quantum dots and nanoplatelets based on II-VI semiconductors. We developed a magneto-optical technique for the quantitative evaluation of the nanoplatelets orientation in ensemble. © 2017 American Chemical Society.

We monitor the phonon sideband emission from paraexcitons confined in a strain trap in cuprous oxide at T = 1.25 K. On the low energy ank of the optical phonon replicas, both of Γ5 − and Γ3 − symmetry (the latter activated by application of a magnetic field), we detect sharp peaks that might represent indications for a paraexciton Bose–Einstein condensate. In contrast, such peaks are absent in the phonon-mediated emission of the orthoexcitons, and they also disappear at elevated temperatures. The results challenge our understanding of the involved physics, e.g., of the Auger recombination of excitons, which has so far been believed to prevent crossing the border to a condensate. © 2018, Pleiades Publishing, Ltd.

Nonlinear optical phenomena are widely used for the study of semiconductor materials. The paper presents an overview of experimental and theoretical studies of excitons by the method of optical second and third harmonics generation in various bulk semiconductors (GaAs, CdTe, ZnSe, ZnO, Cu2O, (Cd,Mn)Te, EuTe, EuSe), and low-dimensional heterostructures ZnSe/BeTe. Particular attention is paid to the role of external electric and magnetic fields that modify the exciton states and induce new mechanisms of optical harmonics generation. Microscopic mechanisms of harmonics generation based on the Stark effect, the spin and orbital Zeeman effects, and on the magneto-Stark effect specific for excitons moving in an external magnetic field are considered. This approach makes it possible to study the properties of excitons and to obtain new information on their energy and spin structure that is not available when the excitons are investigated by linear optical spectroscopy. As a result of these studies, a large amount of information was obtained, which allows us to conclude on the establishing of a new field of research—exciton spectroscopy by the method of optical harmonics generation. © 2018, Pleiades Publishing, Ltd.

A high-amplitude microwave magnetic field localized at the nanoscale is a desirable tool for various applications within the rapidly developing field of nanomagnetism. Here we drive magnetization precession by coherent phonons in a metal ferromagnetic nanograting and generate ac-magnetic induction with extremely high amplitude (up to 10 mT) and nanometer scale localization in the grating grooves. We trigger the magnetization precession by a laser pulse which excites localized surface acoustic waves. The developed technique has prospective uses in several areas of research and technology, including spatially resolved access to spin states for quantum technologies. © 2018 American Physical Society.

We present a spectroscopic technique for second harmonic generation (SHG) using femtosecond laser pulses at a 30-kHz repetition rate, which nevertheless provides high spectral resolution limited only by the spectrometer. The potential of this method is demonstrated by applying it to the yellow exciton series of Cu2O. Besides even parity states with S and D envelopes, we also observe odd parity P excitons with linewidths down to 100 μeV, despite the broad excitation laser spectrum with a full width at half maximum of 14 meV. The underlying light-matter interaction mechanisms of SHG are elaborated by a group theoretical analysis which allows us to determine the linear and circular polarization dependencies, in good agreement with experiment. © 2018 American Physical Society.

We implement the homodyne detection scheme for an increase in the polarimetric sensitivity in spin noise spectroscopy. Controlling the laser intensity of the local oscillator, which is guided around the sample and does not perturb the measured spin system, we are able to improve the signal-to-noise ratio. The opportunity for additional amplification of the measured signal strength allows us to reduce the probe laser intensity incident on the sample and therefore to approach the nonperturbative regime. The efficiency of this scheme with signal enhancement by more than a factor of 3 at low probe powers is demonstrated on bulk n-doped GaAs, where the reduced electron-spin relaxation rate is shown experimentally. Additionally, the control of the optical phase provides us with the possibility to switch between measuring Faraday rotation and ellipticity without changes in the optical setup. © 2018 American Physical Society.

We study the spectrum of the yellow exciton series in crossed electric and magnetic fields. The electric field, applied along the optical axis, tilts the Coulomb potential between electron and hole, so that at sufficiently high fields exciton dissociation becomes possible, roughly when the electric dipole interaction energy exceeds the binding energy of an exciton state with principal quantum number n. For an applied voltage of U = 20 V all excitons above n = 6 are dissociated. Additional application of a magnetic field normal to the optical axis introduces magnetic confinement, due to which above a threshold field strength around B = 2.5 T the exciton lines re-emerge. The complex dispersion with increasing fields suggests quantum chaotic behavior in this crossed field configuration, so that the search for exceptional points may be promising. © 2018, Pleiades Publishing, Ltd.

We show that the dissociation threshold of an exciton, a bound electron-hole pair, by an electric field is mainly determined by its energy: as expected, the dissociation voltage decreases with increasing exciton energy. However, within the multiplet of states belonging to a particular principal quantum number n, the dissociation voltage rises with increasing state energy, in contrast to the expectations based on energy arguments. This behavior is demonstrated for the yellow exciton states of Cu2O and is attributed to the distribution of the wavefunction in the potential landscape, where the lower (higher) lying state in the multiplet is shifted away (towards) the tunnel barrier. © 2018, Pleiades Publishing, Ltd.

The magnetization properties of a ferromagnet-semiconductor Co/CdMgTe/CdTe quantum well hybrid structure are investigated by several techniques. Exploiting the proximity effect between acceptor bound holes and magnetic ions we detect the magnetization curves by measuring the circular polarization of photoluminescence in an out-of-plane magnetic field. We show that magnetization originates from interfacial ferromagnet on Co-CdMgTe interface and the proximity effect is caused by magnetization of interfacial Co-CdMgTe ferromagnetic layer whose magnetic properties are very different from Co. © 2018, Pleiades Publishing, Ltd.

Lately, the yellow series of P-excitons in cuprous oxide could be resolved up to the principal quantum number n = 25. Adding a magnetic field, leads to additional confinement normal to the field. Thereby, the transition associated with the exciton n is transformed into the transition between the electron and hole Landau levels with quantum number n, once the associated magnetic length becomes smaller than the related exciton Bohr radius. The magnetic field of this transition scales roughly as n–3. As a consequence of the extended exciton series, we are able to observe Landau level transitions with unprecedented high quantum numbers of more than 75. © 2018, Pleiades Publishing, Ltd.

We study the coherent dynamics of localized excitons in 100 periods of 2.5-nm-thick (In,Ga)N/GaN quantum wells with 7.5% indium concentration, measured with spectroscopic resolution through two-pulse and three-pulse photon echoes at the temperature of 1.5 K. A long-lived coherent exciton dynamics is observed in the (In,Ga)N quantum wells: When the laser photon energy is tuned across the 43-meV-wide inhomogeneously broadened resonance line, the coherence time T2 varies between 45 and 255 ps, increasing with stronger exciton localization. The corresponding narrow homogeneous linewidths ranging from 5.2 to 29μeV as well as the relatively weak exciton-phonon interaction (0.7μeV/K) confirm a strong, quantum-dot-like exciton localization in a static disordered potential inside the (In,Ga)N quantum well layers. © 2018 American Physical Society.

Periodic laser pulsing of singly charged semiconductor quantum dots in an external magnetic field leads to a synchronization of the spin dynamics with the optical excitation. The pumped electron spins partially rephase prior to each laser pulse, causing a revival of electron spin polarization with its maximum at the incidence time of a laser pulse. The amplitude of this revival is amplified by the frequency focusing of the surrounding nuclear spins. Two complementary theoretical approaches for simulating up to 20 million laser pulses are developed and employed that are able to bridge between 11 orders of magnitude in time: a fully quantum mechanical description limited to small nuclear bath sizes and a technique based on the classical equations of motion applicable for a large number of nuclear spins. We present experimental data of the nonmonotonic revival amplitude as function of the magnetic field applied perpendicular to the optical axis. The dependence of the revival amplitude on the external field with a profound minimum at 4T is reproduced by both of our theoretical approaches and is ascribed to the nuclear Zeeman effect. Since the nuclear Larmor precession determines the electronic resonance condition, it also defines the number of electron spin revolutions between pump pulses, the orientation of the electron spin at the incidence time of a pump pulse, and the resulting revival amplitude. The magnetic field of 4T, for example, corresponds to half a revolution of nuclear spins between two laser pulses. © 2018 American Physical Society.

A time-resolved optical pump-probe technique has been applied for studying the ultrafast dynamics in the magnetic semiconductor EuTe near the absorption band gap. We show that application of external magnetic field up to 6 T results in crossover from the inverse Faraday effect taking place on the femtosecond time scale to the optical orientation phenomenon with an evolution in the picosecond time domain. We propose a model which includes both these processes, possessing different spectral and temporal properties. The circularly polarized optical pumping induces the electronic transition 4 f 7 5 d 0 → 4 f 6 5 d 1 forming the absorption band gap in EuTe. The observed crossover is related to a strong magnetic-field shift of the band gap in EuTe at low temperatures. It was found that manipulation of spin states on intrinsic defect levels takes place on a time scale of 19 ps in the applied magnetic field of 6 T. © 2018 Author(s).

We investigate and compare experimental and numerical excitonic spectra of the yellow series in cuprous oxide (Cu2O) in the Voigt configuration and thus partially extend the results of F. Schweiner et al. [Phys. Rev. B 95, 035202 (2017)2469-995010.1103/PhysRevB.95.035202], who considered only the Faraday configuration. The main difference between the configurations is given by an additional effective electric field in the Voigt configuration, caused by the motion of the exciton through the magnetic field. This magneto-Stark effect was already postulated in 1961 by E. F. Gross et al. [Sov. Phys. Solid State 3, 221 (1961)] and D. G. Thomas and J. J. Hopfield [Phys. Rev. 124, 657 (1961)PHRVAO0031-899X10.1103/PhysRev.124.657]. Group-theoretical considerations show that, most of all, the field significantly increases the number of allowed lines by decreasing the symmetry of the system. This conclusion is supported by both the experimental and numerical data. © 2018 American Physical Society.

Optically detected magnetic resonance (ODMR) is measured for photoexcited electrons in (In,Al)As/AlAs quantum dots having an indirect band gap and a type-I band alignment. A sharp ODMR resonance corresponding to a g factor of 1.97±0.02 is identified, associated with X-valley electrons. Its variation with optical transition energy allows us to distinguish between the spectral regions of direct and indirect QDs, which is in good agreement with the results on exciton recombination dynamics. © 2018 American Physical Society.

We demonstrate spin pumping, i.e., the generation of a pure spin current by precessing magnetization, without the application of microwave radiation commonly used in spin pumping experiments. We use femtosecond laser pulses to simultaneously launch the magnetization precession in each of two ferromagnetic layers of a galfenol-based spin valve and monitor the temporal evolution of the magnetizations. The spin currents generated by the precession cause a dynamic coupling of the two layers. This coupling has a dissipative character and is especially efficient when the precession frequencies in the two layers are in resonance, where coupled modes with strongly different decay rates are formed. © 2018 American Physical Society.

The optical spin Hall effect appears when elastically scattered exciton polaritons couple to an effective magnetic field inside of quantum wells in semiconductor microcavities. Theory predicts an oscillation of the pseudospin of the exciton polaritons in time. Here, we present a detailed analysis of momentum space dynamics of the exciton polariton pseudospin. Compared to what is predicted by theory, we find a higher modulation of the temporal oscillations of the pseudospin. We attribute the higher modulation to additional components of the effective magnetic field which have been neglected in the foundational theory of the optical spin Hall effect. Adjusting the model by adding non-linear polariton-polariton interactions, we find a good agreement in between the experimental results and simulations. © 2018, Pleiades Publishing, Ltd.

We use a time-resolved technique with three laser pulses (pump, orientation and probe) to study the photocharging dynamics with picosecond resolution on a long timescale ranging from ps to ms in CdS colloidal quantum dots. The detection is based on measuring the coherent spin dynamics of electrons, allowing us to distinguish the type of carrier in the dot core (electron or hole). We find that although initially negative photocharging happens because of fast hole trapping on surface states, eventually it evolves to positive photocharging due to electron trapping and hole detrapping. The positive photocharging lasts up to hundreds of microseconds at room temperature. © 2018, Pleiades Publishing, Ltd.

Photon echo from trions and excitons in (In,Ga)As/GaAs quantum dots was studied theoretically and experimentally. Theoretical analysis allowed us to distinguish between photon echo signals from excitons and trions measured in the same range of wavelength using different polarization configurations of laser excitation. The theoretical predictions are in good agreement with the experimental data. © 2018, Pleiades Publishing, Ltd.

An overview on photon echo spectroscopy under resonant excitation of the exciton complexes in semiconductor nanostructures is presented. The use of four-wave-mixing technique with the pulsed excitation and heterodyne detection allowed us to measure the coherent response of the system with the picosecond time resolution. It is shown that, for resonant selective pulsed excitation of the localized exciton complexes, the coherent signal is represented by the photon echoes due to the inhomogeneous broadening of the optical transitions. In case of resonant excitation of the trions or donor-bound excitons, the Zeeman splitting of the resident electron ground state levels under the applied transverse magnetic field results in quantum beats of photon echo amplitude at the Larmor precession frequency. Application of magnetic field makes it possible to transfer coherently the optical excitation into the spin ensemble of the resident electrons and to observe a long-lived photon echo signal. The described technique can be used as a high-resolution spectroscopy of the energy splittings in the ground state of the system. Next, we consider the Rabi oscillations and their damping under excitation with intensive optical pulses for the excitons complexes with a different degree of localization. It is shown that damping of the echo signal with increase of the excitation pulse intensity is strongly manifested for excitons, while on trions and donor-bound excitons this effect is substantially weaker. © 2018, Pleiades Publishing, Ltd.

Manifestations of quantum interference effects in macroscopic objects are rare. Weak localization is one of the few examples of such effects showing up in the electron transport through solid state. Here, we show that weak localization becomes prominent also in optical spectroscopy via detection of the electron spin dynamics. In particular, we find that weak localization controls the free electron spin relaxation in semiconductors at low temperatures and weak magnetic fields by slowing it down by almost a factor of two in n-doped GaAs in the metallic phase. The weak localization effect on the spin relaxation is suppressed by moderate magnetic fields of approximately 1 T, which destroy the interference of electron trajectories, and by increasing the temperature. The weak localization suppression causes an anomalous decrease of the longitudinal electron spin relaxation time T1 with magnetic field, in stark contrast with the well-known magnetic-field-induced increase in T1. This is consistent with transport measurements, which show the same variation of resistivity with magnetic field. Our discovery opens up a vast playground to explore quantum magnetotransport effects optically in the spin dynamics. © 2018 authors. Published by the American Physical Society.

Magneto-optical phenomena such as the Faraday and Kerr effects play a central role in controlling the polarization and intensity of optical fields propagating through a medium. Intensity effects in which the direction of light emission depends on the orientation of the external magnetic field are of particular interest, as they can be harnessed for routing light. Effects known so far for accomplishing such routing all control light emission along the axis parallel to the magnetic field. Here we report a new class of emission phenomena where directionality is established perpendicular to the externally applied magnetic field for light sources located in the vicinity of a surface. As a proof of principle for this effect, which we call transverse magnetic routing of light emission, we demonstrate the routing of emission for excitons in a diluted-magnetic-semiconductor quantum well. In hybrid plasmonic semiconductor structures, we observe significantly enhanced directionality of up to 60%. © 2018, The Author(s).

We study the Rydberg exciton absorption of Cu2O in the presence of free carriers injected by above-band-gap illumination. Already at plasma densities ρEH below one hundredth electron-hole pair per μm3, exciton lines are bleached, starting from the highest observed principal quantum number, while their energies remain constant. Simultaneously, the band gap decreases by correlation effects with the plasma. An exciton line loses oscillator strength when the band gap approaches its energy, vanishing completely at the crossing point. Adapting a plasma-physics description, we describe the observations by an effective Bohr radius that increases with rising plasma density, reflecting the Coulomb interaction screening by the plasma. © 2018 American Physical Society.

We present an investigation of the electron spin dynamics in an ensemble of singly charged semiconductor quantum dots subject to an external magnetic field and laser pumping with circularly polarized light. The spectral laser width is tailored such that ensembles with an increasing number of quantum dots are coherently pumped. Surprisingly, the dephasing time T∗ of the electron spin polarization depends only weakly on the laser spectral width. These findings can be consistently explained by a cluster theory of coupled quantum dots with a long-range electronic spin-spin interaction. We present a numerical simulation of the spin dynamics based on the central spin model that includes a quantum mechanical description of the laser pulses as well as a time-independent Heisenberg interaction between each pair of electron spins. We discuss the individual dephasing contributions stemming from the Overhauser field, the distribution of the electron g factors, and the electronic spin-spin interaction as well as the spectral width of the laser pulse. This analysis reveals counterbalancing effects on the total dephasing time when increasing the spectral laser width. On one hand, the increasing deviations of the electron g factors reduce the dephasing time. On the other hand, more electron spins are coherently pumped and synchronize due to the electronic spin-spin interaction which extends the dephasing time. We find an excellent agreement between the experimental data and the dephasing time in the simulation using an exponential distribution of Heisenberg couplings with a mean value J≈0.26μeV. © 2018 American Physical Society.

We study optical pumping of resident electron spins under resonant excitation of trions in n-type CdTe/(Cd,Mg)Te quantum wells subject to a transverse magnetic field. In contrast to the comprehensively used time-resolved pump-probe techniques with polarimetric detection, we exploit here a single beam configuration in which the time-integrated intensity of the excitation laser light transmitted through the quantum wells is detected. The transmitted intensity reflects the bleaching of light absorption due to optical pumping of the resident electron spins and can be used to evaluate the Larmor precession frequency of the optically oriented carriers and their spin relaxation time. Application of the magnetic field leads to depolarization of the electron spin ensemble so that the Hanle effect is observed. Excitation with a periodic sequence of laser pulses leads to optical pumping in the rotating frame if the Larmor precession frequency is synchronized with the pulse repetition rate. This is manifested by the appearance of Hanle curves every 3.36 or 44.2 mT for pulse repetition rates of 75.8 or 999 MHz, respectively. From the experimental data we evaluate the g factor of |g|=1.61 and the spin relaxation time of 14 ns for the optically pumped resident electrons, in agreement with previous time-resolved pump-probe studies. © 2018 American Physical Society.

The spin dynamics in a broad range of systems can be studied using circularly polarized optical excitation with alternating helicity. The dependence of spin polarization on the frequency of helicity alternation, known as the spin inertia effect, is used here to study the spin dynamics in singly charged (In,Ga)As/GaAs quantum dots (QDs), providing insight into spin generation and accumulation processes. We demonstrate that the dependence of spin polarization in n- and p-type QDs on the external magnetic field has a characteristic V- and M-like shape, respectively. This difference is related to different microscopic mechanisms of the resident carriers' spin orientation. It allows us to determine the parameters of the spin dynamics both for the ground and excited states of singly charged QDs. © 2018 American Physical Society.

Spin-lattice relaxation of the nuclear spin system in p-type GaAs is studied using a three-stage experimental protocol including optical pumping and measuring the difference of the nuclear spin polarization before and after a dark interval of variable length. This method allows us to measure the spin-lattice relaxation time T1 of optically pumped nuclei "in the dark," that is, in the absence of illumination. The measured T1 values fall into the subsecond time range, being three orders of magnitude shorter than in earlier studied n-type GaAs. The drastic difference is further emphasized by magnetic-field and temperature dependencies of T1 in p-GaAs, showing no similarity to those in n-GaAs. This unexpected behavior finds its explanation in the spatial selectivity of the optical pumping in p-GaAs, that is only efficient in the vicinity of shallow donors, together with the quadrupole relaxation of nuclear spins, which is induced by electric fields within closely spaced donor-acceptor pairs. The developed theoretical model explains the whole set of experimental results. © 2018 American Physical Society.

Streak cameras are powerful tools for temporal characterization of ultrafast light pulses, even at the single-photon level. However, the low signal-To-noise ratio in the infrared range prevents measurements on weak light sources in the telecom regime. We present an approach to circumvent this problem, utilizing an up-conversion process in periodically poled waveguides in Lithium Niobate. We convert single photons from a parametric down-conversion source in order to reach the point of maximum detection efficiency of commercially available streak cameras. We explore phase-matching configurations to apply the up-conversion scheme in real-world applications. © 2018 Author(s).

Photon echo from trions and excitons in (In,Ga)As/GaAs quantum dots has been studied theoretically and experimentally. Theoretical analysis allowed us to distinguish photon echo signals from excitons and trions measured in the same range of wavelength using different polarization configurations of laser excitation. The theoretical predictions are in good agreement with the experimental data. © Published under licence by IOP Publishing Ltd.

The spin inertia measurement is a recently emerged tool to study slow spin dynamics, which is based on the excitation of the system by a train of circularly polarized pulses with alternating helicity. Motivated by the experimental results reported by Zhukov et al. [Phys. Rev. B 98, 121304(R) (2018)10.1103/PhysRevB.98.121304], we develop the general theory of spin inertia of localized charge carriers. We demonstrate that the spin inertia measurement in longitudinal magnetic field allows one to determine the parameters of the spin dynamics of resident charge carriers and of photoexcited trions, such as the spin relaxation times, longitudinal g factors, parameters of hyperfine interaction, and nuclear spin correlation times. © 2018 American Physical Society.

We report on a comprehensive experimental and theoretical study of optical third harmonic generation (THG) on the exciton-polariton resonances in the zinc-blende semiconductors GaAs, CdTe, and ZnSe subject to an external magnetic field, representing a topic that had remained unexplored so far. In these crystals, crystallographic THG is allowed in the electric-dipole approximation, so that substantial magnetic-field-induced changes of the THG are unexpected: the symmetry reduction due to magnetic field, corresponding change of the selection rules, and the Zeeman effect are expected to play a minor role. Surprisingly, we observe a strong enhancement of the THG intensity by a factor of 50 for the 1s exciton-polariton in GaAs in magnetic fields up to 10 T. In contrast, the corresponding enhancement is moderate in CdTe and almost absent in ZnSe. In order to explain this strong variation, we develop a microscopic theory accounting for the optical harmonics generation on exciton-polaritons and analyze the THG mechanisms induced by the magnetic field. The calculations show that the increase of THG intensity is dominated by the magnetic field enhancement of the exciton oscillator strength, which is particularly strong for GaAs in the studied range of field strengths. The much weaker increase of THG intensity in CdTe and ZnSe is explained by the considerably larger exciton binding energies, leading to a weaker modification of their oscillator strengths by the magnetic field. © 2018 American Physical Society.

We study the photoluminescence of self-assembled (In,Ga)As/GaAs quantum dot ensembles with varying confinement potential height. The low energy shift of the s-shell emission with increasing excitation power gives a measure of the Coulomb interaction in these structures as it results from carrier–carrier interactions between the optically injected exciton complexes. When dividing this shift by the dot level splitting, determined by the geometric confinement, we obtain a universal function of the number of involved excitons that is independent of the confinement potential height. This shows an identical scaling of Coulomb interaction and geometric quantization with varying confinement. © 2018, Pleiades Publishing, Ltd.

We develop a nanoscopy method with in-depth resolution for layered photonic devices. Photonics often requires tailored light field distributions for the optical modes used, and an exact knowledge of the geometry of a device is crucial to assess its performance. The presented acousto-optical nanoscopy method is based on the uniqueness of the light field distributions in photonic devices: for a given wavelength, we record the reflectivity modulation during the transit of a picosecond acoustic pulse. The temporal profile obtained can be linked to the internal light field distribution. From this information, a reverse-engineering procedure allows us to reconstruct the light field and the underlying photonic structure very precisely. We apply this method to the slow light mode of an AlAs/GaAs micropillar resonator and show its validity for the tailored experimental conditions. © 2017 Optical Society of America.

Travelling coherent phonons can be actively used to manipulate the optical properties of semiconductor nanostructures on the picosecond time scale. Phonon wave packets that interact with a quantum dot (QD) ensemble can significantly vary the output intensity of a laser, which uses the QDs as active medium. Based on a recently developed theoretical model to describe this coupled phonon-QD-photon system, we here study how the laser response on phonon wave packets depends on several parameters, for example phonon pulse properties and laser pump rate. © Published under licence by IOP Publishing Ltd.

We study Rabi oscillations detected in the coherent optical response from various exciton complexes in a 20-nm-thick CdTe/(Cd,Mg)Te quantum well using time-resolved photon echoes. In order to evaluate the role of exciton localization and inhomogeneous broadening we use selective excitation with spectrally narrow ps pulses. We demonstrate that the transient profile of the photon echo from the localized trion (X-) and the donor-bound exciton (D0X) transitions strongly depends on the strength of the first pulse. It acquires a non-Gaussian shape and experiences significant advancement for pulse areas larger than π due to non-negligible inhomogeneity-induced dephasing of the oscillators during the optical excitation. Next, we observe that an increase of the area of either the first (excitation) or the second (rephasing) pulse leads to a significant damping of the photon echo signal, which is strongest for the neutral excitons and less pronounced for the donor-bound exciton complex (D0X). The measurements are analyzed using a theoretical model based on the optical Bloch equations which accounts for the inhomogeneity of optical transitions in order to reproduce the complex shape of the photon echo transients. In addition, the spreading of Rabi frequencies within the ensemble due to the spatial variation of the intensity of the focused Gaussian beams and excitation-induced dephasing are incorporated in our model, which is able to explain the fading and damping of Rabi oscillations. By analyzing the results of the simulation for X- and D0X complexes we are able to establish a correlation between the degree of localization and the transition dipole moments determined as μ(X-)=73 D and μ(D0X)=58 D. © 2017 American Physical Society.

The exchange interaction between magnetic ions and charge carriers in semiconductors is considered to be a prime tool for spin control. Here, we solve a long-standing problem by uniquely determining the magnitude of the long-range p-d exchange interaction in a ferromagnet-semiconductor (FM-SC) hybrid structure where a 10-nm-thick CdTe quantum well is separated from the FM Co layer by a CdMgTe barrier with a thickness on the order of 10 nm. The exchange interaction is manifested by the spin splitting of acceptor bound holes in the effective magnetic field induced by the FM. The exchange splitting is directly evaluated using spin-flip Raman scattering by analyzing the dependence of the Stokes shift ΔS on the external magnetic field B. We show that in a strong magnetic field, ΔS is a linear function of B with an offset of Δpd=50-100μeV at zero field from the FM induced effective exchange field. On the other hand, the s-d exchange interaction between conduction band electrons and FM, as well as the p-d contribution for free valence band holes, are negligible. The results are well described by the model of indirect exchange interaction between acceptor bound holes in the CdTe quantum well and the FM layer mediated by elliptically polarized phonons in the hybrid structure. © 2017 American Physical Society.

The optical properties of colloidal semiconductor nanocrystals are largely influenced by the trapping of charge carriers on the nanocrystal surface. Different concentrations of electron and hole traps and different rates of their capture to the traps provide dynamical charging of otherwise neutral nanocrystals. We study the photocharging formation and evolution dynamics in CdS colloidal quantum dots with native oleic acid surface ligands. A time-resolved technique with three laser pulses (pump, orientation, and probe) is developed to monitor the photocharging dynamics with picosecond resolution on wide time scales ranging from picoseconds to milliseconds. The detection is based on measuring the coherent spin dynamics of electrons, allowing us to distinguish the type of carrier in the QD core (electron or hole). We find that although initially negative photocharging happens because of fast hole trapping, it eventually evolves to positive photocharging due to electron trapping and hole detrapping. The positive photocharging lasts up to hundreds of microseconds at room temperature. These findings give insight into the photocharging process and provide valuable information for understanding the mechanisms responsible for the emission blinking in colloidal nanostructures. © 2017 American Chemical Society.

We study the exciton magnetic polaron (EMP) formation in (Cd,Mn)Se/(Cd,Mg)Se diluted-magnetic-semiconductor quantum wells by using time-resolved photoluminescence (PL). The magnetic-field and temperature dependencies of this dynamics allow us to separate the nonmagnetic and magnetic contributions to the exciton localization. We deduce the EMP energy of 14 meV, which is in agreement with time-integrated measurements based on selective excitation and the magnetic-field dependence of the PL circular polarization degree. The polaron formation time of 500 ps is significantly longer than the corresponding values reported earlier. We propose that this behavior is related to strong self-localization of the EMP, accompanied with a squeezing of the heavy-hole envelope wave function. This conclusion is also supported by the decrease of the exciton lifetime from 600 ps to 200-400 ps with increasing magnetic field and temperature. © 2017 American Physical Society.

We study the time evolution of the optical spin Hall effect, which occurs when exciton polaritons undergo resonant Rayleigh scattering. The resulting spin pattern in momentum space is quantified by calculating the degree of circular polarization of the momentum space image for each point in time. We find the degree of circular polarization performing oscillations, which can be described within the framework of the pseudospin model by Kavokin et al. [A. Kavokin, G. Malpuech, and M. Glazov, Phys. Rev. Lett. 95, 136601 (2005)PRLTAO0031-900710.1103/PhysRevLett.95.136601]. © 2017 American Physical Society.

The spin-flip Raman scattering efficiency of the resident electron is thermally enhanced in singly charged (In,Ga)As/GaAs quantum dots, for probing the s- or p-shell trions. The Raman shift, polarization characteristics, and spectral position of the resonant scattering profile are insensitive to the sample temperature up to 50 K. This indicates a thermally robust mechanism of the coherent electron spin-flip based on exchange interaction. The background of the scattering spectra, whose intensity increases also by about one order of magnitude with temperature, is associated with acoustic phonon scattering. We propose that acoustic phonons enhance the spin-flip probability of the resident electron with growing temperature. The coherent spin-flip Raman scattering is ultimately suppressed at temperatures, which are a few times lower than that required for thermal trion dissociation. © 2017 American Physical Society.

The evolution of the electron spin dynamics as consequence of carrier delocalization in n-type GaAs is investigated by the recently developed extended pump-probe Kerr/Faraday rotation spectroscopy. We find that isolated electrons localized on donors demonstrate a prominent difference between the longitudinal and transverse spin relaxation rates in a magnetic field, which is almost absent in the metallic phase. The inhomogeneous transverse dephasing time T2∗ of the spin ensemble strongly increases upon electron delocalization as a result of motional narrowing that can be induced by increasing either the donor concentration or the temperature. An unexpected relation between T2∗ and the longitudinal spin relaxation time T1 is found, namely, that their product is about constant, as explained by the magnetic field effect on the spin diffusion. We observe a two-stage longitudinal spin relaxation, which suggests the establishment of spin temperature in the system of exchange-coupled donor-bound electrons. © 2017 American Physical Society.

The spin relaxation dynamics of Ce3+ ions in heavily cerium-doped YAG crystals is studied using pulse-electron paramagnetic resonance and time-resolved pump-probe Faraday rotation. Both techniques address the 4f ground state, while pump-probe Faraday rotation also provides access to the excited 5d state. We measure a millisecond spin-lattice relaxation time T1, a microsecond spin coherence time T2, and a ∼10 ns inhomogeneous spin dephasing time T2∗ for the Ce3+ ground state at low temperatures. The spin-lattice relaxation of Ce3+ ions is due to modified Raman processes involving the optical phonon mode at ∼125cm-1. The relaxation at higher temperature goes through a first excited level of the F522 term at about ω≈228cm-1. Effects provided by the hyperfine interaction of the Ce3+ with the Al27 nuclei are observed. © 2017 American Physical Society.

The transferred hyperfine interactions of Ce3+ centers with 27Al neighbor nuclei in heavily cerium doped yttrium aluminum garnet (YAG) are resolved using Pulsed-Electron Nuclear Double Resonance (Pulsed-ENDOR). It is demonstrated that substitution of Ce3+ for Y3+ in YAG crystals leads to the strong reduction of the nuclear quadrupole coupling at the tetrahedral aluminum d-sites (e2qQ/h ≈ 4.67 MHz). © 2017 Author(s).

We report on the observation of a significant increase of the radiative decay rates for exciton transitions in a wide (In,Ga)As/GaAs quantum well (L=95 nm) in magnetic fields up to 6 T applied along the growth axis of the heterostructure. The absolute values of the radiative decay rates are obtained from a quantitative analysis of resonant features in the experimentally measured reflectance spectra in the range of the optical transitions to the quantum-confined exciton states. High crystalline quality of the heterostructure allows us to observe the ground and several excited exciton transitions with the nonradiative broadening comparable to the radiative one. We employ a numerical procedure appropriate for the studied wide quantum well to model the increase of the radiative decay rate in magnetic field. The results of the modeling agree well with the experimental data. © 2017 American Physical Society.

It is demonstrated that the transverse magneto-optical Kerr effect experiences two-order enhancement in the spectral region of the excitonic resonance in the diluted magnetic semiconductor nanostructures. It is studied how the TMOKE depends on the incident angle and extenal magnetic field. The theoretical investigations are in a good agreement with experimental results. © 2017 IEEE.

Currently spin waves are considered for computation and data processing as an alternative to charge currents. Generation of spin waves by ultrashort laser pulses provides several important advances with respect to conventional approaches using microwaves. In particular, focused laser spot works as a point source for spin waves and allows for directional control of spin waves and switching between their different types. For further progress in this direction it is important to manipulate with the spectrum of the optically generated spin waves. Here we tackle this problem by launching spin waves by a sequence of femtosecond laser pulses with pulse interval much shorter than the relaxation time of the magnetization oscillations. This leads to the cumulative phenomenon and allows us to generate magnons in a specific narrow range of wavenumbers. The wavelength of spin waves can be tuned from 15 μm to hundreds of microns by sweeping the external magnetic field by only 10 Oe or by slight variation of the pulse repetition rate. Our findings expand the capabilities of the optical spin pump-probe technique and provide a new method for the spin wave generation and control. © 2017 The Author(s).

Using semiconductor quantum dots in a cavity-quantum electrodynamics laser we show a direct connection between superradiant pulse emission and the photon correlations. This demonstrates the importance of quantum correlations in novel optoelectronic devices. © OSA 2017.

The dynamic polarization of nuclear spins by photoexcited electrons is studied in a high quality GaAs/AlGaAs quantum well. We find a surprisingly high efficiency of the spin transfer from the electrons to the nuclei as reflected by a maximum nuclear field of 0.9 T in a tilted external magnetic field of 1 T strength only. This high efficiency is due to a low leakage of spin out of the polarized nuclear system, because mechanisms of spin relaxation other than the hyperfine interaction are strongly suppressed, leading to a long nuclear relaxation time of up to 1000 s. A key ingredient to that end is the low impurity concentration inside the heterostructure, while the electrostatic potential from charged impurities in the surrounding barriers becomes screened through illumination by which the spin relaxation time is increased compared to keeping the system in the dark. This finding indicates a strategy for obtaining high nuclear spin polarization as required for long-lasting carrier spin coherence. © 2017 American Physical Society.

We have used high-resolution transmission spectroscopy to study the exciton level spectrum in Cu2O subject to a longitudinal external electric field, i.e., in the geometry where the transmitted light is propagating along the field direction. Different experimental configurations given by the field orientation relative to the crystal and the light polarization have been explored. We focus on the range of small principal quantum numbers n≤7. The number of exciton states belonging to a particular principal quantum number increases with n, leading to an enhanced complexity of the spectra. Still, in particular, for n=3,...,5, a spectral separation of the different lines is feasible and identification as well as assignment of the dominant state character are possible. We find a strong dependence of the spectra on the chosen light propagation direction and polarization configuration, reflecting the inadequacy of the hydrogen model for describing the excitons. With increasing the field excitonic states with different parity become mixed, leading to optical activation of states that are dark in zero field. As compared with atoms, due to the reduced Rydberg energy states with different n can be brought into resonance in the accessible electric field strength range. When this occurs, we observe mostly crossing of levels within the experimental accuracy showing that the electron and hole motion remains regular. The observed features are well described by detailed calculations accounting for the spin-orbit coupling, the cubic anisotropy effects, and the symmetry-imposed optical selection rules. © 2017 American Physical Society.

We have used high-resolution transmission spectroscopy to study the exciton level spectrum in Cu2O subject to a longitudinal external electric field, i.e., in the geometry where the transmitted light is propagating along the field direction. Different experimental configurations given by the field orientation relative to the crystal and the light polarization have been explored. We focus on the range of small principal quantum numbers n≤7. The number of exciton states belonging to a particular principal quantum number increases with n, leading to an enhanced complexity of the spectra. Still, in particular, for n=3,...,5, a spectral separation of the different lines is feasible and identification as well as assignment of the dominant state character are possible. We find a strong dependence of the spectra on the chosen light propagation direction and polarization configuration, reflecting the inadequacy of the hydrogen model for describing the excitons. With increasing the field excitonic states with different parity become mixed, leading to optical activation of states that are dark in zero field. As compared with atoms, due to the reduced Rydberg energy states with different n can be brought into resonance in the accessible electric field strength range. When this occurs, we observe mostly crossing of levels within the experimental accuracy showing that the electron and hole motion remains regular. The observed features are well described by detailed calculations accounting for the spin-orbit coupling, the cubic anisotropy effects, and the symmetry-imposed optical selection rules. © 2017 American Physical Society.

Multidimensional coherent optical spectroscopy is one of the most powerful tools for investigating complex quantum mechanical systems. While it was conceived decades ago in magnetic resonance spectroscopy using microwaves and radio waves, it has recently been extended into the visible and UV spectral range. However, resolving MHz energy splittings with ultrashort laser pulses still remains a challenge. Here, we analyze two-dimensional Fourier spectra for resonant optical excitation of resident electrons to localized trions or donor-bound excitons in semiconductor nanostructures subject to a transverse magnetic field. Particular attention is devoted to Raman coherence spectra, which allow one to accurately evaluate tiny splittings of the electron ground state and to determine the relaxation times in the electron spin ensemble. A stimulated steplike Raman process induced by a sequence of two laser pulses creates a coherent superposition of the ground-state doublet which can be retrieved only optically because of selective excitation of the same subensemble with a third pulse. This provides the unique opportunity to distinguish between different complexes that are closely spaced in energy in an ensemble. The related experimental demonstration is based on photon-echo measurements in an n-type CdTe=ðCd; MgÞTe quantum-well structure detected by a heterodyne technique. The difference in the sub-μeV range between the Zeeman splittings of donor-bound electrons and electrons localized at potential fluctuations can be resolved even though the homogeneous linewidth of the optical transitions is larger by 2 orders of magnitude.

Growing competitive pressure forces companies to optimise process productivity and shorten primary production times. At the same time, the resulting manufacturing quality must be kept on a high level. In the automotive sector, deep hole drilling with smallest tool diameters is an important process, e.g. to produce lubrication holes in crankshafts and fuel channels in injectors. A crucial criterion for the achievable productivity and manufacturing quality with respect to the dimensional and shape tolerances as well as the surface quality in smallest diameter deep hole drilling is the chip formation. Therefore, in-depth analyses regarding the mechanisms of chip formation at the cutting edge and the chip removal along the chip flutes are indispensable. To accomplish an in-depth chip formation analysis in smallest diameter deep hole drilling, a new methodology of analysis has been developed. Samples made of the particular test material are inserted into acrylic glass carriers, and the chip formation in the operating zone and the chip removal are documented by high-speed microscopy. In this paper, the experimental setup of the newly developed methodology of analysis and the experimental results for single-lip and twist deep hole drilling of high-strength bainitic steel with smallest diameters are shown. The investigations show the dependence of chip formation on the changes of the microstructure of the cutting edge due to tool wear, and form the basis for an optimization of the tools. In addition to that, a new approach to visualise machining processes running under non-transparent coolant is presented. © 2017 Springer-Verlag London Ltd., part of Springer Nature

Non-magnetic colloidal nanostructures can demonstrate magnetic properties typical for diluted magnetic semiconductors because the spins of dangling bonds at their surface can act as the localized spins of magnetic ions. Here we report the observation of dangling-bond magnetic polarons (DBMPs) in 2.8-nm diameter CdSe colloidal nanocrystals (NCs). The DBMP binding energy of 7 meV is measured from the spectral shift of the emission lines under selective laser excitation. The polaron formation at low temperatures occurs by optical orientation of the dangling-bond spins (DBSs) that result from dangling-bond-assisted radiative recombination of spin-forbidden dark excitons. Modelling of the temperature dependence of the DBMP-binding energy and emission intensity shows that the DBMP is composed of a dark exciton and about 60 DBSs. The exchange integral of one DBS with the electron confined in the NC is ∼0.12 meV. © 2017 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.

Two of the most striking experimental findings when investigating exciton spectra in cuprous oxide using high-resolution spectroscopy are the observability and the fine structure splitting of F excitons reported by J. Thewes et al. [Phys. Rev. Lett. 115, 027402 (2015)PRLTAO0031-900710.1103/PhysRevLett.115.027402]. These findings show that it is indispensable to account for the complex valence band structure and the cubic symmetry of the solid in the theory of excitons. This is all the more important for magnetoexcitons, where the external magnetic field reduces the symmetry of the system even further. We present the theory of excitons in Cu2O in an external magnetic field and especially discuss the dependence of the spectra on the direction of the external magnetic field, which cannot be understood from a simple hydrogenlike model. Using high-resolution spectroscopy, we also present the corresponding experimental spectra for cuprous oxide in Faraday configuration. The theoretical results and experimental spectra are in excellent agreement as regards not only the energies but also the relative oscillator strengths. Furthermore, this comparison allows for the determination of the fourth Luttinger parameter κ of this semiconductor. © 2017 American Physical Society.

Optical tools are promising for spin-wave generation because of the possibilities of ultrafast manipulation and local excitation. However, a single laser pulse can inject spin waves (SWs) only with a broad frequency spectrum, resulting in short propagation distances and low wave amplitudes. Here, we excite a magnetic garnet film by a train of fs-laser pulses with a 1-GHz repetition rate so that the pulse separation is shorter than the decay time of magnetic modes, which allows us to achieve a collective impact on the magnetization and establish a quasistationary source of spin waves, namely, a coherent accumulation of magnons ("magnon cloud"). This approach has several appealing features: (i) The magnon source is tunable, (ii) the SW amplitude can be significantly enhanced, (iii) the SW spectrum is quite narrow, providing long-distance propagation, (iv) the periodic pumping results in an almost constant-in-time SWamplitude for the distances larger than 20 μm away from the source, and (v) the SW emission shows pronounced directionality. These results expand the capabilities of ultrafast coherent optical control of magnetization and pave the way for applications in data processing, including the quantum regime. The quasistationary magnon accumulation might also be of interest for applications in magnon Bose-Einstein condensates.

The optical properties of colloidal cesium lead halide perovskite (CsPbBr3) nanocrystals are examined by time-resolved and polarization-resolved spectroscopy in high magnetic fields up to 30 T. We unambiguously show that at cryogenic temperatures the emission is dominated by recombination of negatively charged excitons with radiative decay time of 300 ps. The additional long-lived emission, which decay time shortens from 40 down to 8 ns and in which the decay time shortens and relative amplitude increases in high magnetic fields, evidences the presence of a dark exciton. We evaluate g-factors of the bright exciton gX = +2.4, the electron ge = +2.18, and the hole gh = -0.22. © 2017 American Chemical Society.

Electron spin dephasing in a singly charged semiconductor quantum dot can partially be suppressed by periodic laser pulsing. We propose a semiclassical approach describing the decoherence of the electron spin polarization governed by the hyperfine interaction with the nuclear spins as well as the probabilistic nature of the photon absorption. We use the steady-state Floquet condition to analytically derive two subclasses of resonance conditions excellently predicting the peak locations in the part of the Overhauser field distribution which is projected in the direction of the external magnetic field. As a consequence of the periodic pulsing, a nonequilibrium distribution develops as a function of time. The numerical simulation of the coupled dynamics reveals the influence of the hyperfine coupling constant distribution onto the evolution of the electron spin polarization before the next laser pulse. Experimental indications are provided for both subclasses of resonance conditions. © 2017 American Physical Society.

The spin noise in singly charged self-assembled quantum dots is studied theoretically and experimentally under the influence of a perturbation, provided by additional photoexcited charge carriers. The theoretical description takes into account generation and relaxation of charge carriers in the quantum dot system. The spin noise is measured under application of above barrier excitation for which the data are well reproduced by the developed model. Our analysis demonstrates a strong difference of the recharging dynamics for holes and electrons in quantum dots. © 2017 American Physical Society.

The spin dynamics of localized donor-bound electrons interacting with the nuclear spin ensemble in n-doped GaAs epilayers is studied using nuclear spin polarization by light with modulated circular polarization. We show that the observed buildup of the nuclear spin polarization is a result of competition between nuclear spin cooling and nuclear spin warmup in the oscillating Knight field. The developed model allows us to explain the dependence of nuclear spin polarization on the modulation frequency and to estimate the equilibration time of the nuclear spin system that appears to be shorter than the transverse relaxation time T2 determined from nuclear magnetic resonance. © 2017 American Physical Society.

We report on the coherent optical response from an ensemble of (In,Ga)As quantum dots (QDs) embedded in a planar Tamm-plasmon microcavity with a quality factor of approximately 100. Significant enhancement of the light-matter interaction is demonstrated under selective laser excitation of those quantum dots which are in resonance with the cavity mode. The enhancement is manifested through Rabi oscillations of the photon echo, demonstrating coherent control of excitons with picosecond pulses at intensity levels more than an order of magnitude smaller as compared with bare quantum dots. The decay of the photon echo transients is weakly changed by the resonator, indicating a small decrease of the coherence time T2 which we attribute to the interaction with the electron plasma in the metal layer located close (40 nm) to the QD layer. Simultaneously we see a reduction of the population lifetime T1, inferred from the stimulated photon echo, due to an enhancement of the spontaneous emission by a factor of 2, which is attributed to the Purcell effect, while nonradiative processes are negligible, as confirmed from time-resolved photoluminescence. © 2017 American Physical Society.

A picosecond acoustic pulse is used to study the photoelastic interaction in single zinc-blende GaN/AlxGa1-xN quantum wells. We use an optical time-resolved pump-probe setup and demonstrate that tuning the photon energy to the quantum well's lowest electron-hole transition makes the experiment sensitive to the quantum well only. Because of the small width, its temporal and spatial resolution allows us to track the few-picosecond-long transit of the acoustic pulse. We further deploy a model to analyze the unknown photoelastic coupling strength of the quantum well for different photon energies and find good agreement with the experiments. © 2017 American Physical Society.

A picosecond acoustic pulse can be used to control the lasing emission from semiconductor nanostructures by shifting their electronic transitions. When the active medium, here an ensemble of (In,Ga)As quantum dots, is shifted into or out of resonance with the cavity mode, a large enhancement or suppression of the lasing emission can dynamically be achieved. Most interesting, even in the case when gain medium and cavity mode are in resonance, we observe an enhancement of the lasing due to shaking by coherent phonons. In order to understand the interactions of the nonlinearly coupled photon-exciton-phonon subsystems, we develop a semiclassical model and find an excellent agreement between theory and experiment. © 2017 American Physical Society. American Physical Society.

We investigate the transition from regular behavior towards chaos for Rydberg excitons in the presence of an external magnetic field. To this end, we develop a measure for chaos that is robust with respect to fluctuations in the density of states. We find that irrespective of whether or not the magnetic field is oriented along a high-symmetry direction of the crystal, all antiunitary symmetries of the system are broken for large field strengths. This result emphasizes the influence of phonons on the symmetry of Rydberg excitons. In addition, we find that the appearance of Landau levels above the band gap results in modified level spacing statistics that resemble systems without broken symmetry. © 2017 American Physical Society.

Circularly polarized optical excitation generates electron spin polarization in the lowest 5d state of rare-earth Ce3+ ions in a YAG crystal. The 5d electron spin dynamics is investigated in transverse and longitudinal magnetic fields by time-resolved pump-probe Faraday rotation. Long lived electron spin coherence with a dephasing time of 2.5 ns is found at room temperature. In a transverse magnetic field of 1 T, the electron spin coherence shows a distinct beating-like amplitude modulation due to several slightly different Larmor frequencies corresponding to different electron g factors of magnetically inequivalent positions of the Ce3+ ions in the crystal lattice. Hyperfine coupling between the 5d electron of Ce3+ ions and environmental nuclear spins dominates the spin relaxation, which can be efficiently suppressed by a longitudinal magnetic field as small as 10 mT. The dependence of electron spin relaxation on both the transverse and longitudinal magnetic fields agrees well with the one predicted theoretically for the hyperfine coupling mechanism. © 2017 Author(s).

Rydberg atoms have attracted considerable interest due to their huge interaction among each other and with external fields. They demonstrate characteristic scaling laws in dependence on the principal quantum number n for features such as the magnetic field for level crossing or the electric field of dissociation. Recently, the observation of excitons in highly excited states has allowed studying Rydberg physics in cuprous oxide crystals. Fundamentally different insights may be expected for Rydberg excitons, as the crystal environment and associated symmetry reduction compared to vacuum give not only optical access to many more states within an exciton multiplet but also extend the Hamiltonian for describing the exciton beyond the hydrogen model. Here we study experimentally and theoretically the scaling of several parameters of Rydberg excitons with n, for some of which we indeed find laws different from those of atoms. For others we find identical scaling laws with n, even though their origin may be distinctly different from the atomic case. At zero field the energy splitting of a particular multiplet n scales as n-3 due to crystal-specific terms in the Hamiltonian, e.g., from the valence band structure. From absorption spectra in magnetic field we find for the first crossing of levels with adjacent principal quantum numbers a Brn-4 dependence of the resonance field strength, Br, due to the dominant paramagnetic term unlike for atoms for which the diamagnetic contribution is decisive, resulting in a Brn-6 dependence. By contrast, the resonance electric field strength shows a scaling as Ern-5 as for Rydberg atoms. Also similar to atoms with the exception of hydrogen we observe anticrossings between states belonging to multiplets with different principal quantum numbers at these resonances. The energy splittings at the avoided crossings scale roughly as n-4, again due to crystal specific features in the exciton Hamiltonian. The data also allow us to assess the susceptibility of Rydberg excitons to the external fields: The crossover field strength in magnetic field from a hydrogenlike exciton to a magnetoexciton dominated by electron and hole Landau level quantization scales as n-3. In electric field, on the other hand, we observe the exciton polarizability to scale as n7. At higher fields, the exciton ionization can be studied with ionization voltages that demonstrate an n-4 scaling law. Particularly interesting is the field dependence of the width of the absorption lines which remains constant before dissociation for high enough n, while for small n12 an exponential increase is found. These results are in excellent agreement with theoretical predictions. © 2017 American Physical Society.

The exciton spin dynamics are investigated both experimentally and theoretically in two-monolayer-thick GaAs/AlAs quantum wells with an indirect band gap and a type-II band alignment. The magnetic field induced circular polarization of photoluminescence Pc is studied as function of the magnetic field strength and direction as well as sample temperature. The observed nonmonotonic behavior of these functions is provided by the interplay of bright and dark exciton states contributing to the emission. To interpret the experiment, we have developed a kinetic master equation model which accounts for the dynamics of the spin states in this exciton quartet, radiative and nonradiative recombination processes, and redistribution of excitons between these states as result of spin relaxation. The model offers quantitative agreement with experiment and allows us to evaluate, for the studied structure, the heavy-hole g factor, ghh=+3.5, and the spin relaxation times of electron, τse=33μs, and hole, τsh=3μs, bound in the exciton. © 2017 American Physical Society.

Photoluminescence polarization is experimentally studied for samples with (In,Ga)As/GaAs self-assembled quantum dots in transverse magnetic field (Hanle effect) under slow modulation of the excitation light polarization from fractions of Hz to tens of kHz. The polarization reflects the evolution of strongly coupled electron-nuclear spin systems in the quantum dots. Strong modification of the Hanle curves under variation of the modulation period is attributed to the peculiarities of the spin dynamics of quadrupole nuclei, which states are split due to deformation of the crystal lattice in the quantum dots. Analysis of the Hanle curves is fulfilled in the framework of a phenomenological model considering a separate dynamics of a nuclear field BNd determined by polarization of the ±1/2 nuclear spin states and of a nuclear field BNq determined by polarization of the split-off states ±3/2, ±5/2, etc. It is found that the characteristic relaxation time for the nuclear field BNd is of order a fraction of a second, while the relaxation of the field BNq is faster by about two orders of magnitude. © 2017 American Physical Society.

We synthesize heterofluorene monomers with Si, Ge, N, As, Se, and Te occupying the 9-position of the fluorene motif, which are then polymerized by Suzuki coupling. The optical properties of the obtained polymers are investigated in their solid state. We compare and elucidate effects in the materials absorption, emission, quantum yield (π), and fluorescence lifetime. Moreover, we determine the refractive indices n and absorption coefficient k by variable angle spectroscopic ellipsometry (VASE). We show that in addition to already known C, Si, and N containing polyfluorenes also Ge and As containing polymers exhibit amplified spontaneous emission. © 2017 American Chemical Society.

Coherent phonons can greatly vary light-matter interaction in semiconductor nanostructures placed inside an optical resonator on a picosecond time scale. For an ensemble of quantum dots (QDs) as active laser medium, phonons are able to induce a large enhancement or attenuation of the emission intensity, as has been recently demonstrated. The physics of this coupled phonon-exciton-light system consists of various effects, which in the experiment typically cannot be clearly separated, in particular, due to the complicated sample structure a rather complex strain pulse impinges on the QD ensemble. Here we present a comprehensive theoretical study how the laser emission is affected by phonon pulses of various shapes as well as by ensembles with different spectral distributions of the QDs. This gives insight into the fundamental interaction dynamics of the coupled phonon-exciton-light system, while it allows us to clearly discriminate between two prominent effects: the adiabatic shifting of the ensemble and the shaking effect. This paves the way to a tailored laser emission controlled by phonons. © 2017 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.

Dynamical strain generated upon excitation of a metallic film by a femtosecond laser pulse may become a versatile tool enabling control of the magnetic state of thin films and nanostructures via inverse magnetostriction on a picosecond time scale. Here, we explore two alternative approaches to manipulate magnetocrystalline anisotropy and excite magnetization precession in a low-symmetry film of a magnetic metallic alloy galfenol (Fe,Ga), either by injecting a picosecond strain pulse into it from a substrate, or by generating dynamical strain of a complex temporal profile in the film directly. In the former case, we realize ultrafast excitation of magnetization dynamics solely by strain pulses. In the latter case, optically-generated strain emerging abruptly in the film modifies its magnetocrystalline anisotropy, competing with heat-induced change of anisotropy parameters. We demonstrate that the optically-generated strain remains efficient for launching magnetization precession, when the heat-induced changes of anisotropy parameters do not trigger the precession any more. We emphasize that in both approaches the ultrafast change of magnetic anisotropy mediating the precession excitation relies on the mixed, compressive, and shear character of the dynamical strain, which emerges due to low-symmetry of the metallic film under study. © 2017 The Royal Swedish Academy of Sciences.

The coherent optical response from 140 nm and 65 nm thick ZnO epitaxial layers is studied using four-wave-mixing spectroscopy with picosecond temporal resolution. Resonant excitation of neutral donor-bound excitons results in two-pulse and three-pulse photon echoes. For the donor-bound A exciton (D0XA) at temperature of 1.8 K we evaluate optical coherence times T2=33-50 ps corresponding to homogeneous line widths of 13-19μeV, about two orders of magnitude smaller as compared with the inhomogeneous broadening of the optical transitions. The coherent dynamics is determined mainly by the population decay with time T1=30-40 ps, while pure dephasing is negligible. Temperature increase leads to a significant shortening of T2 due to interaction with acoustic phonons. In contrast, the loss of coherence of the donor-bound B exciton (D0XB) is significantly faster (T2=3.6ps) and governed by pure dephasing processes. © 2017 American Physical Society.

The electronic band structures of hexagonal ZnO and cubic ZnS, ZnSe, and ZnTe compounds are determined within hybrid-density-functional theory and quasiparticle calculations. It is found that the band-edge energies calculated on the G0 W0 (Zn chalcogenides) or GW (ZnO) level of theory agree well with experiment, while fully self-consistent QSGW calculations are required for the correct description of the Zn 3d bands. The quasiparticle band structures are used to calculate the linear response and second-harmonic-generation (SHG) spectra of the Zn-VI compounds. Excitonic effects in the optical absorption are accounted for within the Bethe-Salpeter approach. The calculated spectra are discussed in the context of previous experimental data and present SHG measurements for ZnO. © 2017 IOP Publishing Ltd.

We use spontaneous (two-pulse) and stimulated (three-pulse) photon echoes for studying the coherent evolution of optically excited ensemble of trions which are localized in semiconductor CdTe/CdMgTe quantum well. Application of transverse magnetic field leads to the Larmor precession of the resident electron spins, which shuffles optically induced polarization between optically accessible and inaccessible states. This results in several spectacular phenomena. First, magnetic field induces oscillations of spontaneous photon echo amplitude. Second, in three-pulse excitation scheme, the photon echo decay is extended by several orders of magnitude. In this study, short-lived optical excitation which is created by the first pulse is coherently transferred into a long-lived electron spin state using the second optical pulse. This coherent spin state of electron ensemble persists much longer than any optical excitation in the system, preserving information on initial optical field, which can be retrieved as a photon echo by means of third optical pulse. © 2016 SPIE.

Spins in semiconductor quantum dots have been considered as prospective quantum bit excitations. Their coupling to the crystal environment manifests itself in a limitation of the spin coherence times to the microsecond range, both for electron and hole spins. This rather short-lived coherence compared to atomic states asks for manipulations on timescales as short as possible. Due to the huge dipole moment for transitions between the valence and conduction band, pulsed laser systems offer the possibility to perform manipulations within picoseconds or even faster. Here, we report on results that show the potential of optical spin manipulations with currently available pulsed laser systems. Using picosecond laser pulses, we demonstrate optically induced spin rotations of electron and hole spins. We further realize the optical decoupling of the hole spins from the nuclear surrounding at the nanosecond timescales and demonstrate an all-optical spin tomography for interacting electron spin sub-ensembles. © 2016, Springer-Verlag Berlin Heidelberg.

We report on a temperature-, time-, and spectrally resolved study of the photoluminescence of type-I InP/ZnS colloidal nanocrystals with varying core size. By studying the exciton recombination dynamics we assess the exciton fine structure in these systems. In addition to the typical bright-dark doublet, the photoluminescence stems from an upper bright state in spite of its large energy splitting (∼100 meV). This striking observation results from dramatically lengthened thermalization processes among the fine structure levels and points to optical-phonon bottleneck effects in InP/ZnS nanocrystals. Furthermore, our data show that the radiative recombination of the dark exciton scales linearly with the bright-dark energy splitting for CdSe and InP nanocrystals. This finding strongly suggests a universal dangling bonds-assisted recombination of the dark exciton in colloidal nanostructures. © 2016 American Chemical Society.

Sophisticated models have been worked out to explain the fast relaxation of carriers into quantum dot ground states after non-resonant excitation, overcoming the originally proposed phonon bottleneck. We apply a magnetic field along the quantum dot heterostructure growth direction to transform the confined level structure, which can be approximated by a Fock-Darwin spectrum, from a nearly equidistant level spacing at zero field to strong anharmonicity in finite fields. This changeover leaves the ground state carrier population rise time unchanged suggesting that fast relaxation is maintained upon considerable changes of the level spacing. This corroborates recent models explaining the relaxation by polaron formation in combination with quantum kinetic effects. © 2016 Author(s).

The phonon properties of films fabricated from colloidal semiconductor nanocrystals play a major role in thermal conductance and electron scattering, which govern the principles for building colloidal-based electronics and optics including thermoelectric devices with a high ZT factor. The key point in understanding the phonon properties is to obtain the strength of the elastic bonds formed by organic ligands connecting the individual nanocrystallites. In the case of very weak bonding, the ligands become the bottleneck for phonon transport between infinitively rigid nanocrystals. In the opposite case of strong bonding, the colloids cannot be considered as infinitively rigid beads and the distortion of the superlattice caused by phonons includes the distortion of the colloids themselves. We use the picosecond acoustics technique to study the acoustic coherent phonons in superlattices of nanometer crystalline CdSe colloids. We observe the quantization of phonons with frequencies up to 30 GHz. The frequencies of quantized phonons depend on the thickness of the colloidal films and possess linear phonon dispersion. The measured speed of sound and corresponding wave modulus in the colloidal films point on the strong elastic coupling provided by organic ligands between colloidal nanocrystals. © 2015 American Chemical Society.

Coherent control of a strongly inhomogeneously broadened system, namely, InAs self-assembled quantum dots, is demonstrated. To circumvent the deleterious effects of the inhomogeneous broadening, which usually masks the results of coherent manipulation, we use prepulse two-dimensional coherent spectroscopy to provide a size-selective readout of the ground, exciton, and biexciton states. The dependence on the timing of the prepulse is due to the dynamics of the coherently generated populations. To further validate the results, we performed prepulse polarization dependent measurements and confirmed the behavior expected from selection rules. All measured spectra can be excellently reproduced by solving the optical Bloch equations for a 4-level system. © 2016 American Physical Society.

The coherent spin dynamics of carriers in the heterostructures that contain an InGaAs/GaAs quantum well (QW) and an Mn δ layer, which are separated by a narrow GaAs spacer 2–10 nm thick, is comprehensively studied by the magnetooptical Kerr effect method at a picosecond time resolution. The exchange interaction of photoexcited electrons in QW with the ferromagnetic Mn δ layer manifests itself in magnetic-field and temperature dependences of the Larmor precession frequency of electron spins and is found to be very weak (several microelectron volts). Two nonoscillating components related to holes exist apart from an electron contribution to the Kerr signal of polarization plane rotation. At the initial stage, a fast relaxation process, which corresponds to the spin relaxation of free photoexcited holes, is detected in the structures with a wide spacer. The second component is caused by the further spin dephasing of energyrelaxed holes, which are localized at strong QW potential fluctuations in the structures under study. The decay of all contributions to the Kerr signal in time increases substantially when the spacer thickness decreases, which correlates with the enhancement of nonradiative recombination in QW. © 2016, Pleiades Publishing, Inc.

Recent high-resolution absorption spectroscopy on highly excited excitons in cuprous oxide (Kazimierczuk et al 2014 Nature 514 343-347) have revealed significant deviations of their spectrum from the ideal hydrogen-like series. In atomic physics, the influence of the ionic core and the resulting modifications of the Coulomb interaction are accounted for by the introduction of a quantum defect. Here we translate this concept to the realm of semiconductor physics and show how the complex band dispersion of a crystal is mirrored in a set of empirical parameters similar to the quantum defect in atoms. Experimental data collected from high-resolution absorption spectroscopy in electric fields allow us to compare results for multiple angular momentum states of the yellow and even the green exciton series of . The agreement between theory and experiment validates our assignment of the quantum defect to the nonparabolicity of the band dispersion. © 2016 IOP Publishing Ltd.

Recent high-resolution absorption spectroscopy on excited excitons in cuprous oxide [Nature (London) 514, 343 (2014)NATUAS0028-083610.1038/nature13832] has revealed significant deviations of their spectrum from that of the ideal hydrogen-like series. Here we show that the complex band dispersion of the crystal, which determines the kinetic energy of electrons and holes, strongly affects the exciton binding energy. Specifically, we show that the nonparabolicity of the band dispersion is the main cause of the deviation from the hydrogen series. Experimental data collected from high-resolution absorption spectroscopy in electric fields validate the assignment of the deviation to the nonparabolicity of the band dispersion. © 2016 American Physical Society.

The electron g factor in an ensemble of InAs/InP quantum dots with emission wavelengths around 1.4 μm is measured using time-resolved pump-probe Faraday rotation spectroscopy in different magnetic field orientations. Thereby, we can extend recent single dot photoluminescence measurements significantly towards lower optical transition energies through 0.86 eV. This allows us to obtain detailed insight into the dispersion of the recently discovered g factor anisotropy in these infrared emitting quantum dots. We find with decreasing transition energy over a range of 50 meV a strong enhancement of the g factor difference between magnetic field normal and along the dot growth axis, namely, from 1 to 1.7. We argue that the g factor cannot be solely determined by the confinement energy, but the dot asymmetry underlying this anisotropy therefore has to increase with increasing dot size. © 2016 Author(s).

The exciton recombination dynamics is studied experimentally and theoretically in two-monolayer-thick GaAs/AlAs quantum wells characterized by an indirect band gap and a type-II band alignment. At cryogenic temperatures, the lifetimes of the excitons that are indirect both in real and k space are in the millisecond range. The exciton recombination time and the photoluminescence (PL) intensity are strongly dependent on strength and orientation of an applied magnetic field. In contrast to the very weak influence of an in-plane field, at 2 K temperature a field applied parallel to the growth axis drastically slows down the recombination and reduces the PL intensity. With increasing temperature the magnetic field effects on PL intensity and decay time are vanishing. The experimental data are well described by a model for the exciton dynamics that takes into account the magnetic-field-induced redistribution of the indirect excitons between their bright and dark states. It allows us to evaluate the lower bound of the heavy-hole longitudinal g factor of 2.5, the radiative recombination time for the bright excitons of 0.34 ms, and the nonradiative recombination time of the bright and dark excitons of 8.5 ms. © 2016 American Physical Society.

We study the dynamics of optically induced nuclear spin polarization in a fluorine-doped ZnSe epilayer via time-resolved Kerr rotation. The nuclear polarization in the vicinity of a fluorine donor is induced by interaction with coherently precessing electron spins in a magnetic field applied in the Voigt geometry. It is detected by nuclei-induced changes in the electron spin coherence signal. This all-optical technique allows us to measure the longitudinal spin relaxation time T1 of the Se77 isotope in a magnetic field range from 10 to 130 mT under illumination. We combine the optical technique with radio frequency methods to address the coherent spin dynamics of the nuclei and measure Rabi oscillations, Ramsey fringes, and the nuclear spin echo. The inhomogeneous spin dephasing time T2∗ and the spin coherence time T2 of the Se77 isotope are measured. While the T1 time is on the order of several milliseconds, the T2 time is several hundred microseconds. The experimentally determined condition T1T2 verifies the validity of the classical model of nuclear spin cooling for describing the optically induced nuclear spin polarization. © 2016 American Physical Society.

Electron spins confined in semiconductor quantum dots have been con- sidered as promising quantum bit candidates. This proposal is based mostly on longitudinal spin relaxation times in the milliseconds range at cryogenic temperatures. Here we substantiate the prospective properties of such quantum bits by addressing both the static and dynamic proper- ties of electron spins in (In,Ga)As/GaAs self-assembled quantum dots. In particular we discuss first the g-factor tensor and turn then to the creation of spin coherence by optical laser pulses. We also discuss the times during which the spin coherence is maintained, and how the spin coherence can be tailored by the details of the laser excitation protocol. © 2008 by Taylor & Francis Group, LLC. All rights reserved.

We demonstrate that in confined plasmonic metal structures subject to ultrafast laser excitation, electron thermal diffusion (ETD) can provide spatial redistribution of excess energy faster than its transfer to the lattice. This relaxation occurs after the excitation of nanometer-sized hot spots in the confined structure, changing sensitively the optical parameters in these regions. The changes become essential when the plasmonic resonance condition is met for both excitation and detection, as evidenced by a pump-probe experiment on plasmonic gold lattices: Subpicosecond relaxation with characteristic times well described by a two-temperature model involving ETD is observed. The results suggest that the dynamical optical response in plasmonic structures can be tuned by the selection of the structural geometry as well as the choice of wavelength and polarization of the excitation and detection light. © 2016 American Physical Society.

We present a detailed investigation of the exciton and trion dynamics in naturally doped MoSe2 and WSe2 single atomic layers as a function of temperature in the range 10-300 K under above band-gap laser excitation. By combining time-integrated and time-resolved photoluminescence (PL) spectroscopy, we show the importance of exciton and trion localization in both materials at low temperatures. We also reveal the transition to delocalized exciton complexes at higher temperatures where the exciton and trion thermal energy exceeds the typical localization energy. This is accompanied by strong changes in PL including suppression of the trion PL and decrease of the trion PL lifetime, as well as significant changes for neutral excitons in the temperature dependence of the PL intensity and the appearance of a pronounced slow PL decay component. In MoSe2 and WSe2 studied here, the temperatures where such strong changes occur are observed at around 100 and 200 K, respectively, in agreement with their inhomogeneous PL linewidth of 8 and 20 meV at T≈10K. The observed behavior is a result of a complex interplay between influences of the specific energy ordering of bright and dark excitons in MoSe2 and WSe2, sample doping, trion, and exciton localization and various temperature-dependent nonradiative processes. © 2016 American Physical Society.

In this paper we demonstrate a versatile concept for a planar cavity polariton beam amplifier using nonresonant excitation. In contrast to resonant excitation schemes, background carriers are injected which form excitons, providing both gain and a repulsive potential for a polariton condensate. Using an attractive potential environment induced by a locally elongated cavity layer, the repulsive potential of the injected background carriers is compensated, and a significant amplification of polariton beams is achieved without beam distortion. © 2016 American Physical Society.

We develop an extended pump-probe Faraday rotation technique to study submicrosecond electron spin dynamics with picosecond time resolution in a wide range of magnetic fields. The electron spin dephasing time T2∗ and the longitudinal spin relaxation time T1, both approaching 250 ns in weak fields, are measured thereby in n-type bulk GaAs. By tailoring the pump pulse train through increasing the contained number of pulses, the buildup of resonant spin amplification is demonstrated for the electron spin polarization. The spin precession amplitude in high magnetic fields applied in the Voigt geometry shows a nonmonotonic dynamics deviating strongly from a monoexponential decay and revealing slow beatings. The beatings indicate a two spin component behavior with a g-factor difference of Δg∼4×10-4, much smaller than the Δg expected for free and donor-bound electrons. This g-factor variation indicates efficient, but incomplete spin exchange averaging. © 2016 American Physical Society.

Light is often characterized only by its classical properties, like intensity or coherence. When looking at its quantum properties, described by photon correlations, new information about the state of the matter generating the radiation can be revealed. In particular the difference between independent and entangled emitters, which is at the heart of quantum mechanics, can be made visible in the photon statistics of the emitted light. The well-studied phenomenon of superradiance occurs when quantum-mechanical correlations between the emitters are present. Notwithstanding, superradiance was previously demonstrated only in terms of classical light properties. Here, we provide the missing link between quantum correlations of the active material and photon correlations in the emitted radiation. We use the superradiance of quantum dots in a cavity-quantum electrodynamics laser to show a direct connection between superradiant pulse emission and distinctive changes in the photon correlation function. This directly demonstrates the importance of quantum-mechanical correlations and their transfer between carriers and photons in novel optoelectronic devices.

The efficiency of the Mn-spin system heating under pulsed laser excitation is studied in diluted magnetic semiconductor heterostructures Zn0.99 Mn0.01 Se/Be0.93 Mn0.07 Te with type-II band alignment by means of time-resolved photoluminescence and pump-probe reflectivity. An essential role in the heating is played by multiple spin-flip scatterings of a hole with localized spins of Mn2+ ions. The efficiency of the spin and energy transfer from photoexcited holes to Mn ions of the Zn0.99 Mn0.01 Se layer considerably depends on the hole lifetime in this layer. This lifetime can be limited by the hole relaxation into the Be0.93 Mn0.07 Te layers and is strongly sensitive to the excitation power and Zn0.99 Mn0.01 Se layer thickness. These dependences allow us to determine a characteristic time of about 20ps for the spin and energy transfer from photoexcited holes to the Mn spin system. © 2014 The Authors. Phys. Status Solidi B is published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

By comparing the worn and untouched locations of a tungsten-carbide/carbon surface of a dry-running twin-screw rotor, we demonstrate that tungsten-oxide Raman modes become observable only at worn locations and the integral intensity of the Raman line at 680 cm-1, which is related to the incipient oxidation of the tungsten-carbide stretching mode, is enhanced. Its frequency and width moreover change significantly, thus indicating the mechanical distortion of the bonding that has been occurred during the wearing process. The shape of the tungsten-oxide Raman lines, resembling the Voigt function, hints at a surface morphology that is a characteristic for an amorphous solid environment. Our Raman scattering results may be exploited to characterize the degree of wear of coated surfaces and to identify signatures of a tribological layer. © 2016 Author(s).

A detailed study of the g-factor anisotropy of electrons and holes in InAs/In0.53Al0.24Ga0.23As self-assembled quantum dots emitting in the telecom spectral range of 1.5-1.6μm (around 0.8 eV photon energy) is performed by time-resolved pump-probe ellipticity technique using a superconducting vector magnet. All components of the g-factor tensors are measured, including their spread in the quantum dot (QD) ensemble. Surprisingly, the electron g factor shows a large anisotropy changing from ge,x=-1.63 to ge,z=-2.52 between directions perpendicular and parallel to the dot growth axis, respectively, at an energy of 0.82 eV. The hole g-factor anisotropy at this energy is even stronger: |gh,x|=0.64 and |gh,z|=2.29. On the other hand, the in-plane anisotropies of electron and hole g factors are small. The pronounced out-of-plane anisotropy is also observed for the spread of the g factors, determined from the spin dephasing time. The hole longitudinal g factors are described with a theoretical model that allows us to estimate the QD parameters. We find that the QD height-to-diameter ratio increases while the indium composition decreases with increasing QD emission energy. © 2016 American Physical Society.

Hybrid structures synthesized from different materials have attracted considerable attention because they may allow not only combination of the functionalities of the individual constituents but also mutual control of their properties. To obtain such a control an interaction between the components needs to be established. For coupling the magnetic properties, an exchange interaction has to be implemented which typically depends on wavefunction overlap and is therefore short-ranged, so that it may be compromised across the hybrid interface. Here we study a hybrid structure consisting of a ferromagnetic Co layer and a semiconducting CdTe quantum well, separated by a thin (Cd,Mg) Te barrier. In contrast to the expected p-d exchange that decreases exponentially with the wavefunction overlap of quantum well holes and magnetic atoms, we find a long-ranged, robust coupling that does not vary with barrier width up to more than 30 nm. We suggest that the resulting spin polarization of acceptor-bound holes is induced by an effective p-d exchange that is mediated by elliptically polarized phonons.

Rydberg excitons are semiconductor analogues to Rydberg atoms, where one electron is promoted to an energy level of large principal quantum number η and which behave in a manner similar to hydrogen. Their huge spatial extent results in giant dipole moments and interaction effects, which can be used to create nonlinearities at the single excitation level. In contrast to hydrogen, the effective masses and Rydberg energies involved are moderately small, so that in contrast to Rydberg atoms the high field limit of Rydberg physics can be studied using fields strengths that can be realized in the lab. Here we investigate the effects of external magnetic fields of up to 7T on Rydberg excitons both in Faraday and Voigt geometry. In both cases complicated splitting patterns emerge. We investigate the differences between the two geometries and highlight spectroscopic features that are especially easy to access using them. We show that the large number of resonances in the spectrum renders a microscopic treatment of each individual resonance implausible. We instead demonstrate general effects introduced by the field like avoided crossings and discuss alternative approaches to the level structure in terms of collective descriptions. © 2016 SPIE.

The optically induced spin polarization in (Cd,Mn)Te/(Cd,Mn,Mg)Te diluted-magnetic-semiconductor quantum wells is investigated by means of picosecond pump-probe Kerr rotation. At 1.8 K temperature, additionally to the oscillatory signals from photoexcited electrons and manganese spins precessing about an external magnetic field, a surprisingly long-lived (up to 60 ns) nonoscillating spin polarization is detected. This polarization is related to optical orientation of equilibrium magnetic polarons involving resident holes. The suggested mechanism for the optical orientation of the equilibrium magnetic polarons indicates that the detected polaron dynamics originates from unexcited magnetic polarons. The polaron spin dynamics is controlled by the anisotropic spin structure of the heavy hole resulting in a freezing of the polaron magnetic moment in one of the two stable states oriented along the structure growth axis. Spin relaxation between these states is prohibited by a potential barrier, which depends on temperature and magnetic field. The magnetic polaron relaxation is accelerated with increasing temperature and in magnetic field. © 2016 American Physical Society.

The spin orientation of electrons is studied in ferromagnet (FM)-semiconductor (SC) hybrid structures composed of a (Ga, Mn)As ferromagnetic layer, which is placed in the direct vicinity of a non-magnetic SC (In, Ga)As quantum well (QW). It is shown that the polarization of carriers in the SC QW is achieved by spin-dependent tunnelling into the magnetized ferromagnetic layer. This leads to dynamical spin polarization of the electrons, which can be directly observed by means of time-resolved photoluminescence. We find that the electron spin polarization grows in time after excitation with an optical pulse and may reach values as large as 30%. The rate of spin-dependent capture grows exponentially steeply with decreasing thickness of the spacer between ferromagnetic layer and QW, and it persists up to the Curie temperature of the (Ga, Mn)As layer. From time-resolved pump-probe Kerr rotation data, we evaluate a value of only a few μeV for the energy splitting between the electron Zeeman sublevels due to interaction with the ferromagnetic (Ga, Mn)As layer, indicating that the equilibrium spin polarization is negligible. Schematic presentation of electron spin orientation in a semiconductor quantum well (QW) under linearly polarized excitation due to spin-dependent capture of electrons in the ferromagnetic layer (FM). The arrows in the FM box indicate the orientation of the magnetization M. The effect is detected by appearance of a circular polarization degree of photoluminescence after pulsed optical excitation (right). The data are shown for a spacer thickness of 5 nm. © 2014 The Authors. Phys. Status Solidi B is published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

An ensemble of quantum dot excitons may be used for coherent information manipulation. Due to the ensemble inhomogeneity any optical information retrieval occurs in the form of a photon echo. We show that the inhomogeneity can lead to a significant deviation from the conventional echo timing sequence. Variation of the area of the initial rotation pulse, which generates excitons in a dot subensemble only, reveals this complex picture of photon echo formation. We observe a retarded echo for π/2 pulses, while for 3π/2 the echo is advanced in time as evidenced through monitoring the Rabi oscillations in the time-resolved photon echo amplitude from (In,Ga)As/GaAs self-assembled quantum dot structures and confirmed by detailed calculations. © 2016 American Physical Society.

Symmetries are the underlying principles of fundamental interactions in nature. Chaos in a quantum system may emerge from breaking these symmetries. Compared to vacuum, crystals are attractive for studying quantum chaos, as they not only break spatial isotropy, but also lead to novel quasiparticles with modified interactions. Here we study yellow Rydberg excitons in cuprous oxide which couple strongly to the vacuum light field and interact significantly with crystal phonons, leading to inversion symmetry breaking. In a magnetic field, time-reversal symmetry is also broken and the exciton states show a complex splitting pattern, resulting in quadratic level repulsion for small splittings. In contrast to atomic chaotic systems in a magnetic field, which show only a linear level repulsion, this is a signature of a system where all anti-unitary symmetries are broken simultaneously. This behaviour can otherwise be found only for the electro-weak interaction or engineered billiards.

The term quantum physics refers to the phenomena and characteristics of atomic and subatomic systems which cannot be explained by classical physics. Quantum physics has had a long tradition in Germany, going back nearly 100 years. Quantum physics is the foundation of many modern technologies. The first generation of quantum technology provides the basis for key areas such as semiconductor and laser technology. The “new” quantum technology, based on influencing individual quantum systems, has been the subject of research for about the last 20 years. Quantum technology has great economic potential due to its extensive research programs conducted in specialized quantum technology centres throughout the world. To be a viable and active participant in the economic potential of this field, the research infrastructure in Germany should be improved to facilitate more investigations in quantum technology research. © 2016, Springer-Verlag Berlin Heidelberg.

The dynamics of spin-lattice relaxation in the magnetic Mn2+ ion system of (Zn,Mn)Se/(Zn,Be)Se quantum-well structures are studied using optical methods. Pronounced cusps are found in the giant Zeeman shift of the quantum-well exciton photoluminescence at specific magnetic fields below 10 T, when the Mn spin system is heated by photogenerated carriers. The spin-lattice relaxation time of the Mn ions is resonantly accelerated at the cusp magnetic fields. Our theoretical analysis demonstrates that a cusp occurs at a spin-level mixing of single Mn2+ ions and a quick-relaxing cluster of nearest-neighbor Mn ions, which can be described as intrinsic cross-relaxation resonance within the Mn spin system. © 2016 American Physical Society.

Coherent optical control of individual particles has been demonstrated both for atoms and semiconductor quantum dots. Here we demonstrate the emergence of quantum coherent effects in semiconductor Rydberg excitons in bulk Cu2O. Because of the spectral proximity between two adjacent Rydberg exciton states, a single-frequency laser may pump both resonances with little dissipation from the detuning. As a consequence, additional resonances appear in the absorption spectrum that correspond to dressed states consisting of two Rydberg exciton levels coupled to the excitonic vacuum, forming a V-type three-level system, but driven only by one laser light source. We show that the level of pure dephasing in this system is extremely low. These observations are a crucial step towards coherently controlled quantum technologies in a bulk semiconductor. © 2016 American Physical Society.

Spin coherence of resident electrons and holes is measured in ZnSe-based quantum wells by means of time-resolved Kerr rotation technique. At a temperature of 1.8K spin dephasing time for localized electrons can be as long as 33ns, and for holes of 0.8ns. Electron spin precession is clearly observed in a wide temperature range up to 230K. The electron spin dephasing becomes much shorter of 0.2ns for the quantum well with a high-density electron gas. Using vector magnet all components of the g-factors tensor are evaluated for the electrons and heavy-holes, revealing strong anisotropy for the heavy-holes. © 2014 The Authors. Phys. Status Solidi B is published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

We report on experimental study of the exciton-polariton emission (PE) polarization noise below and above the polariton lasing threshold under continuous-wave nonresonant excitation. The experiments were performed with a high-Q graded 5λ/2 GaAs/AlGaAs microcavity with four sets of three quantum wells in the strong-coupling regime. The PE polarization noise substantially exceeded in magnitude the shot-noise level and, in the studied frequency range (up to 650 MHz), had a flat spectrum. We have found that the polarization and intensity noise dependences on the pump power are strongly different. This difference is ascribed to the bosonic stimulation effect in spin-dependent scattering of the polaritons to the condensate. A theoretical model describing the observed peculiarity of the PE polarization noise is proposed. © 2016 American Physical Society.

The spin fluctuations of electron and hole doped self-assembled quantum dot ensembles are measured optically in the low-intensity limit of a probe laser for absence and presence of longitudinal or transverse magnetic fields. The experimental results are modeled by two complementary approaches based either on a semiclassical or quantum mechanical description. This allows us to characterize the hyperfine interaction of electron and hole spins with the surrounding bath of nuclei on time scales covering several orders of magnitude. Our results demonstrate (i) the intrinsic precession of the electron spin fluctuations around the effective Overhauser field caused by the host lattice nuclear spins, (ii) the comparably long time scales for electron and hole spin decoherence, as well as (iii) the dramatic enhancement of the spin lifetimes induced by a longitudinal magnetic field due to the decoupling of nuclear and charge carrier spins. © 2016 American Physical Society.

The excitation of coherent optical phonons in solids provides a pathway for ultrafast modulation of light on a sub-ps time scale. Here, we report on efficient 3.6 THz modulation of light reflected from hybrid metal/semiconductor plasmonic crystals caused by lattice vibrations in a few-nm-thick layer of elemental tellurium. We observe that surface plasmon polaritons contribute significantly to the photoinduced formation of the tellurium layer at the interface between a telluride-based II-VI semiconductor, such as (Cd,Mg)Te or (Cd,Mn)Te, and a one-dimensional gold grating. The change in interface composition is monitored via the excitation and detection of coherent optical tellurium phonons of A1 symmetry by femtosecond laser pulses in a pump-probe experiment. The patterning of a plasmonic grating onto the semiconductor enhances the transient signal which originates from the interface region. This allows one to monitor the layer formation and observe the shift of the phonon frequency caused by confinement of the lattice vibrations in the nm-thick segregated layer. Efficient excitation and detection of coherent optical phonons by means of surface plasmon polaritons are evidenced by the dependence of the signal strength on polarization of the pump and probe pulses and its spectral distribution. © 2016 American Physical Society.

Thermal dissociation of free and acceptor-bound quasi-two-dimensional positive trions is investigated by measuring the temperature dependence of the integrated emission intensity in magnetic fields up to 17 T in high quality GaAs/AlxGa1-xAs quantum wells. Three distinct dissociation processes are observed for the well-resolved hole cyclotron replicas ("shake-up") of positive trions bound to neutral acceptors in the spin-doublet state (SU-A0Xd+). To demonstrate that the hole involved in the shake-up process is not bound by the Coulomb interaction to the charged A0X+ complex, we calculate the valence Landau levels using the Luttinger model beyond the axial approximation. The calculated value of the hole cyclotron energy agrees well with the experimental data for the energy separation of the A0X+ and SU-A0X+ lines, determined from the emission spectra. At low temperatures, below 6 K, the dominant dissociation results in a free hole and an exciton bound to the neutral acceptor in the spin-singlet or -triplet state, (A0Xd+→A0Xs+h or A0Xt+h). At higher temperatures, above 9 K, the dissociation into the free positive trion and the neutral acceptor (A0Xd+→A0+X+) predominates. From the temperature evolution of the integrated emission of the free trion lines (X+) we evaluate the transition energy between the two triplet trion states, the dark one (Xtd+) and the bright one (Xtb+). The ionization energies of all detected dissociation processes are compared with the spectral positions of the relevant radiative recombination lines from which excellent quantitative agreement is achieved. © 2016 American Physical Society.

The precise adjustment of the polariton condensate flow under incoherent excitation conditions is an indispensable prerequisite for polariton-based logic gate operations. In this report, an all-optical approach for steering the motion of a polariton condensate using only nonresonant excitation is demonstrated. We create arbitrarily shaped functional potentials by means of a spatial light modulator, which allow for tailoring the condensate state and guiding a propagating condensate along reconfigurable pathways. Additional numerical simulations confirm the experimental observations and elucidate the interaction effects between background carriers and polariton condensates. © 2015 American Physical Society.

An electric field applied to a semiconductor reduces its crystal symmetry and modifies its electronic structure which is expected to result in changes of the linear and nonlinear response to optical excitation. In GaAs, we observe experimentally strong electric field effects on the optical second (SHG) and third (THG) harmonic generation. The SHG signal for the laser-light k vector parallel to the [001] crystal axis is symmetry forbidden in the electric-dipole approximation, but can be induced by an applied electric field in the vicinity of the 1s exciton energy. Surprisingly, the THG signal, which is allowed in this geometry, is considerably reduced by the electric field. We develop a theory which provides good agreement with the experimental data. In particular, it shows that the optical nonlinearities for the 1s exciton resonance are modified in an electric field by the Stark effect, which mixes the 1s and 2p exciton states of opposite parity. This mixing acts in opposite way on the SHG and THG processes, as it leads to the appearance of forbidden SHG in (001)-oriented GaAs and decreases the crystallographic THG. © 2015 American Physical Society.

We extend the range of quantum dot (QD) emission energies where electron and hole g factors have been measured to the practically important telecom range. The spin dynamics in InAs/In0.53Al0.24Ga0.23As self-assembled QDs with emission wavelengths at about 1.6 μm grown on InP substrate is investigated by pump-probe Faraday rotation spectroscopy in a magnetic field. Pronounced oscillations on two different frequencies, corresponding to the QD electron and hole spin precessions about the field, are observed from which the corresponding g factors are determined. The electron g factor of about -1.9 has the largest negative value so far measured for III-V QDs by optical methods. This value, as well as the g factors reported for other III-V QDs, differ from those expected for bulk semiconductors at the same emission energies, and this difference increases significantly for decreasing energies. © 2015 American Physical Society.

The exciton spin dynamics and polarization properties of the related emission are investigated in colloidal CdSe/CdS dot-in-rod (DiR) and spherical core/shell nanocrystal (NC) ensembles by magneto-optical photoluminescence (PL) spectroscopy in magnetic fields up to 15 T. It is shown that the degree of circular polarization (DCP) of the exciton emission induced by the magnetic field is affected by the NC geometry as well as the exciton fine structure and can provide information on nanorod orientation. A theory to describe the circular and linear polarization properties of the NC emission in a magnetic field is developed. It takes into account phonon mediated coupling between the exciton fine structure states as well as the dielectric enhancement effect resulting from the anisotropic shell of DiR NCs. This theoretical approach is used to model the experimental results and allows us to explain most of the measured features. The spin dynamics of the dark excitons is investigated in magnetic fields by time-resolved photoluminescence. The results highlight the importance of confined acoustic phonons in the spin relaxation of dark excitons. The bare core surface as well as the core/shell interface give rise to an efficient spin-relaxation channel, while the surface of core/shell NCs seems to play only a minor role. © 2015 American Physical Society.

We present a systematic experimental study along with theoretical modeling of the energy transfer in an ensemble of closely packed CdTe colloidal nanocrystals identified as the Förster resonant energy transfer (FRET). We prove that at low temperature of 4.2 K, mainly the ground dark exciton states in the initially excited small-size (donor) nanocrystals participate in the dipole-dipole FRET leading to additional excitation of the large-size (acceptor) nanocrystals. The FRET becomes possible due to the weak admixture of the bright exciton states to the dark states. The admixture takes place even in zero magnetic field and allows the radiative recombination of the dark excitons. An external magnetic field considerably enhances this admixture, thus increasing the energy transfer rate by a factor of 2-3 in a field of 15 T, as well as the radiative rates of the dark excitons in the donor and acceptor nanocrystals. The theoretical modeling allows us to determine the spectral dependence of the probability for the NC to serve as a donor for larger nanocrystals, to evaluate the energy transfer rates as well as to predict their dependencies on the magnetic field, to describe the spectral shift of the photoluminescence maximum due to the energy transfer, and to reproduce the experimentally observed spectral dependencies of the photoluminescence recombination dynamics in the magnetic field. © 2015 American Physical Society.

We report on the ground- and excited-state properties of Fe3+ centers in hydrothermally and chemical-vapor-transport grown single ZnO crystals studied by continuous-wave electron-paramagnetic resonance (EPR) under dark and laser-illuminated conditions, pulsed-EPR and magneto-photoluminescence. By use of EPR experiments, the fine-structure parameters of the Fe3+ spin Hamiltonian are determined. Three types of charge-compensated Fe3+ centers are identified and the charge conversion from Fe2+ to Fe3+ is highlighted. The magneto-optical studies of the Zeeman components of the spin-forbidden electric-dipole transitions from excited T14(G) to ground A16(S6) states of the Fe3+ center indicate the trigonal symmetry of the fine structure of the lowest Γ8(T14) excited state. The energy positions of the Zeeman components are measured in the external magnetic field of 8 T rotated in (12¯10) and (0001) crystal planes. The angular variation of the Zeeman lines exhibits two magnetically nonequivalent Fe3+ centers. These features result from the contribution of high-rank Zeeman terms of dimension BJ3 in the spin Hamiltonian. For the electron spin S=5/2 system of the trigonal Fe3+ ion, we further demonstrate the tuning of one-photon Rabi oscillations by means of electron spin-echo measurements. ©2015 American Physical Society.

We use a picosecond acoustics technique to modulate the laser output of electrically pumped GaAs/AlAs micropillar lasers with InGaAs quantum dots. The modulation of the emission wavelength takes place on the frequencies of the nanomechanical extensional and breathing (radial) modes of the micropillars. The amplitude of the modulation for various nanomechanical modes is different for every micropillar which is explained by a various elastic contact between the micropillar walls and polymer environment. © 2015 AIP Publishing LLC.

The real-time spin dynamics and the spin noise spectra are calculated for p and n-charged quantum dots within an anisotropic central spin model extended by additional nuclear electric quadrupolar interactions and augmented by experimental data. Using realistic estimates for the distribution of coupling constants including an anisotropy parameter, we show that the characteristic long time scale is of the same order for electron and hole spins strongly determined by the quadrupolar interactions even though the analytical form of the spin decay differs significantly consistent with our measurements. The low frequency part of the electron spin noise spectrum is approximately 1/3 smaller than those for hole spins as a consequence of the spectral sum rule and the different spectral shapes. This is confirmed by our experimental spectra measured on both types of quantum dot ensembles in the low power limit of the probe laser. © 2015 American Physical Society.

Optically induced nuclear spin polarization in a fluorine-doped ZnSe epilayer is studied by time-resolved Kerr rotation using resonant excitation of donor-bound excitons. Excitation with helicity-modulated laser pulses results in a transverse nuclear spin polarization, which is detected as a change of the Larmor precession frequency of the donor-bound electron spins. The frequency shift in dependence on the transverse magnetic field exhibits a pronounced dispersion-like shape with resonances at the fields of nuclear magnetic resonance of the constituent zinc and selenium isotopes. It is studied as a function of external parameters, particularly of constant and radio frequency external magnetic fields. The width of the resonance and its shape indicate a strong spatial inhomogeneity of the nuclear spin polarization in the vicinity of a fluorine donor. A mechanism of optically induced nuclear spin polarization is suggested based on the concept of resonant nuclear spin cooling driven by the inhomogeneous Knight field of the donor-bound electron. © 2015 American Physical Society.

The spin dynamics of strongly localized donor-bound electrons in fluorine-doped ZnSe epilayers is studied using pump-probe Kerr rotation techniques. A method exploiting the spin inertia is developed and used to measure the longitudinal spin relaxation time T1 in a wide range of magnetic fields, temperatures, and pump densities. The T1 time of the donor-bound electron spin of about 1.6 μs remains nearly constant for external magnetic fields varied from zero up to 2.5 T (Faraday geometry) and in a temperature range 1.8-45 K. These findings impose severe restrictions on possible spin relaxation mechanisms. In our opinion they allow us to rule out scattering between free and donor-bound electrons, jumping of electrons between different donor centers, scattering between phonons and donor-bound electrons, and with less certainty charge fluctuations in the environment of the donors caused by the 1.5 ps pulsed laser excitation. © 2015 American Physical Society.

We study the interplay between the dynamic nuclear spin polarization and resonant spin-flip Raman scattering of the resident electron in an ensemble of singly charged (In,Ga)As/GaAs quantum dots by using a two-color laser excitation scheme. The shift of the electron spin-flip Raman line gives a direct measure of the optically induced Overhauser shift, while the linewidth indicates nuclear spin fluctuations. The dynamic nuclear spin polarization leads only to a reduction in the electron spin splitting induced by wetting-layer excitation that is copolarized with the resonant quantum dot excitation. The respective mechanism of the two-color spin-flip Raman scattering is discussed together with the electron-nuclear hyperfine interaction and Pauli exclusion principle. The temporal evolution of the Overhauser shift further demonstrates a nuclear spin depolarization within several seconds depending strongly on the temperature. © 2015 American Physical Society.

The recent observation of dipole-allowed P excitons up to principal quantum numbers of n=25 in cuprous oxide has given insight into exciton states with unprecedented spectral resolution. While so far the exciton description as a hydrogenlike complex has been fully adequate for cubic crystals, we demonstrate here distinct deviations: The breaking of rotational symmetry leads to mixing of high angular momentum F and H excitons with the P excitons so that they can be observed in absorption. The F excitons show a threefold splitting that depends systematically on n, in agreement with theoretical considerations. From detailed comparison of experiment and theory we determine the cubic anisotropy parameter of the Cu2O valence band. © 2015 American Physical Society. © 2015 American Physical Society.

Gallium chalcogenides are promising building blocks for novel van der Waals heterostructures. We reportonthe low-temperature micro-photoluminescence (PL) of GaTe and GaSefilms with thicknesses ranging from 200 nm toasingle unit cell.Inboth materials,PL shows adramatic decrease by 104-105 when film thicknessis reduced from 200to10 nm. Basedon evidence from continuouswave (cw) and time-resolved PL, wepropose amodel explaining the PLdecrease as a result of nonradiative carrier escape via surface states. Our results emphasize the need for special passivation of two-dimensional films for optoelectronic applications. © 2015 IOP Publishing Ltd.

We introduce photon-statistics excitation spectroscopy and exemplarily apply it to a quantum-dot micropillar laser. Both the intensity and the photon number statistics of the emission from the micropillar show a strong dependence on the photon statistics of the light used for excitation of the sample. The results under coherent and pseudothermal excitation reveal that a description of the laser properties in terms of mean input photon numbers is not sufficient. It is demonstrated that the micropillar acts as a superthermal light source when operated close to its threshold. Possible applications for important spectroscopic techniques are discussed. © 2015 American Physical Society. © 2015 American Physical Society.

We overview the results of three recently performed experiments, where the picosecond acoustic technique was applied to semiconductor devices with quantum wells or quantum dots embedded in an optical microcavity. In these experiments, high amplitude picosecond strain pulses are injected into such a device and the resulting changes in the response of the optical resonance are monitored. First, in quantum well devices we observe the generation of THz sidebands in optical reflectivity near the polariton resonance. Second, for certain conditions we detect the destruction and recurrence of excitons by acoustic shock waves on picosecond time scales. Third, in a vertical cavity surface emitting laser with a quantum dot layer the injection of the picosecond strain pulses induces the giant increase of the laser output. All these effects are governed by nonadiabatic processes in the interaction between a strain pulse and the electronic quantum confined states. Their observation became possible due to the possibility of generating very short strain pulses with sufficiently high amplitude. © 2014 Elsevier B.V. All rights reserved.

We realize resonant driving of the magnetization precession by monochromatic phonons in a thin ferromagnetic layer embedded into a phononic Fabry-Pérot resonator. A femtosecond laser pulse excites resonant phonon modes of the structure in the 10-40GHz frequency range. By applying an external magnetic field, we tune the precession frequency relative to the frequency of the phonons localized in the cavity and observe an enormous increase in the amplitude of the magnetization precession when the frequencies of free magnetization precession and phonons localized in the cavity are equal. © 2015 American Physical Society. ©2015 American Physical Society.

We investigate the response of a polariton laser driven slightly off-resonantly using light fields differing from the routinely studied coherent pump sources. The response to driving light fields with thermal and displaced thermal statistics with varying correlation times shows significant differences in the transmitted intensity, its noise, and the position of the nonlinear threshold. We predict that adding more photons on average may actually reduce the transmission through the polariton system. ©2015 American Physical Society.

The ability to store optical information is important for both classical and quantum communication. Achieving this in a comprehensive manner (converting the optical field into material excitation, storing this excitation, and releasing it after a controllable time delay) is greatly complicated by the many, often conflicting, properties of the material. More specifically, optical resonances in semiconductor quantum structures with high oscillator strength are inevitably characterized by short excitation lifetimes (and, therefore, short optical memory). Here, we present a new experimental approach to stimulated photon echoes by transferring the information contained in the optical field into a spin system, where it is decoupled from the optical vacuum field and may persist much longer. We demonstrate this for an n-doped CdTe/(Cd,Mg)Te quantum well, the storage time of which could be increased by more than three orders of magnitude, from the picosecond range up to tens of nanoseconds. © 2014 Macmillan Publishers Limited. All rights reserved.

We demonstrate the potential of a periodic laser-pulse protocol for dynamic decoupling of hole spins in (In,Ga)As quantum dots from surrounding baths. When doubling the repetition rate of inversion laser pulses between two reference pulses for orienting the spins, we find that the spin coherence time is increased by a factor of 2. © 2014 American Physical Society.

Resonant cooling of different nuclear isotopes manifested in optically induced nuclear magnetic resonances (NMR) is observed in n-doped CdTe/(Cd,Mg)Te and ZnSe/(Zn,Mg)Se quantum wells and for donor-bound electrons in ZnSe:F and GaAs epilayers. By time-resolved Kerr rotation used in the regime of resonant spin amplification, we can expand the range of magnetic fields where the effect can be observed up to nuclear Larmor frequencies of 170 kHz. The mechanism of the resonant cooling of the nuclear spin system is analyzed theoretically. The developed approach allows us to model the resonant spin amplification signals with NMR features. © 2014 American Physical Society.

We demonstrate the basic features of an all-optical spin tomography on picosecond time scale. The magnetization vector associated with a mode-locked electron spin ensemble in singly charged quantum dots is traced by ellipticity measurements using picosecond laser pulses. After optical orientation the spins precess about a perpendicular magnetic field. By comparing the dynamics of two interacting ensembles with the dynamics of a single ensemble we find buildup of a spin component along the magnetic field in the two-ensemble case. This component arises from a Heisenberg-like spin-spin interaction. © 2014 American Physical Society.

We present zero-, one-, and two-quantum two-dimensional coherent spectra of excitons and trions in a CdTe/(Cd,Mg)Te quantum well. The set of spectra provides a unique and comprehensive picture of the coherent nonlinear optical response. Distinct peaks in the spectra are manifestations of exciton-exciton and exciton-trion coherent coupling. Excellent agreement using density matrix calculations highlights the essential role of many-body effects on the coupling. Strong exciton-trion coherent interactions open up the possibility for novel conditional control schemes in coherent optoelectronics. © 2014 American Physical Society.

We exploit the flexibility offered by an (In,Ga)As/GaAs quantum dot spin ensemble to demonstrate that complex dynamic evolutions can be excited in the ensemble magnetization and accessed by tailored pulsed laser protocols. The modes for spin precession about a magnetic field are adapted to the periodic excitation protocol such that at specific times the magnetization can effectively be decomposed in two, three, or four equal components with angles of π,2π/3, or π/2 between them. Optical orientation of these components by an additional laser pulse leads to the generation of higher harmonics in the spin precession, as evidenced by time-resolved ellipticity measurements. © 2014 American Physical Society.

The recombination and spin dynamics of excitons in colloidal CdTe nanocrystals (NCs) are studied by time-resolved photoluminescence in high magnetic fields up to 15 T and at cryogenic temperatures. The recombination decay shows a nonexponential temporal behavior, with the longest component corresponding to the dark excitons having 260-ns decay time at zero magnetic field and 4.2-K temperature. This long component shortens to 150 ns at 15 T due to the magnetic-field-induced mixing of the bright- and dark-exciton states. The spin dynamics, assessed through the evolution of the magnetic-field-induced circular polarization degree of the photoluminescence, has a fast component shorter than 1 ns related to the bright excitons and a slow component of 5-10 ns associated with the dark excitons. The latter shortens with increasing magnetic field, which is characteristic for a phonon-assisted spin-relaxation mechanism. The relatively low saturation level of the associated magnetic-field-induced circular polarization degree of -30% is explained by a model that suggests the CdTe NCs to constitute an ensemble of prolate and oblate NCs, both having a structural quantization axis. The exciton g factor of 2.4-2.9 evaluated from fitting the experimental data in the frame of the suggested approach is in good agreement with the expected value for the dark excitons in CdTe NCs. © 2014 American Physical Society.

A highly excited atom having an electron that has moved into a level with large principal quantumnumberis a hydrogen-like object, termed a Rydberg atom. The giant size of Rydberg atoms leads to huge interaction effects. Monitoring these interactions has provided insights into atomic andmolecular physics on the single-quantum level. Excitons-the fundamental optical excitations in semiconductors, consisting of an electron and a positively charged hole-are the condensed-matter analogues of hydrogen. Highly excited excitons with extensions similar to those of Rydberg atoms are of interest because they can be placed and moved in a crystal with high precision using microscopic energy potential landscapes. The interaction of such Rydberg excitons may allow the formation of ordered exciton phases or the sensing of elementary excitations in their surroundings on a quantum level. Here we demonstrate the existence of Rydberg excitons in the copper oxide Cu2O, with principal quantum numbers as large as n 525. These states have giant wave function extensions (that is, the average distance between the electron and the hole) of more than two micrometres, compared to about a nanometre for the ground state. The strong dipoledipole interaction between such excitons is indicated by a blockade effect in which the presence of one exciton prevents the excitation of another in its vicinity. © 2014 Macmillan Publishers Limited. All rights reserved.

We report on low temperature, polarization resolved, high magnetic field (up to 23T) photoluminescence experiments on high mobility asymmetric GaAs quantum wells. At high magnetic fields, we detect two strong emission lines of the neutral and positively charged excitons (X and X+) and a series of weaker lines of the excitons bound to ionized acceptors (AX-). From polarization energy splittings of these lines, we determine the hole Landé factors (gh) of different complexes. For X and X+, ghinitially grows with magnetic field and then saturates at gh=0.88 and 1.55, respectively; for AX-'s, ghbegins from a high value (from 6 to 11 at zero field) and decreases with the field growth. This contrasting behavior is traced to the structure of valence band Landau levels, calculated numerically in the Luttinger model, beyond axial approximation. This points to the coexistence (in the same well) of mobile X and X+with localized and interface-pressed AX-states. © 2014 AIP Publishing LLC.

We use the picosecond acoustic pump-probe technique to study the elastic properties of monodisperse mesoporous silica spheres filled with nickel and deposited in the form of opal-like films on silica substrates. The picosecond pump-probe optical transmission signal shows harmonic oscillations corresponding to the lower energy radial Lamb mode in the vibrational spectrum of the spheres. These oscillations, with a frequency of several gigahertz last for several nanoseconds in the spheres with diameter 1050 nm, showing high homogeneity of the sphere parameters. By analysis of the oscillation spectrum of films with different sphere diameters and nickel content we obtain the elastic moduli of the mesoporous silica spheres. © 2014 IOP Publishing Ltd.

One-dimensional polariton condensates (PoCos) in a photonic wire are generated through nonresonant laser excitation, by which also a reservoir of background carriers is created. Interaction with this reservoir may affect the coherence of the PoCo, which is studied here by injecting a condensate locally and monitoring the coherence along the wire. While the incoherent reservoir is mostly present within the excitation laser spot, the condensate can propagate ballistically through the wire. Photon correlation measurements show that far from the laser spot the second-order correlation function approaches unity value, as expected for the coherent condensed state. When approaching the spot, however, the correlation function increases up to values of 1.2 showing the addition of noise to the emission due to interaction with the reservoir. This finding is substantiated by measuring the first-order coherence by a double-slit experiment, which shows a reduced visibility of interference at the excitation laser spot. © 2014 American Physical Society.

Planar microcavities with distributed Bragg reflectors (DBRs) host, besides confined optical modes, also mechanical resonances due to stop bands in the phonon dispersion relation of the DBRs. These resonances have frequencies in the 10- to 100-GHz range, depending on the resonatorâ optical wavelength, with quality factors exceeding 1,000. The interaction of photons and phonons in such optomechanical systems can be drastically enhanced, opening a new route towards the manipulation of light. Here we implemented active semiconducting layers into the microcavity to obtain a vertical-cavity surface-emitting laser (VCSEL). Thereby, three resonant excitations - photons, phonons and electrons - can interact strongly with each other providing modulation of the VCSEL laser emission: a picosecond strain pulse injected into the VCSEL excites long-living mechanical resonances therein. As a result, modulation of the lasing intensity at frequencies up to 40â €‰GHz is observed. From these findings, prospective applications of active optomechanical resonators integrated into nanophotonic circuits may emerge. © 2014 Macmillan Publishers Limited. All rights reserved.

We present a systematic investigation of two-photon excitation processes in a GaAs-based microcavity in the strong-coupling regime. We observe second harmonic generation resonant to the upper and lower polariton level, which exhibits a strong dependence on the photonic fraction of the corresponding polariton. In addition, we have performed two-photon excitation spectroscopy to identify 2p exciton states which are crucial for the operation as a terahertz lasing device, which was suggested recently [A. V. Kavokin, Phys. Rev. Lett. 108, 197401 (2012)PRLTAO0031-900710.1103/PhysRevLett.108.197401]. However, no distinct signatures of a 2p exciton state could be identified, which indicates a low two-photon pumping efficiency. © 2014 American Physical Society.

The photoluminescence polarizations of (In,Ga)As/GaAs quantum dots annealed at different temperatures are studied as a function of external magnetic field (Hanle curves). In these dependencies, remarkable resonant features appear due to all-optical nuclear magnetic resonances (NMR) for optical excitation with modulated circular polarization. Application of an additional radio-frequency field synchronously with the polarization modulation strongly modifies the NMR features. The resonances can be related to transitions between different nuclear spin states split by the strain-induced gradient of the crystal field and by the externally applied magnetic field. A theoretical model is developed to simulate quadrupole and Zeeman splittings of the nuclear spins in a strained quantum dot. Comparison with the experiment allows us to uniquely identify the observed resonances. The large broadening of the NMR resonances is attributed to variations of the quadrupole splitting within the quantum dot volume, which is well described by the model. © 2014 American Physical Society.

The experimentally measured input-output characteristics of optically pumped semiconductor microcavities exhibits unexpected oscillations modifying the fundamentally linear slope in the excitation power regime below lasing. A systematic microscopic analysis reproduces these oscillations, identifying them as a genuine quantum-memory effect, i.e., a photon-density correlation accumulated during the excitation. With the use of projected quantum measurements, it is shown that the input-output oscillations can be controlled and enhanced by an order of magnitude when the quantum fluctuations of the pump are adjusted. © 2014 American Physical Society.

We report a time-resolved study of the photoluminescence of CdSe colloidal nanoplatelets with two different thicknesses. By studying the exciton recombination dynamics we assess the exciton fine structure in these systems. The splitting between bright and dark excitons is enhanced compared to epitaxial quantum well structures as result of dielectric confinement. Despite of strong variations in the absolute magnitude, by comparison with literature data we find a relatively slightly varying bright-dark exciton lifetime ratio in very different CdSe-based colloidal nanostructures, regardless of growth technique and of core and shell properties such as materials, dimensions, etc. This finding points to a universal mechanism in the dark exciton recombination. © 2014 American Chemical Society.

A comprehensive overview of coherent spin manipulation in (In, Ga)As quantum dots, singly charged with resident electrons or holes, is given, from orientation to rotation of the spins. These operations are performed by excitation of the charged exciton complex with laser pulses. The specifics of the approach is performing the manipulations on dot ensembles, potentially giving robustness to the coherent spin dynamics. In particular, we focus on the spin mode-locking regime, in which the precession of the spins about an external magnetic field is synchronized with the periodic pulsed laser excitation initiating the operations. The periodic excitation protocol introduces a frequency comb through which detrimental effects of the inhomogeneities in the spin system can be avoided. © 2014 The Authors. Phys. Status Solidi B is published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Per the fluctuation-dissipation theorem, the information obtained from spin fluctuation studies in thermal equilibrium is necessarily constrained by the system's linear response functions. However, by including weak radio frequency magnetic fields, we demonstrate that intrinsic and random spin fluctuations even in strictly unpolarized ensembles can reveal underlying patterns of correlation and coupling beyond linear response, and can be used to study nonequilibrium and even multiphoton coherent spin phenomena. We demonstrate this capability in a classical vapor of K41 alkali atoms, where spin fluctuations alone directly reveal Rabi splittings, the formation of Mollow triplets and Autler-Townes doublets, ac Zeeman shifts, and even nonlinear multiphoton coherences. © 2014 American Physical Society.

The band structure of type-I (In,Al)As/AlAs quantum dots with band gap energy exceeding 1.63 eV is indirect in momentum space, leading to long-lived exciton states with potential applications in quantum information. Optical access to these excitons is provided by mixing of the Γ- and X-conduction-band valleys, for which their spins may be oriented by resonant spin-flip Raman scattering. This access is used to study the exciton spin-level structure by accurately measuring the anisotropic hole and isotropic electron g factors. The spin-flip mechanisms for the indirect exciton and its constituents as well as the underlying optical selection rules are determined. The spin-flip intensity is a reliable measure of the strength of Γ-X-valley mixing, as evidenced by both experiment and theory. © 2014 American Physical Society.

We report on a time-resolved study of the photoluminescence of core/shell CdSe/CdS dot-in-rod colloidal nanocrystals having various geometries. By studying the exciton recombination dynamics, we unveil a quadratic dependence of the bright-dark exciton energy splitting and the dark exciton radiative recombination rate on the inverse CdS rod width, regardless of the CdSe core size. We also evidence a strong dependence of the spin-flip rate between bright and dark exciton states on the shell thickness that suggests an acoustic phonon bottleneck. This work highlights the possibility to fully control and tune the optical properties of colloidal nanocrystals by shape engineering of the CdS shell. © 2014 American Chemical Society.

Spin noise spectroscopy is an optical technique for probing electron and hole spin dynamics that is based on detecting their intrinsic fluctuations while in thermal equilibrium. Here we show that fluctuation correlations can be further exploited in multi-probe noise studies to reveal information that in general cannot be accessed by conventional linear optical spectroscopy, such as the underlying homogeneous linewidths of individual constituents within inhomogeneously broadened systems. This is demonstrated in singly charged (In,Ga)As quantum-dot ensembles using two weak probe lasers: When the lasers have similar wavelengths, they probe the same quantum dots in the ensemble and show correlated spin fluctuations. In contrast, mutually detuned probe lasers measure different subsets of quantum dots, giving uncorrelated fluctuations. The noise correlation versus laser detuning directly reveals the quantum dot homogeneous linewidth even in the presence of a strong inhomogeneous broadening. Such noise-based correlation techniques are not limited to semiconductor spin systems, but are applicable to any system with measurable intrinsic fluctuations. © 2014 Macmillan Publishers Limited.

Magneto-optical effects in ferrimagnetic or ferromagnetic materials are usually too weak for potential applications. The transverse magneto-optical Kerr effect (TMOKE) in ferromagnetic films is typically on the order of 0.1%. Here, we demonstrate experimentally the enhancement of TMOKE due to the interaction of particle plasmons in gold nanowires with a photonic waveguide consisting of magnetooptical material, where hybrid waveguide-plasmon polaritons are excited. We achieve a large TMOKE that modulates the transmitted light intensity by 1.5%, accompanied by high transparency of the system. Our concept may lead to novel devices of miniaturized photonic circuits and switches, which are controllable by an external magnetic field.

The effects of confinement on biexciton renormalization in self-assembled InAs and interfacial GaAs quantum dot (QD) ensembles are studied using two-dimensional Fourier-transform spectroscopy. We find that in thermally annealed InAs QDs, changes in the biexciton transition energy are strongly correlated with those of the exciton and that the biexciton binding energy is similar for all QDs in the ensemble. These results are in contrast to those from GaAs QDs formed from interfacial fluctuations of a narrow quantum well (QW). In both the GaAs QW and QDs, correlation is reduced and the biexciton binding exhibits a strong dependence on localization. Comparison with simulations reveals how confinement and Coulomb interactions modify biexciton renormalization. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Experimental results for the carrier capture and relaxation dynamics in self-organized semiconductor quantum dots are analyzed using a microscopic theory. Time-resolved differential transmission changes of the quantum-dot transitions after ultrafast optical excitation of the barrier states are studied in a wide range of carrier temperatures and excitation densities. The measurements can be explained by quantum-dot polaron scattering and their excitation-dependent renormalization due to additional Coulomb scattering processes. Results of configuration-picture and single-particle-picture descriptions, both with nonperturbative transition rates, show good agreement with the experiments while Boltzmann scattering rates lead to a different excitation density and temperature dependence. © 2013 American Physical Society.

Compressive sensing is considered a huge breakthrough in signal acquisition. It allows recording an image consisting of N-2 pixels using much fewer than N-2 measurements if it can be transformed to a basis where most pixels take on negligibly small values. Standard compressive sensing techniques suffer from the computational overhead needed to reconstruct an image with typical computation times between hours and days and are thus not optimal for applications in physics and spectroscopy. We demonstrate an adaptive compressive sampling technique that performs measurements directly in a sparse basis. It needs much fewer than N-2 measurements without any computational overhead, so the result is available instantly.

Exchange-mediated fine-structure splitting of bright excitons in an ensemble of InAs quantum dots is studied using optical two-dimensional Fourier-transform spectroscopy. By monitoring the non-radiative coherence between the bright states, we find that the fine-structure splitting decreases with increasing exciton emission energy at a rate of 0.1μeV/meV. Dephasing rates are compared to population decay rates to reveal that pure dephasing causes the exciton optical coherences to decay faster than the radiative limit at low temperature, independent of excitation density. Fluctuations of the bright state transition energies are nearly perfectly correlated, protecting the non-radiative coherence from interband dephasing mechanisms. © 2013 Elsevier Ltd. All rights reserved.

We report on a systematic study of the phase transitions to polariton condensation (PC) and further to cavity lasing in a GaAs-based microcavity with respect to exciton-cavity detuning and lattice temperature. Using far field and time-resolved spectroscopy, we determined the parameter space in which PC can be achieved and the corresponding variation of PC threshold power. We found a lower bound of -12 meV for the exciton-cavity detuning and an upper bound of 90 K for the lattice temperature. © 2013 American Institute of Physics.

Exciton, trion, and biexciton dephasing rates are measured for an ensemble of InAs quantum dots using two-dimensional Fourier-transform spectroscopy. The two-dimensional spectra reveal that the dephasing rate of each excitonic state is similar for all dots in the ensemble and the rates are independent of excitation density. An additional spectral feature (too weak to be observed in the time-integrated four-wave mixing signal) appears at high excitation density and is attributed to the χ(5) biexcitonic nonlinear response. ©2013 American Physical Society.

The role of nuclear spin fluctuations in the dynamic polarization of nuclear spins by electrons is investigated in (In,Ga)As/GaAs quantum dots. The photoluminescence polarization under circularly polarized optical pumping in transverse magnetic fields (Hanle effect) is studied. A weak additional magnetic field parallel to the optical axis is used to control the efficiency of nuclear spin cooling and the sign of nuclear spin temperature. The shape of the Hanle curve is drastically modified when changing this control field, as observed earlier in bulk semiconductors and quantum wells. However, the standard nuclear spin cooling theory, operating with the mean nuclear magnetic field (Overhauser field), fails to describe the experimental Hanle curves in a certain range of control fields. This controversy is resolved by taking into account the nuclear spin fluctuations owed to the finite number of nuclei in the quantum dot. We propose a model considering cooling of the nuclear spin system by electron spins experiencing fast vector precession in the random Overhauser fields of nuclear spin fluctuations. The model allows us to accurately describe the measured Hanle curves and to evaluate the parameters of the electron-nuclear spin system of the studied quantum dots. © 2013 American Physical Society.

The emission energy dependence of the biexciton binding energy is investigated in three semiconductor quantum dot (QD) systems that exhibit different quantum well → QD confinement. Using two-dimensional Fourier-transform spectroscopy, we demonstrate that in strongly confining InAs QDs, the binding energy is independent of exciton emission energy and fluctuations in the ground state → exciton transition energy are strongly correlated with those of the exciton → biexciton. In contrast, the biexciton binding energy increases with emission energy in weakly confining interfacial GaAs QDs, and the level of correlation of exciton-biexciton broadening is reduced. A comparison with simulations reveals the significance of the strength and nature of confinement on Coulomb interactions responsible for biexciton renormalization. ©2013 American Physical Society.

We study the polarization dynamics of exciton-polaritons propagating in sub-mm-thick (Cd,Zn)Te bulk crystals using polarimetric time-of-flight techniques. The application of a magnetic field in Faraday geometry leads to synchronous temporal oscillations of all Stokes parameters of an initially linearly or circularly polarized, spectrally broad optical pulse of 150-fs duration propagating through the crystal. Strong dispersion for photon energies close to the exciton resonance leads to stretching of the optical pulse to a duration of 200-300 ps and enhancement of magneto-optical effects such as the Faraday rotation and the nonreciprocal birefringence. The oscillation frequency of the exciton-polariton polarization increases with magnetic field B, reaching 10 GHz at B∼5 T. Surprisingly, the relative contributions of Faraday rotation and nonreciprocal birefringence undergo strong changes with photon energy, which is attributed to a nontrivial spectral dependence of Faraday rotation in the vicinity of the exciton resonance. This leads to polarization nutation of the transmitted optical pulse in the time domain. The results are well explained by a model that accounts for Faraday rotation and magnetospatial dispersion in zinc-blende crystals. We evaluate the exciton g factor |g exc|=0.2 and the magnetospatial constant |V|=5×10-12 eV cm T-1. © 2013 American Physical Society.

Quasilongitudinal and quasitransverse picosecond strain pulses injected into a ferromagnetic (311) (Ga,Mn)As film induce dynamical shear strain in the film, thereby modulating the magnetic anisotropy and inducing resonant precession of the magnetization at a frequency ∼10 GHz. The modulation of the out-of-plane magnetization component by the quasitransverse strain reaches amplitudes as large as 10% of the equilibrium magnetization. Our theoretical analysis is in good agreement with the observed results, thus providing a strategy for ultrafast magnetization control in ferromagnetic films by strain pulses. © 2013 American Physical Society.

The magneto-Stark effect of excitons is demonstrated to be an efficient source of optical nonlinearity in hexagonal ZnO. Strong resonant second harmonic generation signals induced by an external magnetic field are observed in the spectral range of 2s and 2p excitons. The microscopic theoretical analysis shows that for excitons with a finite wave vector, exciton states of opposite parity are mixed by an effective odd parity electric field induced by the magnetic field despite its even parity. The field, spectral, and polarization dependencies of the second harmonic generation intensity validate the proposed mechanism. The observed phenomenon is not limited to a certain symmetry class and therefore must be effective in other semiconductors. © 2013 American Physical Society.

The time of spin relaxation of excitons in (In,Al)As/AlAs quantum dots with an indirect bandgap and type-I band alignment is determined by measuring the dynamics of photoluminescence circular polarization induced by a magnetic field B. The spin relaxation time τ S increases with decreasing magnetic field in proportion to B -5; its value is ~40 μs in a magnetic field of 6 T at a temperature of 1.8 K. As the temperature T increases in a magnetic field of 7 T, the value of τ S decreases as T -1.1. The character of the dependences of τ S on the magnetic field and temperature evidences that spin relaxation of excitons is provided by a process with participation of one acoustic phonon. © 2013 Allerton Press, Inc.

Spontaneous fluctuations of the magnetization of a spin system in thermodynamic equilibrium (spin noise) manifest themselves as noise in the Faraday rotation of probe light. We show that the correlation properties of this noise over the optical spectrum can provide clear information about the composition of the spin system that is largely inaccessible for conventional linear optics. Such optical spectroscopy of spin noise, e.g., allows us to clearly distinguish between optical transitions associated with different spin subsystems, to resolve optical transitions that are unresolvable in the usual optical spectra, to unambiguously distinguish between homogeneously and inhomogeneously broadened optical bands, and to evaluate the degree of inhomogeneous broadening. These new possibilities are illustrated by theoretical calculations and by experiments on paramagnets with different degrees of inhomogeneous broadening of optical transitions [atomic vapors of K41 and singly charged (In,Ga)As quantum dots]. © 2013 American Physical Society.

Coherent high-amplitude precession of the magnetization and spin waves with frequencies up to 40 GHz are generated by injecting picosecond compressive and shear acoustic pulses into nanometer-sized galfenol (Fe81Ga 19) films. The magnetization modulation is due to the picosecond inverse magnetostrictive effect. The oscillations of the magnetization measured by magneto-optical Kerr rotation last for several nanoseconds, and the maximum modulation of the in-plane effective magnetic field is as high as 40 mT. These results in combination with a comprehensive theoretical analysis show that galfenol films possess excellent properties for ultrafast magnetization control based on the picosecond inverse magnetostrictive effect. © 2013 AIP Publishing LLC.

By means of a metal opto-acoustic transducer we generate quasilongitudinal and quasi-transverse picosecond strain pulses in a (311)-GaAs substrate and monitor their propagation by picosecond acoustic interferometry. By probing at the sample side opposite to the transducer the signals related to the compressive and shear strain pulses can be separated in time. In addition to conventional monitoring of the reflected probe light intensity we monitor also the polarization rotation of the optical probe beam. This polarimetric technique results in improved sensitivity of detection and provides comprehensive information about the elasto-optical anisotropy. The experimental observations are in a good agreement with a theoretical analysis. © 2013 Optical Society of America.

Magnetic field control of light is among the most intriguing methods for modulation of light intensity and polarization on sub-nanosecond timescales. The implementation in nanostructured hybrid materials provides a remarkable increase of magneto-optical effects. However, so far only the enhancement of already known effects has been demonstrated in such materials. Here we postulate a novel magneto-optical phenomenon that originates solely from suitably designed nanostructured metal-dielectric material, the so-called magneto-plasmonic crystal. In this material, an incident light excites coupled plasmonic oscillations and a waveguide mode. An in-plane magnetic field allows excitation of an orthogonally polarized waveguide mode that modifies optical spectrum of the magneto-plasmonic crystal and increases its transparency. The experimentally achieved light intensity modulation reaches 24%. As the effect can potentially exceed 100%, it may have great importance for applied nanophotonics. Further, the effect allows manipulating and exciting waveguide modes by a magnetic field and light of proper polarization. © 2013 Macmillan Publishers Limited. All rights reserved.

We attract attention to the fact that the ultimate (shot-noise-limited) polarimetric sensitivity can be enhanced by orders of magnitude leaving the photon flux incident onto the photodetector on the same low level. This opportunity is of crucial importance for present-day spin noise spectroscopy, where a direct increase of sensitivity by increasing the probe beam power is strongly restricted by the admissible input power of the broadband photodetectors. The gain in sensitivity is achieved by replacing the 45° polarization geometry commonly used in conventional schemes with balanced detectors by geometries with stronger polarization extinction. The efficiency of these high-extinction polarization geometries with enhancement of the detected signal by more than an order of magnitude is demonstrated by measurements of the spin noise spectra of bulk n:GaAs in the spectral range 835-918 nm. It is shown that the inevitable growth of the probe beam power with the sensitivity gain makes spin noise spectroscopy much more perturbative, but, at the same time, opens up fresh opportunities for studying nonlinear interactions of strong light fields with spin ensembles. © 2013 American Physical Society.

Nonlinear optics of semiconductors is an important field of fundamental and applied research, but surprisingly the role of excitons in the coherent processes leading to harmonics generation has remained essentially unexplored. Here we report results of a comprehensive experimental and theoretical study of the three-photon process of optical second-harmonic generation (SHG) involving the exciton resonances of the noncentrosymmetric hexagonal wide-band-gap semiconductor ZnO in the photon energy range of 3.2-3.5 eV. Resonant crystallographic SHG is observed for the 1s(A,B), 2s(A,B), 2p(A,B), and 1s(C) excitons. We show that SHG signals at these exciton resonances are induced by the application of a magnetic field when the incident and the SHG light wave vectors are along the crystal z axis where the crystallographic SHG response vanishes. A microscopic theory of SHG through excitons is developed, which shows that the nonlinear interaction of coherent light with excitons has to be considered beyond the electric-dipole approximation. Depending on the particular symmetry of the exciton states SHG can originate from the electric- and magnetic-field-induced perturbations of the excitons due to the Stark effect, the spin as well as orbital Zeeman effects, or the magneto-Stark effect. The importance of each mechanism is analyzed and discussed by confronting experimental data and theoretical results for the dependences of the SHG signals on photon energy, magnetic field, electric field, crystal temperature, and light polarization. Good agreement is obtained between experiment and theory proving the validity of our approach to the complex problem of nonlinear interaction of light with ZnO excitons. This general approach can be applied also to other semiconductors. © 2013 American Physical Society.

The spin dynamics in chemically synthesized CdSe/CdS core/shell nanocrystals (NCs) are studied by polarization- and time-resolved photoluminescence (PL) techniques in high magnetic fields and at low temperatures. Analysis of the recombination dynamics shows that the emission of thin-shell NCs is contributed by neutral excitons, while for thick-shell NCs it is dominated by charged excitons (trions). The sign of the PL polarization unambiguously demonstrates that these trions are negatively charged. A theoretical model of the PL polarization in an ensemble of randomly oriented NCs describes well magnetic field and time dependences of the PL polarization degree and allows us to determine the hole g factor in CdSe/CdS NCs, g h=-0.54. From direct measurements of the spin relaxation rate dependences on magnetic field and temperature, we identify the mechanism of the negative trion spin relaxation as two-phonon-assisted Raman scattering between the hole spin sublevels mixed by the applied magnetic field. © 2013 American Physical Society.

Spin-flip Raman scattering of electrons and heavy holes is studied for resonant excitation of neutral and charged excitons in a CdTe/Cd 0.63Mg0.37Te quantum well. The spin-flip scattering is characterized by its dependence on the incident and scattered light polarization as well as on the magnetic field strength and orientation. Model schemes of electric-dipole-allowed spin-flip Raman processes in the exciton complexes are compared to the experimental observations, from which we find that lowering the exciton symmetry, time of carrier spin relaxation, and mixing between electron states and, respectively, light- and heavy-hole states play an essential role in the scattering. At the exciton resonance, anisotropic exchange interaction induces heavy-hole spin-flip scattering, while acoustic phonon interaction is mainly responsible for the electron spin-flip. In resonance with the positively and negatively charged excitons, anisotropic electron-hole exchange as well as mixed electron states allow spin-flip scattering. Variations in the resonant excitation energy and lattice temperature demonstrate that localization of resident electrons and holes controls the Raman process probability and is also responsible for symmetry reduction. We show that the intensity of the electron spin-flip scattering is strongly affected by the lifetime of the exciton complex, and in tilted magnetic fields it is affected by the angular dependence of the anisotropic electron-hole exchange interaction. © 2013 American Physical Society.

The temperature dependence of the coherence time of hole spins confined in self-assembled (In,Ga)As/GaAs quantum dots is studied by spin-mode-locking and spin-echo techniques. Coherence times limited to about a microsecond are measured for temperatures below 8 K. For higher temperatures, a fast drop occurs down to a few nanoseconds over a 10-K range. The hole-nuclear hyperfine interaction appears too weak to account for these limitations. We suggest that spin-orbit-related interactions are the decisive sources for hole spin decoherence. © 2013 American Physical Society.

We report a study of the luminescence due to Hg in ZnO, concentrating on the main zero phonon line (ZPL) at 3.2766(1) eV and its associated phonon sidebands. For a sample implanted with radioactive 192Hg, the ZPL intensity, normalised to that of shallow bound exciton emission, is observed to decrease with an equivalent half-life of 4.5(1) h, very close to the 4.85(20) h half-life of 192Hg. ZnO implanted with stable Hg impurities produces the same luminescence spectrum. Temperature dependent measurements confirm that the zero phonon line is a thermalizing doublet involving one allowed and one largely forbidden transition from excited states separated by 0.91(1) meV to a common ground state. Uniaxial stress measurements show that the allowed transition takes place from an orbitally degenerate excited state to a non-degenerate ground state in a centre of trigonal (C3v) symmetry while the magneto-optical properties are characteristic of electron-hole pair recombination at an isoelectronic defect. The doublet luminescence is assigned to bound exciton recombination involving exchange-split Γ5 and Γ1,2 excited states (using C6v symmetry labels; Γ3 and Γ1,2 using C3v labels) at isoelectronic Hg impurities substituting for Zn in the crystal. The electron and hole g values deduced from the magneto-optical data indicate that this Hg impurity centre in ZnO is hole-attractive. © 2013 AIP Publishing LLC.

The spectral properties of the transverse magneto-optical Kerr effect (TMOKE) in periodic metal-dielectric hybrid structures are studied, in particular with respect to the achievable magnitude. It is shown that the TMOKE is sensitive to the magneto-optical activity of the bismuth-substituted rare-earth iron garnet, which is used as a dielectric material in the investigated structures. For samples with larger Bi substitution level and, consequently, larger gyration constant, the magnitude of the TMOKE increases and reaches 13% in the case of a Bi1.8Lu1.2Fe 3.6Al1.4O12 magnetic film. Further, it is demonstrated that the TMOKE vanishes at the high-symmetry points of the Brillouin zone (at the Γ and X points). The main enhancement of the TMOKE takes place near the resonances of the surface plasmon polaritons (SPPs) at the metal/magnetic-dielectric interface. However, near the degenerate resonances of the SPPs at the air/metal and metal/magnetic-dielectric interfaces the TMOKE is increased by the air/metal SPPs as well. This phenomenon is explained in terms of a coupled oscillator model. © IOP Publishing and Deutsche Physikalische Gesellschaft.

Femtosecond laser spectroscopy is an important tool for investigating fundamental processes of light-matter interaction. Here, we report on transient optical anisotropy in the multiferroic hexagonal manganite YMnO3 excited by linearly or circularly polarized laser pulses with ∼17 fs and ∼34 fs duration. Spectral dependencies of photoinduced optical rotation and ellipticity at different temperatures are analyzed by considering ultrafast population and relaxation processes near the interband transitions from the hybridized O(2p)-Mn(3d) to the Mn(3d) states at photon energies around 1.6 eV. A Raman coherence time of ∼10 fs between the excited Γ5|x and Γ5|y states and a relaxation time of ∼500 fs between the Γ5|x,y and Γ1|g states for the electronic transition Γ1→Γ5 are determined in that way. The ultrashort Raman coherence time is related to the strong electron Coulomb interaction for the Γ5|x,y states. © 2013 American Physical Society.

Recently observed photoluminescence (PL) in ZnO, positioned at 3.324 eV and known to be related to Ge impurities, is investigated here by uniaxial stress and Zeeman spectroscopy measurements. The 3.324-eV PL line shifts but does not split under uniaxial stress both parallel and perpendicular to the c axis, indicating trigonal defect symmetry. This reinforces the findings of prior work that the defect center is related to a substitutional Ge impurity in ZnO. Applied magnetic fields result in linear splittings of the line into two components for fields parallel and perpendicular to the c axis. This result, combined with the temperature dependence of the Zeeman spectra, enables the line to be assigned to neutral donor bound-exciton recombination, most likely at a partially compensated GeZn double-donor impurity. © 2013 American Physical Society.

Here we demonstrate a simple and reconfigurable way to create a polariton condensate in well defined discrete momentum states, allowing us to manipulate the local polariton flow. To this end, we created a spatially varying potential formed in the presence of noncondensed carriers by subjecting a microcavity to spatially modulated nonresonant optical excitation. The choice of the spatial shape of this potential allows us to tailor the properties of the polariton condensate in momentum space. Our results demonstrate a way to prepare a polariton condensate in an adjustable momentum state and provide a first step toward the creation of functional all-optical elements for polaritonic logic circuits on demand by projecting circuits onto an unprocessed planar sample. © 2012 American Physical Society.

We compare two regimes indicative of polariton lasing and photon lasing of a planar GaAs/GaAlAs microcavity with zero detuning between the bare cavity mode and the quantum-well exciton. In particular, we investigate the cavity emission subsequent to nonresonant pulsed excitation. For the ground state emission from the lower energy-momentum dispersion branch we find a two-threshold behavior in the input-output curve where each transition is accompanied by characteristic changes of the in-plane mode dispersion. We demonstrate that the thresholds are unambiguously evidenced in the photon statistics of the emission based on the second-order correlation function. Moreover, the distinct two-threshold behavior is confirmed in the evolution of the emission pulse duration. Our findings show that a comprehensive study of spectral and temporal characteristics of the emission from a semiconductor microcavity can be used to characterize the different emission regimes. © 2012 American Physical Society.

We present a simple technique for measuring coherence times for stationary light fields using a single detector with tunable time resolution. By measuring the equal-time second-order correlation function at varying instrument response functions it is possible to determine the coherence time and also the shape of the temporal decay without the need to record time-resolved data. The technique is demonstrated for pseudothermal light. Possible applications for dynamic light scattering and photon statistics measurements are discussed.© 2012 Optical Society of America.

The coherent spin dynamics of resident carriers, electrons, and holes in semiconductor nanostructures is studied theoretically under the conditions of periodical optical excitation using short laser pulses and in an external magnetic field. The generation and dephasing of spin polarization in an ensemble of carrier spins, for which the relaxation time of individual spins exceeds the repetition period of the laser pulses, are analyzed. Accumulation of the spin polarization is manifested either as resonant spin amplification or as mode locking of carrier spin coherences. It is shown that both regimes have the same origin, while their appearance is determined by the optical pump power and the spread of spin precession frequencies in the ensemble. © 2012 American Physical Society.

High intensity ultrafast acoustics is a powerful tool to modulate excitons in semiconductors on picosecond time scales. Here we give a particular example for the potential of this technique by demonstrating ultrafast destruction of exciton states with their subsequent, similarly fast recurrence, making the method's impact volatile, in stark contrast to other techniques. The origin of the destruction is strain modulation of single particle states on time scales so short that their contribution to exciton formation is hindered. This is demonstrated for an exciton confined in a quantum well, which is placed in a high quality microresonator. For this system the strong light-matter interaction leads to the formation of polaritons. The itinerant exciton destruction enforces a transfer to the weak coupling regime with fast subsequent recovery of strong coupling. Thereby a tool for controlling light-matter coupling in condensed matter systems has been realized, which may be of interest for emerging fields such as exciton polaritonics and quantum optics. © 2012 American Physical Society.

Integration of magnetism into semiconductor electronics would facilitate an all-in-one-chip computer. Ferromagnet/bulk semiconductor hybrids have been, so far, mainly considered as key devices to read out the ferromagnetism by means of spin injection. Here we demonstrate that a Mn-based ferromagnetic layer acts as an orientation-dependent separator for carrier spins confined in a semiconductor quantum well that is set apart from the ferromagnet by a barrier only a few nanometers thick. By this spin-separation effect, a non-equilibrium electron-spin polarization is accumulated in the quantum well due to spin-dependent electron transfer to the ferromagnet. The significant advance of this hybrid design is that the excellent optical properties of the quantum well are maintained. This opens up the possibility of optical readout of the ferromagnet's magnetization and control of the non-equilibrium spin polarization in non-magnetic quantum wells. © 2012 Macmillan Publishers Limited. All rights reserved.

The properties of electron and hole spins in InP/(Ga,In)P self-assembled quantum dots are studied through their coherent dynamics using time-resolved Kerr rotation. From these studies information about the g factor and dephasing of the spin excitations is extracted. The electron spin shows a behavior similar to that of electron spins in quantum dots of different material: The g factor is isotropic in the dot plane and with increasing applied magnetic field the spin dephasing accelerates due to variations in the dot ensemble. On the other hand, the hole spin demonstrates a behavior different from other dot systems. Namely, the signal decay on a time scale of about 100 ps does not depend on magnetic field, and the g factor is isotropic in the dot plane. These findings underline previous suggestions that only the electrons are well localized in the InP/(Ga,In)P quantum dots, while the holes are weakly confined and carry bulklike character. © 2012 American Physical Society.

We report the excitation of spin waves in ferromagnetic semiconductor (Ga,Mn)As films by picosecond strain pulses. The strain pulse with a broad acoustic spectrum excites a number of magnon modes, which contribute to the precession of magnetization. The spectrum of the excited spin waves shows two well-resolved peaks with intensities dependent on the applied magnetic field. For a certain range of magnetic fields only the low-frequency spin wave is detected. We present the theoretical analysis and compare it with the experimental results, addressing the spatial overlap of the magnon and phonon eigenfunctions. Depending on the boundary conditions and the spectrum of the spin waves the spatial matching of the spin wave and resonance phonon eigenfunctions may provide high excitation efficiency for only one magnon mode, while other modes are not excited. © 2012 American Physical Society.

Coherent collective phenomena of hole spins confined in an ensemble of (In,Ga)As quantum dots are studied by monitoring their Larmor precession about a magnetic field. Variation of the field strength drives the hole spins from a regime of resonant spin amplification, in which their precession frequency is a unique multiple of the laser repetition rate, to spin mode locking, in which several precession modes are commensurable with the laser repetition rate. In this regime the spin coherence time of individual holes is determined to be 0.7 μs. In contrast, electron spins in the quantum dots are always trapped in the mode-locking regime due to the strong hyperfine interaction with nuclear spins. © 2012 American Physical Society.

The population dynamics of dark and bright excitons in (In,Ga)As/GaAs quantum dots is studied by two-color pump-probe spectroscopy in an external magnetic field. With the field applied in Faraday geometry and at T<20 K, the dark excitons decay on a ten nanoseconds time scale unless the magnetic field induces a resonance with a bright exciton state. At these crossings their effective lifetime is drastically shortened due to spin flips of either electron or hole by which the dark excitons are converted into bright ones. Due to the quasielastic character we attribute the origin of these flips to the hyperfine interaction with the lattice nuclei. We compare the exciton spin relaxation times in the two resonances and find that the spin flip involving an electron is approximately 25 times faster than the one of the hole. A temperature increase leads to a considerable, nonmonotonic decrease of the dark exciton lifetime. Here phonon-mediated spin flips due to the spin-orbit interaction gradually become more important. © 2012 American Physical Society.

The problem of how single central spins interact with a nuclear spin bath is essential for understanding decoherence and relaxation in many quantum systems, yet is highly nontrivial owing to the many-body couplings involved. Different models yield widely varying time scales and dynamical responses (exponential, power-law, Gaussian, etc.). Here we detect the small random fluctuations of central spins in thermal equilibrium [holes in singly charged (In,Ga)As quantum dots] to reveal the time scales and functional form of bath-induced spin relaxation. This spin noise indicates long (400 ns) spin correlation times at a zero magnetic field that increase to ∼5μs as dominant hole-nuclear relaxation channels are suppressed with small (100G) applied fields. Concomitantly, the noise line shape evolves from Lorentzian to power law, indicating a crossover from exponential to slow [∼1/log(t)] dynamics. © 2012 American Physical Society.

Semiconductor light emission can be changed considerably in an optical resonator. Prerequisite is that the electronic transitions involved in light generation are in resonance with a cavity mode. Although resonance can be arranged through dedicated fabrication, there are cases where this is virtually impossible. As an example, we study a planar microcavity containing an inhomogeneous quantum dot ensemble with a spectral broadening much larger than the optical mode width, so that resonance is achieved for a tiny dot fraction only. Still, the laser threshold can be crossed at moderate optical pumping. We demonstrate that strain pulses generated by ultrafast acoustics techniques can be used to modulate the transition energies so that resonance with the optical mode is dynamically induced for a much larger dot fraction. As a result, the emission output can be enhanced by more than two orders of magnitude, which is potentially useful for modulating light sources.

We report on magnetic field-induced oscillations of the photon echo signal from negatively charged excitons in a CdTe/(Cd,Mg)Te semiconductor quantum well. The oscillatory signal is due to Larmor precession of the electron spin about a transverse magnetic field and depends sensitively on the polarization configuration of the exciting and refocusing pulses. The echo amplitude can be fully tuned from the maximum down to zero depending on the time delay between the two pulses and the magnetic-field strength. The results are explained in terms of the optical Bloch equations accounting for the spin level structure of electrons and trions. © 2012 American Physical Society.

Coherent sub-THz phonons incident on a gold grating that is deposited on a dielectric substrate undergo diffraction and thereby induce an alteration of the surface plasmon-polariton resonance. This results in efficient high-frequency modulation (up to 110 GHz) of the structure's reflectivity for visible light in the vicinity of the plasmon-polariton resonance. High modulation efficiency is achieved by designing a periodic nanostructure which provides both plasmon-polariton and phonon resonances. Our theoretical analysis shows that the dynamical alteration of the plasmon-polariton resonance is governed by modulation of the slit widths within the grating at the frequencies of higher-order phonon resonances. © 2012 American Physical Society.

Miniaturization is an important aspect of device fabrication. Despite the advancements of modern top-down approaches, scaling-down to the sub-nanometer size is still a challenge. As an alternative, bottom-up approaches, such as the use of DNA as an engineering material, are therefore emerging, allowing control of matter at the single-molecule level. A DNA-based self-assembly method for the construction of switchable DNA devices is descrbied here based on G-quadruplex moieties, which are patterned on quasi-planar DNA arrays with nanoscale precision. The reversible switching of the devices is triggered by addition of DNA sequences ('fuels') and translated into linear extension/contractile movements. The conformational change of the devices was visualized by atomic force microscopy and FRET spectroscopy. Steady state fluorescence spectroscopy indicated that scaffolding of the G4 motors to either individual tiles or extended superlattices had no significant impact on the switching and optical performance of the system. However, time-resolved spectroscopy revealed that ordering in the microstructural environment enhances the fraction of molecules subject to FRET. Altogether, our study confirms that DNA superstructures are well-suited scaffolds for accommodation of mechanically switchable units and thus opens the door to the development of more sophisticated nanomechanical devices. Ordered planar nanoarrays bearing DNA-responsive devices are produced by DNA self-assembly procedures. The nanomotors display extension and contraction movements in response to addition of DNA fuels with efficient cycling operation even after repetitive cycles. This DNA-based assembly of nanomechanical units offers full control over the spatial arrangement of each single molecule and opens the door to the development of more sophisticated nanomechanical devices. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Two-color pump probe experiments reveal the possibility to use non-resonant pulsed laser excitation to create mode-locking of the ground state electron spin precessions in an ensemble of singly charged (In,Ga)As/GaAs quantum dots. The mode-locking shows a resonance for excitation into the first excited shell while for excitation into higher shells or barriers it disappears; however, spin coherence can still be induced. We conclude that the optically excited carriers relax spin-conserved from the p-shell into their ground states on a picosecond time scale, much shorter than the spin revolution period about the magnetic field. © 2012 American Institute of Physics.

We report on optical studies of the thin films of multiferroic hexagonal (P.G. 6mm) rare-earth orthoferrites RFeO 3 (R = Ho, Er, Lu) grown epitaxially on a (111)-surface of ZrO 2(Y 2O 3) substrate. The optical absorption study in the range of 0.6-5.6 eV shows that the films are transparent below 1.9 eV; above this energy four broad intense absorption bands are distinguished. The absorption spectra are analyzed taking into account the unusual fivefold coordination of the Fe 3+ ion. Temperature dependence of the optical absorption at 4.9 eV shows anomaly at 124 K, which we attribute to magnetic ordering of iron sublattices. © 2012 American Institute of Physics.

Third harmonic generation (THG) has been studied in europium selenide EuSe in the vicinity of the band gap at 2.1-2.6 eV and at higher energies up to 3.7 eV. EuSe is a magnetic semiconductor crystalizing in centrosymmetric structure of rock-salt type with the point group m3m. For this symmetry the crystallographic and magnetic-field-induced THG nonlinearities are allowed in the electric-dipole approximation. Using temperature, magnetic field, and rotational anisotropy measurements, the crystallographic and magnetic-field-induced contributions to THG were unambiguously separated. Strong resonant magnetic-field-induced THG signals were measured at energies in the range of 2.1-2.6 eV and 3.1-3.6 eV for which we assign to transitions from 4f7 to 4f65d1 bands, namely involving 5d(t 2g) and 5d(e g) states. © 2012 American Physical Society.

This chapter discusses the characteristics of semiconductor nanostructures as light emitters in terms of their coherence properties. These are evidenced by investigating the emitted photon statistics. Special emphasis is placed on second- and higher-order correlation functions. Several experimental approaches to measure them on different timescales for stationary as well as for non-stationary fields are introduced and discussed. Three model systems - one in the weak-coupling regime, one in the strong-coupling regime and one that shows a transition from one to the other - are discussed in detail and their equal-time correlation functions are analyzed. It is shown that transitions from predominant spontaneous emission towards coherent light emission can be identified by changes in the correlation functions. © 2012 Woodhead Publishing Limited. All rights reserved.

The ever-growing demand for fast optical data transmission calls for lasers offering high modulation rates and low energy consumption at the same time. Advances in growth and processing methods make quantum dot (QD) based lasers better candidates for this challenge than ever before. Placed in microresonators able to confine light in regions roughly the size of their wavelength, QDs pave the way to ultra-low threshold lasing. The most common resonator geometries aimed at three-dimensional light confinement are microdisks, photonic crystal membrane cavities and micropillars. The latter are especially good candidates for realizing microlasers suitable for applications as they offer directed emission and allow for parallel device processing. However, this increased efficiency also results in modified emission properties of QD lasers [8]. Semiconductor-specific processes like Pauli-blocking of states, the composite nature of the carriers involved and Coulomb interactions between carriers cause deviations from the standard atomistic laser picture. The main aim of our studies is to characterize microlaser emission in terms of photon statistics and coherence properties. Following Glauber, the most detailed description of a light field is given in a series of correlation functions describing coherence in different orders [10].This chapter is organized as follows. Section 10.2 contains a brief review on the characteristic properties of micropillar lasers and discusses the emission properties of microlasers operated below and above threshold. Section 10.2.1 focuses on photon statistics and the classification of light fields. © Cambridge University Press 2012.

We demonstrate that the dispersion of surface plasmon polaritons in a periodically perforated gold film can be efficiently manipulated by femtosecond laser pulses in spectral regions far from the intrinsic gold resonances. Using a time- and frequency-resolved pump-probe technique we observe shifting of the plasmon polariton resonances with response times from 200 to 800 fs depending on the probe photon energy, by which we obtain comprehensive insight into the electron dynamics in gold. We show that Wood anomalies in the optical spectra provide pronounced resonances in differential transmission and reflection with magnitudes up to 3% for moderate pump fluences of 0.5 mJ/cm2. © 2012 American Physical Society.

We create an optically imprinted gain-trapped polariton wire with a length of 15 μm and a width of 2μm by modulating the shape of a non-resonant excitation spot using a high-resolution spatial light modulator (SLM). We study the spatially, spectrally and temporally resolved emission from the wire and find that the system passes several regimes, starting with an intense emission peak originating from the wire center and develops towards longer living emission from its side. The temporal development of the emission wavelengths corresponding to these peaks allow us to characterize these different emission regimes in terms of photon lasing and polariton condensation. © 2012 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE).

Electron spin coherence in CdTe/(Cd,Mg)Te quantum wells is studied experimentally and theoretically in tilted external magnetic fields generated by a superconducting vector magnet. The long-lived spin coherence is measured by pump-probe Kerr rotation in the resonant spin amplification (RSA) regime. The shape of RSA signals is very sensitive to weak magnetic field components deviating from the Voigt or Faraday geometries. © 2012 American Physical Society.

The spatially, spectrally, and temporally resolved emission from a nonsymmetric polariton condensate prepared in a staircase shape is investigated. The results show a strong dependence of the spatial emission profile on the excitation density due to polariton redistribution. At large excitation densities the onset of conventional photon lasing has a crucial effect on the system. It is possible to vary the position from which the peak emission originates, and an emission profile showing switching of the predominant emission position from one end of the pump spot to the other is demonstrated. The findings are compared to a numerical simulation. © 2012 American Physical Society.

The spin coherence of an ensemble of electrons bound to fluorine donors in epitaxially grown ZnSe layers is studied by time-resolved pump-probe Kerr rotation. Long-lived spin dephasing with decay times up to T2*=33 ns is found for a sample with a low fluorine concentration of 1×1015 cm -3 at cryogenic temperatures. The time is close to the limit set by nuclear-spin fluctuations, for which we measure a strength of 1.65 mT. We find T2* to be constant up to 40 K, with a strong drop for higher temperatures. The dephasing time also shortens with increasing fluorine concentration, indicating an interaction between the spins at different fluorine centers. © 2012 American Physical Society.

Spin dynamics of negatively charged excitons is experimentally studied in (In,Al)As/AlAs quantum dots with indirect band gap and type-I band alignment. At low temperatures of 1.8 K, the spin relaxation time is 55 μ s in a magnetic field of 3 T. It decreases with increasing magnetic field as B -5, which evidences that the spin relaxation of the negatively charged excitons is provided by an one-acoustic-phonon process. © 2012 American Institute of Physics.

Optical femtosecond laser pulses diffracted into a crystalline substrate by a gold grating on top interact with gigahertz coherent phonons propagating towards the grating from the opposite side. As a result, Brillouin oscillations are detected for diffracted light. The experiment and theoretical analysis show that the amplitude of the oscillations for the first order diffracted light exceeds that of the zero order signal by more than ten times. The results provide a method for internal probing of the optical far-field inside materials containing periodic nanostructures. © 2012 American Institute of Physics.

The second-order correlation function g (2)(τ = 0), input-output curves and pulse duration of the emission from a microcavity exciton-polariton system subsequent to picosecond-pulsed excitation are measured for different temperatures. At low temperatures a two-threshold behaviour emerges, which has been attributed to the onset of polariton lasing and conventional lasing at the first and the second threshold, respectively. We observe that polariton lasing is stable up to temperatures comparable with the exciton binding energy. At higher temperatures a single threshold displays the direct transition from thermal emission to photon lasing. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.

The g-factor tensors of electron and hole in self-assembled (In,Ga)As/GaAs quantum dots are studied by time-resolved ellipticity measurements in a three dimensional vector magnet system. Both g-factor tensors show considerable deviations from isotropy. These deviations are much more pronounced for the hole than for the electron and are described by different anisotropy factors, which can even have opposite signs. © 2011 American Institute of Physics.

The electron spin precession about an external magnetic field was studied by Faraday rotation on an inhomogeneous ensemble of singly charged, self-assembled (In,Ga)As/GaAs quantum dots. From the data the dependence of electron g -factor on optical transition energy was derived. A comparison with literature reports shows that the electron g -factors are quite similar for quantum dots with very different geometrical parameters, and their change with transition energy is almost identical. © 2011 American Institute of Physics.

Plasmonics allows light to be localized on length scales much shorter than its wavelength, which makes it possible to integrate photonics and electronics on the nanoscale. Magneto-optical materials are appealing for applications in plasmonics because they open up the possibility of using external magnetic fields in plasmonic devices. Here, we fabricate a new magneto-optical material, a magnetoplasmonic crystal, that consists of a nanostructured noble-metal film on top of a ferromagnetic dielectric, and we demonstrate an enhanced Kerr effect with this material. Such magnetoplasmonic crystals could have applications in telecommunications, magnetic field sensing and all-optical magnetic data storage.

The dynamics of exciton recombination in an ensemble of indirect band-gap (In,Al)As/AlAs quantum dots with type-I band alignment is studied. The lifetime of confined excitons that are indirect in momentum space is mainly influenced by the sharpness of the heterointerface between the (In,Al)As quantum dot and the AlAs barrier matrix. Time-resolved photoluminescence experiments and theoretical model calculations reveal a strong dependence of the exciton lifetime on the thickness of the interface diffusion layer. The lifetime of excitons with a particular optical transition energy varies because this energy is obtained for quantum dots differing in size, shape, and composition. The different exciton lifetimes, which result in photoluminescence with nonexponential decay obeying a power-law function, can be described by a phenomenological distribution function G(τ), which allows one to fit the photoluminescence decay with one parameter only. © 2011 American Physical Society.

Exciton states have been studied experimentally in strained ZnSe/(Zn,Mg)Se quantum well structures with a Mg content of only 3.6% by means of magnetoreflectivity and ultrafast piezospectroscopy. The small intrinsic band gap difference and the built-in strain in barriers and quantum wells lead to a shallow confinement potential for heavy holes which is smaller than the Coulomb electron-hole interaction. An exciton state formed by a confined electron and heavy-hole continuum states is identified. The experimental findings are supported by numerical model calculations. © 2011 American Physical Society.

Bose-Einstein condensation (BEC) is a thermodynamic phase transition of an interacting Bose gas. Its key signatures are remarkable quantum effects like superfluidity and a phonon-like Bogoliubov excitation spectrum, which have been verified for atomic BECs. In the solid state, BEC of exciton-polaritons has been reported. Polaritons are strongly coupled light-matter quasiparticles in semiconductor microcavities and composite bosons. However, they are subject to dephasing and decay and need external pumping to reach a steady state. Accordingly the polariton BEC is a nonequilibrium process of a degenerate polariton gas in self-equilibrium, but out of equilibrium with the baths it is coupled to and therefore deviates from the thermodynamic phase transition seen in atomic BECs. Here we show that key signatures of BEC can even be observed without fulfilling the self-equilibrium condition in a highly photonic quantum degenerate nonequilibrium system.

Using optical pulses of variable duration up to 80 ps, we report on spin coherence initialization and its subsequent detection in n-type singly charged quantum dots, subject to a transverse magnetic field, by pump-probe techniques. We demonstrate experimentally and theoretically that the spin coherence generation and readout efficiencies are determined by the ratio of laser pulse duration to spin precession period: An increasing magnetic field suppresses the spin coherence signals for a fixed duration of pump and/or probe pulses, and this suppression occurs for smaller fields, the longer is the pulse duration. The reason for suppression is the varying spin orientation due to precession during pulse action. © 2011 American Physical Society.

The electron spin coherence in n-doped and undoped, self-assembled CdSe/Zn(S,Se) quantum dots has been studied by time-resolved pump-probe Kerr rotation. Long-lived spin coherence persisting up to 13 ns after spin orientation has been found in the n-doped quantum dots, outlasting significantly the lifetimes of charge neutral and negatively charged excitons of 350-530 ps. The electron spin dephasing time as long as 5.6 ns has been measured in a magnetic field of 0.25 T. Hyperfine interaction of resident electrons with nuclear spin fluctuations is suggested as the main limiting factor for the dephasing time. The efficiency of this mechanism in II-VI and III-V quantum dots is analyzed. © 2011 American Physical Society.

We propose photon-statistics excitation spectroscopy as an adequate tool to describe the optical response of a nonlinear system. To this end we suggest to use optical excitation with varying photon statistics as another spectroscopic degree of freedom to gather information about the system in question. The responses of several simple model systems to excitation beams with different photon statistics are discussed. Possible spectroscopic applications in terms of identifying lasing operation are pointed out. © 2011 American Physical Society.

Coherent interactions between spins in quantum dots are a key requirement for quantum gates. We have performed pump-probe experiments in which pulsed lasers emitting at different photon energies manipulate two distinct subsets of electron spins within an inhomogeneous InGaAs quantum dot ensemble. The spin dynamics are monitored through their precession about an external magnetic field. These measurements demonstrate spin precession phase shifts and modulations of the magnitude of one subset of oriented spins after optical orientation of the second subset. The observations are consistent with results from a model using a Heisenberg-like interaction with μeV strength. © 2011 American Physical Society.

We report on optical orientation of Mn2+ ions in bulk GaAs subject to weak longitudinal magnetic fields (B≤100mT). A manganese spin polarization of 25% is directly evaluated by using spin-flip Raman scattering. The dynamical Mn2+ polarization occurs due to the s-d exchange interaction with optically oriented conduction band electrons. Time-resolved photoluminescence reveals a nontrivial electron spin dynamics, where the oriented Mn2+ ions tend to stabilize the electron spins. © 2011 American Physical Society.

In this work, we study the temperature-dependent spin relaxation of exciton-bound carriers in (In,Ga)As/GaAs quantum dots. The exciton population mapped by a time-resolved differential transmission signal reveals a decay on two different time scales, reflecting the fractions of optically active and inactive excitons. The underlying exciton states are split from each other by the exchange interaction. The phonon-assisted spin-orbit interaction induces spin flips of an exciton-bound electron or hole which convert the exciton populations into each other. The temperature-dependent relaxation rate follows a thermal phonon distribution. Deviations indicate that two-phonon processes involving higher orbitals may contribute significantly to the total relaxation process. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

A magneto-optical study of the energy and spin structure of charged excitons in a 20-nm-thick CdTe/Cd0.65Mg0.35Te quantum well is performed in strong magnetic fields up to 51 T. The type of resident carriers (holes or electrons) in the quantum well is controlled optically by above-barrier illumination, permitting a direct comparison of positively (T +) versus negatively (T-) charged excitons. The binding energies of the singlet states of these complexes behave qualitatively differently with increasing magnetic field B; namely, the binding energy decreases for T+ and increases for T- with B. The triplet state of T+ is identified in strong fields with a binding energy smaller than that of the T- triplet state. © 2011 American Physical Society.

We investigate the effect of molecular doping on the recombination of electrons and holes localized at conjugated-polymer-fullerene interfaces. We demonstrate that a low concentration of p-type dopant molecules (<4% weight) reduces the interfacial recombination via charge transfer excitons and results in a favored formation of separated carriers. This is observed by the ultrafast quenching of photoluminescence from charge transfer excitons and the increase in photoinduced polaron density by ∼70%. The results are consistent with a reduced formation of emissive charge transfer excitons, induced by state filling of tail states. © 2011 American Physical Society.

We report on the observation of resonant optical pumping of nuclear spin polarization in an ensemble of singly charged (In,Ga)As/GaAs quantum dots subject to a transverse magnetic field. Electron spin orientation by circularly polarized light with the polarization modulated at the nuclear spin transition frequency is found to create a significant nuclear spin polarization, precessing about the magnetic field. Synchronous rf field application along the optical excitation axis considerably enhances the effect. Nuclear spin resonances for all isotopes in the quantum dots are found in that way. In particular, transitions between states split off from the |±1/2 doublets by the nuclear quadrupole interaction are identified. © 2011 American Physical Society.

A theoretical model of the coherent precession of magnetization excited by a picosecond acoustic pulse in a ferromagnetic semiconductor layer of (Ga,Mn)As is developed. The short strain pulse injected into the ferromagnetic layer modifies the magnetocrystalline anisotropy resulting in a tilt of the equilibrium orientation of magnetization and subsequent magnetization precession. We derive a quantitative model of this effect using the Landau-Lifshitz equation for the magnetization that is precessing in the time-dependent effective magnetic field. After developing the general formalism, we then provide a numerical analysis for a certain structure and two typical experimental geometries in which an external magnetic field is applied either along the hard or the easy magnetization axis. As a result we identify three main factors, which determine the precession amplitude: the magnetocrystalline anisotropy of the ferromagnetic layer, its thickness, and the strain pulse parameters. © 2011 American Physical Society.

We show that the magnetization of a thin ferromagnetic (Ga,Mn)As layer can be modulated by picosecond acoustic pulses. In this approach a picosecond strain pulse injected into the structure induces a tilt of the magnetization vector M, followed by the precession of M around its equilibrium orientation. This effect can be understood in terms of changes in magnetocrystalline anisotropy induced by the pulse. A model where only one anisotropy constant is affected by the strain pulse provides a good description of the observed time-dependent response. © 2010 The American Physical Society.

The magnetization dynamics of the Mn spin system in an undoped (Zn,Mn)Se/BeTe type-II quantum well was studied by a time-resolved pump-probe photoluminescence technique. The Mn spin temperature was evaluated from the giant Zeeman shift of the exciton line in an external magnetic field of 3 T. The relaxation dynamics of the Mn spin temperature to the equilibrium temperature of the phonon bath after the pump-laser-pulse heating can be accelerated by the presence of free electrons. These electrons, generated by a control laser pulse, mediate the spin and energy transfer from the Mn spin system to the lattice and bypass the relatively slow direct spin-lattice relaxation of the Mn ions. © 2010 The American Physical Society.

We have studied the Faraday rotation and ellipticity signals in ensembles of singly charged (In,Ga)As/GaAs quantum dots by pump-probe spectroscopy. For degenerate pump and probe we observe that the Faraday rotation signal amplitude first grows with increasing the time separation between pump and probe before a decay is observed for large temporal separations. The temporal behavior of the ellipticity signal, on the other hand, is regular: its amplitude decays with the separation. By contrast, for detuned pump and probe the Faraday rotation and ellipticty signals both exhibit similar and conventional behavior. The experimental results are well described in the frame of a recently developed microscopic theory. The comparison between calculations and experimental data allows us to provide insight into the spectral dependence of the electron spin precession frequencies and extract the electron g factor dependence on energy. © 2010 The American Physical Society.

We study exciton magnetic polaron (EMP) formation in (Cd,Mn)Se/(Cd,Mg)Se diluted magnetic semiconductor quantum wells (QWs) using time-resolved photoluminescence (PL). Magnetic field dependence of the transients allows us to separate the non-magnetic and magnetic contributions of the exciton localization. We find binding energy of 18 meV and formation time of 500 ps for the EMP. We propose that long polaron formation time is related to auto-localization process, accompanied with the squeezing of the heavy-hole envelope wave function. This conclusion is supported by a strong reduction of the exciton radiative lifetime from 600 to 200 ps with increase of magnetic field. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

We report experiments in which high quality silica opal films are used as three-dimensional hypersonic crystals in the 10 GHz range. Controlled sintering of these structures leads to well-defined elastic bonding between the submicrometer-sized silica spheres, due to which a band structure for elastic waves is formed. The sonic crystal properties are studied by injection of a broadband elastic wave packet with a femtosecond laser. Depending on the elastic bonding strength, the band structure separates long-living surface acoustic waves with frequencies in the complete band gap from bulk waves with band frequencies that propagate into the crystal leading to a fast decay. © 2010 American Chemical Society.

We present a detailed discussion of a recently demonstrated experimental technique capable of measuring the correlation function of a pulsed light source with picosecond time resolution. The measurement involves a streak camera in single photon counting mode, which is modified such that a signal at a fixed repetition rate, and well defined energy, can be monitored after each pulsed laser excitation. The technique provides further insight into the quantum optical properties of pulsed light emission from semiconductor nanostructures, and the dynamics inside a pulse, on the subnanosecond time scale. © 2010 Optical Society of America.

Spectroscopic study of diamagnetic, diluted magnetic, and magnetically ordered semiconductors reveals several novel mechanisms of optical harmonics generation. It is found that Landau-level orbital quantization of the band energy is a key mechanism for magnetic-field-induced second harmonic generation (MSHG) in diamagnetic semiconductors GaAs and CdTe. The giant Zeeman spin-splitting of electronic states is essential for MSHG in diluted magnetic semiconductors (Cd,Mn)Te. Both mechanisms involving the optical nonlinearities of electric-dipole type take place in noncentrosymmetric semiconductors. Spin-induced second harmonic generation (SHG) is observed at the band gap in magnetic centrosymmetric semiconductors EuTe and EuSe. This mechanism involving the optical nonlinearities of magnetic-dipole type is essential for centrosymmetric semiconductors. The magnetic field and temperature dependencies demonstrate that the nonlinear processes arise due to novel types of optical nonlinearities caused by the external magnetic field. The observed mechanisms of optical nonlinearities open access to a wide class of centrosymmetric and noncentrosymmetric systems by harmonics generation spectroscopy. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

We present time-resolved reflectivity spectra of the cavity mode in a II-VI semiconductor based planar microcavity that is modulated by a strain pulse injected using an ultrafast acoustics technique. The modulation occurs when the strain pulse passes interfaces of the layered cavity structure at which the electric field has an antinode. Maximum modulation is reached when the pulse enters or leaves the central cavity layer. The mode shifts in the cavity with a finesse of about 2000 are comparable to its mode linewidth, which shows that the proposed technique is prospective for ultrafast optical switching. © 2010 The American Physical Society.

Optical control of the spin coherence of quantum well electrons by short laser pulses with circular or linear polarization is studied experimentally and theoretically. For that purpose the coherent electron spin dynamics in a n -doped CdTe/(Cd,Mg)Te quantum well structure was measured by time-resolved pump-probe Kerr rotation, using resonant excitation of the negatively charged exciton (trion) state. The amplitude and phase shifts of the electron spin beat signal in an external magnetic field, that are induced by laser control pulses, depend on the pump-control delay and polarization of the control relative to the pump pulse. Additive and nonadditive contributions to pump-induced signal due to the control are isolated experimentally. These contributions can be well described in the framework of a two-level model for the optical excitation of the resident electron to the trion. © 2010 The American Physical Society.

The magnetic europium chalcogenide semiconductors EuTe and EuSe are investigated by the spectroscopy of second harmonic generation (SHG) in the vicinity of the optical band gap formed by transitions involving the 4f and 5d electronic orbitals of the magnetic Eu2+ ions. In these materials with centrosymmetric crystal lattice the electric-dipole SHG process is symmetry forbidden so that no signal is observed in zero magnetic field. Signal appears, however, in applied magnetic field with the SHG intensity being proportional to the square of magnetization. The magnetic field and temperature dependencies of the induced SHG allow us to introduce a type of nonlinear optical susceptibility determined by the magnetic-dipole contribution in combination with a spontaneous or induced magnetization. The experimental results can be described qualitatively by a phenomenological model based on a symmetry analysis and are in good quantitative agreement with microscopic model calculations accounting for details of the electronic energy and spin structure. © 2010 The American Physical Society.

EuTe possesses the centrosymmetric crystal structure m3m of rocksalt type in which the second-harmonic generation is forbidden in electric dipole approximation but the third-harmonic generation (THG) is allowed. We studied the THG spectra of this material and observed several resonances in the vicinity of the band gap at 2.2-2.5 eV and at higher energies up to 4 eV, which are related to four-photon THG processes. The observed resonances are assigned to specific combinations of electronic transitions between the ground 4 f7 state at the top of the valence band and excited 4 f6 5 d1 states of Eu2⊃+ ions, which form the lowest energy conduction band. Temperature, magnetic field, and rotational anisotropy studies allowed us to distinguish crystallographic and magnetic-field-induced contributions to the THG. A strong modification of THG intensity for the 2.4 eV band and suppression of the THG for the 3.15 eV band was observed in applied magnetic field. Two main features of the THG spectra were assigned to 5d (t2g) and 5d (eg) subbands at 2.4 eV and 3.15 eV, respectively. A microscopic quantum-mechanical model of the THG response was developed and its conclusions are in qualitative agreement with the experimental results. © 2010 The American Physical Society.

Nuclear-magnetic resonances were detected optically in an ensemble of singly charged (In,Ga)As/GaAs quantum dots. The resonances were found in the magnetic field dependence of photoluminescence polarization (Hanle curve) when applying a radio frequency (RF) field. The frequency dependences of the resonances allow us to ascribe them to transitions between Zeeman states of the G 71 a and A 75 s nuclei perturbed by quadrupole interaction. Applying an RF-field sweep over a wide frequency range results in a notable narrowing of the Hanle curve, which can be explained by suppression of dynamic nuclear polarization that otherwise is stabilized by the quadrupole splitting. © 2010 The American Physical Society.

ZnO has attracted increasing interest as promising material for optoelectronics and spintronics. Magnetic properties of ZnO can be controlled by doping with transition metal ions among which Fe3+ is one of the interesting candidates. We report on the properties of Fe3+ centres in hydrothermally and chemical vapour transport grown ZnO single crystals investigated by photo-electron paramagnetic resonance (EPR) and optical spectroscopy. Detailed magneto-optical studies of Zeeman components of spin-forbidden electric dipole transitions 4T1(G) ! 6A1(6S) of Fe3+ centre in ZnO reveal the trigonal symmetry of fine structure of lowest G8 (4T1) excited state. The studies were performed in external magnetic fields up to 10 T and in the temperature range from 2 to 30 K. The energy positions of the Zeeman components at 8 T were measured as a function of the direction of applied magnetic field in (12-10) and (0001) planes. The detailed check of the angular variation of Zeeman lines shows two magnetically non-equivalent Fe3+ centres. These special features were accounted by contribution of higher rank Zeeman terms of dimension BJ3. The photo- generated EPR spectra of trigonal Fe3+ centres were detected in hydrothermally grown ZnO single crystals in addition to three types of charge compensated iron centres presented in these samples in the dark conditions. © 2010 WILEY-VCH Verlag GmbH &amp; Co. KGaA, Weinheim.

The magnetization dynamics in diluted magnetic semiconductor heterostructures based on (Zn,Mn)Se and (Cd,Mn)Te were studied optically and simulated numerically. In samples with inhomogeneous magnetic ion distribution, these dynamics are contributed by spin-lattice relaxation and spin diffusion in the Mn spin system. A spin-diffusion coefficient of 7× 10-8 cm2 /s was evaluated for Zn0.99 Mn0.01 Se from comparison of experiment and theory. Calculations of the exciton giant Zeeman splitting and the magnetization dynamics in ordered alloys and digitally grown parabolic quantum wells show perfect agreement with the experimental data. In both structure types, spin diffusion contributes essentially to the magnetization dynamics. © 2010 The American Physical Society.

The carrier spin dynamics in a n -doped (In,Ga)As/GaAs quantum well has been studied by time-resolved Faraday rotation and ellipticity techniques in the temperature range down to 430 milliKelvin. These techniques give data with very different spectral dependencies, from which nonetheless consistent information on the spin dynamics can be obtained, in agreement with theoretical predictions. The mechanisms of long-lived spin coherence generation are discussed for the cases of trion and exciton resonant excitation. We demonstrate that carrier localization leads to a saturation of spin relaxation times at 45 ns for electrons below 4.5 K and at 2 ns for holes below 2.3 K. The underlying spin relaxation mechanisms are discussed. © 2010 The American Physical Society.

We measure the frequency spectra of random spin fluctuations, or "spin noise," in ensembles of (In,Ga)As/GaAs quantum dots (QDs) at low temperatures. We employ a spin noise spectrometer based on a sensitive optical Faraday rotation magnetometer that is coupled to a digitizer and field-programmable gate array, to measure and average noise spectra from 0-1 GHz continuously in real time with subnanoradian/Hz sensitivity. Both electron and hole spin fluctuations generate distinct noise peaks, whose shift and broadening with magnetic field directly reveal their g factors and dephasing rates within the ensemble. A large, energy-dependent anisotropy of the in-plane hole g factor is clearly exposed, reflecting systematic variations in the average QD confinement potential. © 2010 The American Physical Society.

We study the excitonic polariton relaxation and propagation in bulk Cd0.88Zn0.12 Te crystals using time-resolved photoluminescence and time-of-flight techniques. Propagation of picosecond optical pulses through a 745-μm -thick crystal results in time delays up to 350 ps, depending on the photon energy. Optical pulses with 150 fs duration become strongly stretched while moving through the sample. The spectral dependence of the group velocity is consistent with the dispersion of the lower excitonic polariton branch. The lifetimes of excitonic polaritons in the upper and lower branches are 1.5 ns and 3 ns, respectively. © 2010 The American Physical Society.

We report on polariton propagation beats of the yellow 1 S paraexciton in Cu2 O as function of magnetic field, laser energy, and temperature. The propagation beats are fitted by a model including the inhomogeneity of the sample, which enables us to distinguish between homogeneous damping and inhomogeneous broadening. By use of a H3 e insert we achieved bath temperatures down to 0.35 K. Propagation beats of a 13 ns Gaussian laser pulse were observable up to 180 ns after the pulse leading to a remarkably small homogeneous damping of about 9 neV at the lowest temperature. The corresponding dephasing time is 150 ns decreasing rapidly to about 5 ns at 2.1 K. These results are confirmed by absorption spectra of the same sample. The temperature dependence of the homogeneous damping is discussed in terms of longitudinal-acoustic phonon scattering. © 2010 The American Physical Society.

Ultrafast changes in the statistical properties of light emission are studied for quantum-dot micropillar lasers. Using pulsed excitation with varying power, we follow the time evolution of the second-order correlation function g (2) (t,τ=0) reflecting two-photon coincidences and compare it to that of the output intensity. The previously impossible time resolution of a few picoseconds gives insight into the dynamical transition between thermal and coherent light emission. The g (2) results allow us to isolate the spontaneous and stimulated-emission contributions within an emission pulse, not accessible via the emission-intensity dynamics. Results of a microscopic theory confirm the experimental findings. © 2010 The American Physical Society.