The drift of electron spin helices in an external in-plane electric field in GaAs quantum wells is studied by means of time-resolved magneto-optical Kerr microscopy. The evolution of the spin distribution measured for different excitation powers reveals that, for short delay times and higher excitation powers, the spin helix drift slows down while its envelope becomes anisotropic. The effect is understood as a local decrease of the electron gas mobility due to electron collisions with nonequilibrium holes within the excitation spot and is reproduced well in the kinetic theory framework. For larger delay times, when the electrons constituting the spin helix and nonequilibrium holes are separated by an electric field, the spin helix drift accelerates and the mobility reaches its unperturbed value again. © 2021 American Physical Society.

We report on a compact optical frequency comb, operating in the wavelength range from 670 to 1500 nm, based on diode-pumped low-noise femtosecond Yb:CALGO amplified laser system. Both the carrier-envelope offset and repetition rate are phase-locked to reference synthesizers. A full characterization of the frequency comb, in terms of frequency stability, phase noise analysis, and optical beating against a single-frequency non-planar ring oscillator Nd:YAG laser, is presented, showing the excellent properties of the Yb:CALGO comb. © 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

We investigate the non-degenerate two-photon absorption coefficient β(ω1, ω2) as a function of the non-degeneracy parameter ω1/ω2 for bulk GaAs and Si at a constant transition energy ω1 + ω2 = 1.57 eV. In both materials, the two-photon absorption strength increases with increasing ω1/ω2 regardless of the direct and indirect character of the bandgap. The GaAs measurement data agrees well with corresponding theoretical predictions for direct semiconductors. The Si data reveals similar trends albeit with smaller overall absorption strength. In addition, different crystallographic orientations and polarization configurations are analyzed. © 2021 Optical Society of America.

We experimentally and theoretically investigate the nondegenerate two-photon absorption coefficient β(ω1,ω2) in the prototypical semiconductor ZnSe. In particular, we provide a comprehensive data set on the dependence of β(ω1,ω2) on the nondegeneracy parameter ω1/ω2 with the total frequency sum ω1+ω2 kept constant. We find a substantial increase in the two-photon absorption strength with increasing ω1/ω2. In addition, different crystallographic orientations and polarization configurations are investigated. The nonlinear optical response is analyzed theoretically by evaluating the multiband semiconductor Bloch equations including inter- and intraband excitations in the length gauge. The band structure and the matrix elements are taken as an eight-band k·p model. The simulation results are in very good agreement with the experiment. © 2021 American Physical Society.

We realize and investigate a nonlinear metasurface taking advantage of intersubband transitions in ultranarrow GaN/AlN multi-quantum well heterostructures. Owing to huge band offsets, the structures offer resonant transitions in the telecom window around 1.55 µm. These heterostructures are functionalized with an array of plasmonic antennas featuring cross-polarized resonances at these near-infrared wavelengths and their second harmonic. This kind of nonlinear metasurface allows for substantial second-harmonic generation at normal incidence which is completely absent for an antenna array without the multi-quantum well structure underneath. While the second harmonic is originally radiated only into the plane of the quantum wells, a proper geometrical arrangement of the plasmonic elements permits the redirection of the second-harmonic light to free-space radiation, which is emitted perpendicular to the surface. © 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

We investigate the nonlinear optical response of a thin film of ion-implanted VO2 nanocrystals with open aperture z-scans involving femtosecond near-infrared pulses. Beyond the established nonlinearity related to the insulator-metal phase transition of VO2, the metallic state features a pronounced saturable absorption for 100 fs pulses from a modelocked Yb:fiber source at λ = 1036 nm. In contrast, we find a pronounced reverse saturable absorption for 90 fs pulses in the telecom window at λ = 1550 nm. We attribute these nonlinearities to a transient red-shift of the plasmonic resonance of the nanocrystals, in line with the temperature dependence of the linear absorption and the theoretical expectation for electronic heating. Details of the transmissivity characteristics can be tailored by the lattice temperature and/or the size of the nanocrystals. The results hold promise for the use of VO2 nanocrystals as a saturable absorber, e.g., to mode-locked near-infrared lasers. © 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement.

Spin-orbit interactions in solids have inspired innovative physics for spin-based technologies. One such example is the persistent spin helix, where spin-orbit interactions from the semiconductor lattice are balanced with those in asymmetric quantum wells, to create long-lived spin textures. Spin transport in the presence of the momentum-dependent spin-orbit interactions lead to Larmor precession and subsequent dephasing that challenges the design of current spin-based information processing devices. We demonstrate that external magnetic fields can be applied to overcome this issue for spin-polarized charge carriers transported by in-plane electric fields. A frame of reference picture is introduced to describe the emergence and dynamics of the polarization-locked spin-wave packet after optical excitation. Applying well-matched magnetic fields maintains the persistent spin-helix profile regardless of whether the frame of reference is in motion or not. Monte Carlo simulations allow this traveling persistent spin-helix concept to be extended to a proposed spin Hall-effect transistor to ease design requirements. © 2020 American Physical Society.

This review article summarizes recent developments related to the dynamical formation of persistent spin helices in GaAs- A nd CdTe-based heterostructures. We start with fundamental aspects of spin-orbit interaction in quantum wells, in particular the Dresselhaus and Rashba terms and their relation to the bulk and structural inversion asymmetries, respectively. In the regime of balanced interactions, their combined impact gives rise to the formation of the persistent spin helix, i.e., a regime where a unidirectional spin grating with enhanced coherence time is established. The experimental scheme relies on ultrafast Kerr microscopy and permits to excite the spin polarization and detect it with a simultaneous spatial and temporal resolution of micrometers and picoseconds, respectively. For a microscopic understanding and a description of the results, kinetic theory of spatio-temporal spin dynamics of two-dimensional electrons is presented. In addition, Monte Carlo simulations of the spin distribution function are performed. Based on these concepts we discuss three areas with recent advances in the field of spin helices. (i) Anisotropic spin transport and spin helix dynamics in a modulation-doped GaAs quantum well is analyzed. It is observed that application of an out-of-plane electric field changes spin-orbit interaction through the Rashba component and the cubic Dresselhaus term. Remarkably, a weak in-plane electric field substantially increases spin diffusion and also affects the spin helix wavelength. (ii) In-plane magnetic fields applied in two perpendicular orientations allow for the extraction of the individual spin-orbit coupling parameters. (iii) Finally, we explore the influence of optical doping on the spin helix in a CdTe quantum well. Most importantly, a non-uniform spatio-temporal precession pattern is observed. The kinetic theory of spin diffusion allows us to model this finding by incorporating a dependence on the photo-carrier density into the Rashba and the Dresselhaus parameters. © 2019 IOP Publishing Ltd.

The nonlinear optical response of direct-gap semiconductors is investigated with a focus on non-degenerate multiphoton absorption processes. The theoretical approach is based on the semiconductor Bloch equations and yields the absorption rate either perturbatively or non-perturbatively in the incident light intensities. We describe the semiconductor by a two-band model and consider a pump-probe scheme where the weak probe pulse provides one of the simultaneously absorbed photons. The perturbative response can be described analytically within some approximations and we give simple expressions for two, three, and four-photon absorption coefficients. These are compared with numerical results for the absorption of pulses with a finite duration, where the influence of dephasing and relaxation as well as higher-order corrections are also investigated. For strong pump fields that are treated non-perturbatively we demonstrate non-trivial dependencies of the absorption on the time delay between the pulses. In the non-perturbative response of a single light pulse characteristic modulations appear in the absorption dependence on the field strength that may be interpreted as multi-photon Rabi oscillations. Finally, we present measurements of the non-degenerate two-photon absorption coefficient of bulk GaAs via time-delay and polarization-dependent transmissivity changes in a pump probe setup. The observed strong increase of the absorption coefficient with frequency ratios deviating from unity qualitatively agrees with theoretical expectations. Copyright © 2019 SPIE.

Time-resolved magneto-optic Kerr microscopy measures the effect of in-plane electric fields on the dynamics of a photoexcited spin distribution in a modulation-doped GaAs quantum well. The structure features nearly equal Dresselhaus and Rashba coefficients, such that there is negligible impact of spin-orbit coupling for electrons moving along the [110] or [110] directions. Meanwhile, spin texture emerges for electrons moving in the [110] or [110] directions. The overall spin pattern resembles a persistent spin helix. An in-plane electric field, applied transverse to the spin texture (along the [110] or [110] directions), introduces a drift of the spin packet and additional Larmor precessions, i.e., a marked decrease of the spatial periodicity of the spin pattern. The in-plane electric field also increases the temporal frequency of the evolving spin distribution, which is directly linked to the cubic Dresselhaus spin-orbit coupling term. Moreover, the in-plane field increases the diffusion coefficient by more than an order of magnitude. We attribute this effect to carrier heating and the separation of the photogenerated electron-hole dipole. © 2019 American Physical Society.

Understanding and controlling the spin degree of freedom in two-dimensional transition metal dichalcogenides offers the potential for designing functional quantum materials. This work investigates the dynamics of photo- and resident carrier spins in an encapsulated MoSe2 monolayer using non-degenerate time-resolved Kerr-rotation microscopy. The lightly doped monolayer exhibits clear exciton and trion resonances with spin-polarizations that are characterized by a fast (~20 ps) decay attributed to photocarrier relaxation and recombination, followed by a slow (~690 ps) decay associated with resident carrier depolarization. Dual-frequency Kerr-rotation spectra directly reveal exciton-trion coupling on ultrashort timescales and within the spin coherence time of the system. Moreover, the distribution of the exciton-trion coupling features exposes inhomogeneous broadening likely arising from different domains within the excitation spot. © 2018 IOP Publishing Ltd.

Electron spin transport and dynamics are investigated in a single, high-mobility, modulation-doped, GaAs quantum well using ultrafast two-color Kerr-rotation microspectroscopy, supported by qualitative kinetic theory simulations of spin diffusion and transport. Evolution of the spins is governed by the Dresselhaus bulk and Rashba structural inversion asymmetries, which manifest as an effective magnetic field that can be extracted directly from the experimental coherent spin precession. A spin-precession length λSOI is defined as one complete precession in the effective magnetic field. It is observed that application of (i) an out-of-plane electric field changes the spin decay time and λSOI through the Rashba component of the spin-orbit coupling, (ii) an in-plane magnetic field allows for extraction of the Dresselhaus and Rashba parameters, and (iii) an in-plane electric field markedly modifies both the λSOI and diffusion coefficient. © 2018 American Physical Society.

Time-resolved Kerr-rotation microscopy explores the influence of optical doping on the persistent spin helix in a [001]-grown CdTe quantum well at cryogenic temperatures. Electron spin-diffusion dynamics reveal a momentum-dependent effective magnetic field providing SU(2) spin-rotation symmetry, consistent with kinetic theory. The Dresselhaus and Rashba spin-orbit coupling parameters are extracted independently from rotating the spin helix with external magnetic fields applied parallel and perpendicular to the effective magnetic field. Most importantly, a nonuniform spatiotemporal precession pattern is observed. The kinetic-theory framework of spin diffusion allows for modeling of this finding by incorporating the photocarrier density into the Rashba (α) and the Dresselhaus (β3) parameters. Corresponding calculations are further validated by an excitation-density-dependent measurement. This work shows universality of the persistent spin helix by its observation in a II-VI compound and the ability to fine-tune it by optical doping. © 2018 American Physical Society.

While two-photon emission processes are firmly established in atomic physics, their observation and use in semiconductor physics remains elusive. Here, we experimentally investigate stimulated two-photon emission in photoexcited bulk CdSe and identify requirements for the observation of stimulated two-photon emission. In particular, this process requires population inversion as well as two-photon transition energies close to the bandgap energy. In any regime investigated in the present study, net optical gain is not achieved, as the free-carrier absorption intrinsically linked to the photoexcitation completely masks the twophoton gain. The results are well in line with a recent study on nondegenerate versions of two-photon emission in GaAs and place clear limits for the practical use of two-photon emission in optically excited semiconductors. © 2018 Optical Society of America.

The linear and nonlinear behaviour of intersubband transitions of cubic GaN/AlN multi quantum well (QW) structures in the IR spectral region is investigated. In this study photoluminescence, IR absorption as well as pump-probe measurements are done. Two cubic GaN/AlN multi quantum wells with Si content of NSi ~1019 cm-3 in the cubic GaN quantum wells were grown on 3C-SiC (001) substrate by radio-frequency plasma-assisted molecular beam epitaxy. A broad IR absorption with a FWHM of 370meV was found with a maximum at 0.7eV, corresponding to the intersubband transition of the multi quantum wells. The nonlinear optical measurement reveals a clear change of transmission for a pump pulse with an angle of incidence of 65°. Furthermore, transmission electron microscopy measurements are used to determine the real layer thicknesses. These thickness values are exploited in the calculation with the Schrödinger-Poisson solver nextnano3. The simulated transition energies agree very well with the experimental data for the photoluminescence and the absorption measurement. © 2017 Elsevier B.V.

Up to 400 mW of near-IR (1370-1500 nm) femtosecond pulses are generated from an optical parametric amplifier directly driven by a Yb:fiber oscillator delivering 100 fs pulses at 1036 nm. The process is seeded by a stable supercontinuum obtained from a photonic crystal fiber. We use a single pass through a 3 mm, magnesium oxide-doped, periodically poled LiNbO3 downconversion crystal to produce a near-IR pulse train with a remarkable power stability of 1.4% (RMS) during one hour. Tuning is achieved by the temperature and the poling period of the nonlinear crystal. © 2017 Optical Society of America.

Exciton, trion, and electron spin dynamics in a 20-nm-wide modulation-doped GaAs single quantum well are investigated using resonant ultrafast two-color Kerr rotation spectroscopy. Excitons and trions are selectively detected by resonant probe pulses while their relative spectral weight is controlled by adjusting the gate voltage which tunes the carrier density. Tuning the carrier density markedly influences the spin decay time of the two-dimensional electron gas. The spin decay time can be enhanced by a factor of 3 at an intermediate carrier concentration in the quantum well where excitons and trions coexist in the system. In addition, we explore the capability to tune the g factor of the electron gas via the carrier density. © 2016 American Physical Society.

We present a comprehensive study of coherently controlled charge currents in electrically contacted GaAs microdevices. Currents are generated all-optically by phase-related femtosecond (Formula presented.) pulse pairs and are often linked to the third-order optical nonlinearity (Formula presented.). Here, we first focus on elevated irradiances where absorption saturation and ultimately the onset of Rabi oscillations contribute to the optical response. In particular, we identify clear departures of the injected current from the (Formula presented.) -expectation (Formula presented.). Theoretical simulations for the coherently controlled current based on the semiconductor Bloch equations agree well with the experimental trends. We then move on to investigate spectroscopic applications of the quantum interference control technique. In particular, we implement a versatile scheme to analyze the phase structure of femtosecond pulses. It relies on phase-sensitive (Formula presented.) -current injection driven by two time-delayed portions of the (Formula presented.) / (Formula presented.) pulse pair. Most strikingly, the group velocity dispersions of both the (Formula presented.) and (Formula presented.) components can be unambiguously determined from a simple Fourier transform of the resulting current interferogram. Finally, we aim to use femtosecond (Formula presented.) pulse pairs to demonstrate a theoretically proposed scheme for all-optical current detection in thin GaAs membranes. However, we find the signal to be superimposed by second harmonic generation related to the electric field inducing the current. As a result, the currents’ signature cannot be unambiguously identified. © 2016, Springer-Verlag Berlin Heidelberg.

We propose and implement a new concept for thermochromic plasmonic elements. It is based on vanadium dioxide (VO2) nanocrystals located in the near field of surface plasmon polaritons supported by an otherwise unstructured gold thin film. When the VO2 undergoes the metal-insulator phase transition, the coupling conditions for conversion of light into propagating surface plasmon polaritons change markedly. In particular, we realize thermochromic plasmonic grating couplers with substantial switching contrast as well as tunable plasmonic couplers in a Kretschmann configuration. The use of VO2 nanocrystals permits highly repetitive switching and room temperature operation. Simulations based on the actual dielectric function of our VO2 nanocrystals agree well with the experiment. © 2016 Optical Society of America.

We investigate the transient optical response in high-quality Cd0.88Zn0.12Te crystals in the regime of slow light propagation on the lower exciton-polariton branch. Femtosecond photoexcitation leads to very substantial transmission changes in a ∼10-meV broad spectral range within the transparency window of the unexcited semiconductor. These nonlinear optical signatures decay on picosecond time scales governed by carrier thermalization and recombination. The temporal and spectral dependence indicate the dynamical optical response as arising from excitation-induced dephasing and perturbed free induction decay. Model simulations for the optical response taking into account the actual exciton-polariton dispersion and excitation-induced dephasing of a nonlinearly driven two-level system support this interpretation. © 2016 American Physical Society.

The impact of travelling surface acoustic waves on plasmonic devices is analyzed with a novel stroboscopic technique. Most strikingly, our electro-mechanical approach deliberately modulates the efficiency of a surface plasmon polariton launcher on sub-nanosecond timescales. © 2015 IEEE.

We investigate the linear and dynamical nonlinear optical properties of a superlattice composed of ultra-narrow n-doped GaN/AlN quantum wells. Owing to huge band offsets, the structures feature a broad inter-miniband transition in the telecom window at 1.55 μm. Resonant pump-probe experiments directly reveal ultrafast intersubband relaxation occurring within < 100 fs. We estimate an associated third order nonlinear optical susceptibility of Im (χ (3)) ∼ 1.1 × 10 - 20 m2/V2. The polarization and angular dependences of the optical response confirm the nonlinearity as originating from inter-miniband transitions in the heterostructure. © 2015 AIP Publishing LLC.

BACKGROUND: Bilberries (Vaccinium myrtillus L.) have been suggested to have preventive properties against diseases associated with oxidative stress such as colon cancer or inflammatory bowel diseases. Therefore the gastrointestinal tract is regarded as a potential target for prevention. In this study the antioxidative properties of a commercially available anthocyanin-rich bilberry extract (BE) were investigated in comparison with four different BE-loaded microcapsule systems. As markers to describe the antioxidant status in this cellular system, intracellular reactive oxygen species (ROS) levels, oxidative DNA damage and total glutathione (tGSH) levels were monitored. RESULTS: Incubations with the BE-loaded capsule systems showed an increase in cellular glutathione levels and reduction of ROS levels at high BE concentrations (100-500 μg mL-1) and a positive effect on the formation of DNA strand breaks (5-10 μg mL-1 BE). The biological properties of BE-loaded pectin amide core-shell capsules, whey protein matrix capsules and coated apple pectin matrix capsules were comparable to those of the non-encapsulated BE. CONCLUSION: Overall, the BE and the encapsulated BE types tested have antioxidative activity under the studied assay conditions in terms of the prevention of oxidative DNA damage, the reduction of intracellular ROS and the enhancement of cellular tGSH. © 2014 Society of Chemical Industry.

Recent experiments have revealed the possibility to optically orient both electron and hole spins in bulk germanium. Here we discuss the wavelength dependence of this spin injection process using time-resolved Faraday rotation. Significant hole spin polarization is found only when addressing indirect optical transitions. In contrast, electron spins can be oriented via both direct and indirect optical transitions and even with excess energies much larger than the spin-orbit coupling energy. For photon energies very close to the indirect bandgap, we find indications that the degree of electron spin polarization is significantly enhanced - a trend in line with theoretical predictions. © 2014 SPIE.

We investigate the optical and thermal hysteresis of single-domain vanadium dioxide nanocrystals fabricated by ion beam synthesis in a fused silica matrix. The nanocrystals exhibit a giant hysteresis, which permits to optically generate a long-time stable supercooled metallic phase persistent down to practically room temperature. Spatial patterns of supercooled and insulating nanocrystals feature a large dielectric contrast, in particular, for telecom wavelengths. We utilize this contrast to optically imprint reconfigurable photonic elements comprising diffraction gratings as well as on- and off-axis zone plates. The structures allow for highly repetitive (>104) cycling through the phase transition without structural damage. © 2014 AIP Publishing LLC.

We propose and implement a versatile scheme to analyze the phase structure of femtosecond pulses. It relies on second harmonic generation in combination with phase-sensitive X(3)-current injection driven by two time-delayed portions of the emerging ω/2ω pulse pair. Most strikingly, the group velocity dispersions of both the ω and 2ω components can be unambiguously determined from a simple Fourier transformation of the resulting current interferogram. We test the concept for 45 fs pulses at 1.45 μmand directly compare it to second harmonic frequency resolved optical gating. By choosing appropriate frequency doublers and semiconductor detectors, the scheme is applicable to many wavelength regimes and to pulses as short as a few optical cycles. © 2014 Optical Society of America.

We explore the impact of ∼500 MHz surface acoustic waves traveling across a commensurable plasmonic grating coupler. A stroboscopic technique involving surface acoustic waves synchronized to a modelocked optical source allows to time-resolve the dynamical impact of the electromechanically induced perturbation. The surface acoustic wave periodically enhances or decreases the surface ripple of the static grating. Most remarkably, the dynamic surface deformation deliberately modulates the coupler's efficiency by ±2% during the ∼2 ns acoustic cycle. (Figure presented) © 2014 American Chemical Society.

Recent experiments have revealed the possibility of optically orienting electron spins in bulk germanium via indirect optical transitions. However, the temporal limitations to both the spin lifetime and the coherence of photogenerated electrons have remained unexplored so far. Here we demonstrate resonant spin amplification in intrinsic bulk germanium using a 90 MHz femtosecond pulse train at 0.8 eV central photon energy. Most importantly, we find remarkably long spin lifetimes exceeding 50 ns at temperatures of up to 60 K, limited by Elliott-Yafet type processes. Consistent with model simulations we also find pronounced signatures of the g-factor anisotropy in germanium in the resonant spin amplification data. © 2014 American Physical Society.

In cell culture were compared the different release rates of anthocyanins from a bilberry pomace extract encapsulated either in food grade whey protein-based matrix capsules (WPC) or in pectin amid-based hollow spherical capsules (PHS). The impact of the formulations on typical anthocyanin-associated biological end points such as inhibition of the epidermal growth factor receptor (EGFR) and suppression of cell growth in HT29 colon carcinoma cells was assessed. The purpose was to find whether the release rates are sufficient to maintain biological activity and whether encapsulation affected EGFR inhibitory and growth suppressive properties of the extract. Even though anthocyanin release from extract-loaded capsules was proven under cell culture conditions, the inhibitory potential toward the EGFR was diminished. However, nonencapsulated extract as well as both extract-loaded encapsulation systems diminished the growth of HT29 cells to a comparable extent. The loss of EGFR inhibitory properties by encapsulation despite anthocyanin release indicates substantial contribution of other further constituents not monitored so far. Taken together, both applied encapsulation strategies allowed anthocyanin release and maintained biological activity with respect to growth inhibitory properties. However, the loss of EGFR inhibitory effects emphasizes the need for biological profiling to estimate process-induced changes of plant constituent's beneficial potencies. © 2013 American Chemical Society.

We present a comprehensive experimental and theoretical study of the power dependence of coherently controlled currents in bulk GaAs. Currents are optically induced by phase-stable femtosecond ω/2ω pulse pairs. For moderate irradiances, these currents are linked to the third-order optical nonlinearity χ(3)(0;ω,ω,-2ω). Here we focus on elevated irradiances where absorption saturation and ultimately the onset of Rabi oscillations contribute to the optical response. Current diagnostics is achieved electrically by recording the photoresponse of contacted specimens of low temperature grown GaAs as a function of the relative phase of the ω and 2ω pulses. For stronger ω irradiance we find the magnitude of the coherently controlled current to be markedly reduced when compared to the χ(3) expectation dJ/dta Eω2E2ω. Additional pump-probe type experiments corroborate that this current saturation is indeed predominantly linked to macroscopic band filling. Theoretical simulations for the coherently controlled current based on a 14 band k·p model agree well with the experimental trends. © 2013 American Physical Society.

Ultrafast pump-probe spectroscopy is employed to directly monitor the tunneling of charge carriers from single and vertically coupled quantum dots and probe intra-molecular dynamics. Immediately after resonant optical excitation, several peaks are observed in the pump-probe spectrum arising from Coulomb interactions between the photogenerated charge carriers. The influence of few-Fermion interactions in the photoexcited system and the temporal evolution of the optical response is directly probed in the time domain. In addition, the tunneling times for electrons and holes from the QD nanostructure are independently determined. In polarization resolved measurements, near perfect Pauli-spin blockade is observed in the spin-selective absorption spectrum as well as stimulated emission. While electron and hole tunneling from single quantum dots is shown to be well explained by the WKB formalism, for coupled quantum dots pronounced resonances in the electron tunneling rate are observed arising from elastic and inelastic electron tunneling between the different dots. © 2012 by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

We employ ultrafast pump-probe spectroscopy to directly monitor electron tunneling between discrete orbital states in a pair of spatially separated quantum dots. Immediately after excitation, several peaks are observed in the pump-probe spectrum due to Coulomb interactions between the photogenerated charge carriers. By tuning the relative energy of the orbital states in the two dots and monitoring the temporal evolution of the pump-probe spectra the electron and hole tunneling times are separately measured and resonant tunneling between the two dots is shown to be mediated both by elastic and inelastic processes. Ultrafast (<5ps) interdot tunneling is shown to occur over a surprisingly wide bandwidth, up to ∼8meV, reflecting the spectrum of exciton-acoustic phonon coupling in the system. © 2012 American Physical Society.

We employ ultrafast pump-probe spectroscopy with photocurrent readout to directly probe the dynamics of a single hole spin in a single, electrically tunable self-assembled quantum dot molecule formed by vertically stacking (In,Ga)As quantum dots. Excitons with defined spin configurations are initialized in one of the two dots using circularly polarized picosecond pulses. The time-dependent spin configuration is probed by the spin selective optical absorption of the resulting few fermion complex. Taking advantage of sub-5 ps electron tunneling to an orbitally excited state of the other dot, we initialize a single hole spin with a purity of >96%, i.e., much higher than demonstrated in previous single dot experiments. Measurements in a lateral magnetic field monitor the coherent Larmor precession of the single hole spin with no observable loss of spin coherence within the ∼300 ps hole lifetime. Thereby, the purity of the hole spin initialization remains unchanged for all investigated magnetic fields. © 2012 American Physical Society.

We present a comprehensive study of electron spin decoherence in bulk germanium. As a first cornerstone, the effective g tensor of L-valley electrons in germanium is deduced from time-domain magneto-optical experiments. The decay of the ensemble spin polarization is then found to be closely related to intervalley scattering. In particular, the angular and temperature dependences of the experiment are well described in a model including Elliott-Yafet-type spin-flip processes as well as dephasing due to scattering between valleys exhibiting different effective g factors. © 2012 American Physical Society.

Bilberries (Vaccinium myrtillus L.) and their major polyphenolic constituents, anthocyanins, have preventive activities inter alia against colon cancer and inflammatory bowel diseases. However, anthocyanins are sensitive to environmental conditions; thus their bioavailability in the gastrointestinal tract is an important determinant of their in vivo activity. In the study reported here, the potential benefits of encapsulating an anthocyanin rich bilberry extract (BE) on anthocyanin stability were investigated. Nonencapsulated BE and three different BE loaded microcapsule systems were incubated in simulated gastric fluid (SGF) and fed state simulated intestinal fluid (FeSSIF). After exposure to these media, released anthocyanins were identified and quantified by HPLC with UV/Vis detection. Although a rapid release of anthocyanins was observed within the first 20 min, encapsulation of anthocyanins doubled the amount of available anthocyanins after 150 min of incubation. These results illustrate the ability of encapsulation to inhibit early degradation of anthocyanins in the intestinal system. © 2011 American Chemical Society.

We apply ultrafast pump-probe photocurrent spectroscopy to directly probe few Fermion charge and spin dynamics in an artificial molecule formed by vertically stacking a pair of InGaAs self-assembled quantum dots. As the relative energy of the orbital states in the two dots are energetically tuned by applying static electric fields, pronounced anticrossings are observed arising from electron tunnel couplings. Time resolved photocurrent measurements performed in the vicinity of these anticrossings provide direct information on the comparative roles of elastic and inelastic resonant tunneling processes between the two quantum dots forming the molecule. Resonant pumping of the neutral exciton in the upper dot with circularly polarized light facilitates ultrafast initialization of hole spin qubits over timescales limited only by the laser pulse duration (<5ps) and a near perfect Pauli spin-blockade with a near unity suppression of absorption (>96%) for spin forbidden transitions. Such a spin selective photocurrent response opens the way to probe spin dynamics in the system over ultrafast timescales. © 2012 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE).

Femtosecond ω / 2 ω pulse pairs derived from a compact Er:fibre source induce coherently controlled currents in low temperature grown GaAs. They are characterized by analyzing charge accumulation at contacts closeby. We focus on the photoresponse of bowtie optical antennas integrated into such metal-semiconductor-metal structures. Antennas are designed to enhance the ω field and to confine it into the 50 nm antenna gap. The coherently controlled current is markedly enhanced by the plasmonic nanostructure. However, we find an only unpronounced dependence on the antenna length which is probably related to the large refractive index of GaAs and intricate resonance conditions for ultrabroadband excitation light. © 2012 American Institute of Physics.

We implement a versatile concept to time-resolve optical nonlinearities of semiconductors in amplitude and phase. A probe pulse transmitted through the optically pumped sample is superimposed with first subharmonic spectral components derived from the same laser source. This effective ω/2ω pulse pair induces a coherently controlled current in a time-integrating semiconductor detector. Current interferograms obtained by scanning the ω/2ω time delay then reveal the electric field of the 2ω part as well as its pump-induced modifications. As a paradigm we analyze the excitonic optical nonlinearity of a CdTe thin film at frequencies around 385 THz. We then move on to resolve the pump-induced amplitude-and phase-distortions of a probe pulse related to two-photon absorption and cross-phase modulation in ZnSe. © 2012 Optical Society of America.

Quantum interference control of electrical currents is well established in bulk semiconductors. It arises from the interference of one- and two-photon absorption pathways. Here, the concept is transferred to one-dimensional semiconductor nanostructures. First, currents are optically injected into aligned single-walled carbon nanotube ensembles by phase-related 700 and 1400 nm, 150 fs pulses. These transient currents are detected via the emitted THz radiation. In a second set of experiments, a phase-stable superposition of ∼100 fs pulses from a compact erbium-doped fiber source and their second harmonic is shown to induce ultrashort ∼μA current bursts in single unbiased GaAs nanowires. The current flow is characterized by charge accumulation and the related potential difference between the contacted ends of the ∼10 μm long wires. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

We optically probe the spectrum of ground and excited state transitions of an individual, electrically tunable self-assembled quantum dot molecule. Photocurrent absorption measurements show that the spatially direct neutral exciton transitions in the upper and lower dots are energetically separated by only ∼2 meV. Excited state transitions ∼8-16 meV to higher energy exhibit pronounced anticrossings as the electric field is tuned due to the formation of hybridized electron states. We show that the observed excited state transitions occur between these hybridized electronic states and different hole states in the upper dot. By simultaneously pumping two different excited states with independent laser fields we demonstrate a strong (88% on-off contrast) laser-induced switching of the optical response. The results represent an electrically tunable, discrete coupled quantum system with a conditional optical response. © 2011 American Physical Society.

We propose and demonstrate an ultrabroadband concept to characterize amplitude and phase changes of femtosecond pulses. The radiation is superimposed with the first subharmonic spectral components from the same laser source. This effective ω/2ω pulse pair induces a coherently controlled charge current in a time-integrating semiconductor detector. An interferometric variation of the time delay between the harmonically related components then reveals the electric field of the 2ω part. This method is realized with the second harmonic of a compact Er:fiber source centered at 390 THz and a GaAs-based detector. Most strikingly, it is sensitive to ∼π/20 phase changes and can be utilized to analyze femtojoule pulses. © 2011 Optical Society of America.

We review a comprehensive study of the ultrafast optoelectronic properties of a single self-assembled InGaAs/GaAs quantum dot. While manipulation of the artificial atom relies on two widely and independently tunable picoseconds pulse trains, sensitive readout is achieved via the ~pA photocurrent of the diode device. In particular, the absorption changes after occupation of an s-shell exciton reveal a biexciton absorption line as well as previously unobserved p-shell transitions in the presence of s-shell population. In addition, time-resolved data directly maps the picosecond tunneling times of electrons and holes out of the dot. Beyond these incoherent phenomena, we also realize coherent QD manipulations. Those comprise well-known excitonic Rabi-oscillations as well as single-pulse biexciton generation and conditional Rabi-oscillations of the exciton-biexciton transition after deterministic exciton preparation. © 2011 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE).

Circularly polarized light is demonstrated to inject partially spin-polarized electrons and holes in bulk germanium via both direct and indirect optical transitions. While the degree of spin polarization is markedly reduced when compared to prototypical III-V semiconductors, coherent spin precessions in an external magnetic field are well resolved in ultrafast magneto-optics. At cryogenic temperatures, hole (electron) spins exhibit remarkably long coherence times of ∼ 100 ps (∼ 1 ns). © 2011 American Physical Society.

A phase-stable superposition of femtosecond pulses from a compact erbium-doped fiber source and their second harmonic is shown to induce ultrashort ∼μA current bursts in single unbiased GaAs nanowires. Current injection relies on a quantum interference of one- and two-photon absorption pathways. The vector direction of the current is solely dictated by the polarization and relative phase of the harmonically related light components while its power dependence is consistent with a third order optical nonlinearity. © 2010 American Chemical Society.

We propose and demonstrate a surface acoustic wave-driven converter of light into surface plasmon polaritons. An otherwise unstructured thin metal film is deformed by traveling acoustic waves on a piezoelectric substrate underneath. This spatially periodic corrugation enables to bridge the momentum gap between free-space radiation and surface-bound modes. This principle is realized with plain gold films on a LiNbO3 wafer where surface acoustic waves induce surface ripples in the metal. For near-infrared light we observe efficiencies of order 10-4 for exciting surface plasmon polaritons. © 2010 The American Physical Society.

We propose and demonstrate an ultrafast all-optical method to couple light to surface plasmon polaritons on planar gold films. By interfering two 150 fs, 810 nm pulses we excite a transient grating in the temperature of the free electrons of the metal, resulting in a grating in the dielectric function, and leading to a 1 ps launch window for plasmonic excitation. We use pump-probe experiments to identify these ultrashort plasmonic excitations between 520 and 570 nm. © 2010 The American Physical Society.

We report a comprehensive study of the ultrafast optoelectronic response of a single self-assembled InGaAs/GaAs quantum dot embedded in a n-i -Schottky photodiode device. While manipulation of the artificial atom relies on two independently tunable picosecond pulse trains, sensitive all electrical readout is achieved via photocurrent. We apply our methods to probe the temporal evolution of the quantum dot absorption spectrum following coherent optical generation of a s -shell exciton. Our measurements reveal the biexciton absorption as well as previously unobserved p -shell transitions that appear in the presence of s -shell population. Furthermore, time-resolved measurements allow us to directly monitor the picosecond tunneling times of electrons and holes out of the dot. Beyond these incoherent phenomena, we also demonstrate the potential of our techniques for coherent quantum dot manipulations. Beginning with excitonic Rabi oscillations driven by a single pulse tuned to the neutral exciton we move on to demonstrate the coherent generation of the biexciton via a two-photon nonlinearity, before realizing conditional Rabi oscillations in the exciton-biexciton state manifold. The results provide significant potential for realizing and utilizing conditional coherent dynamics in quantum dot nanostructures for all optical, ultrafast quantum information processing. © 2010 The American Physical Society.

We experimentally demonstrate a general optical pump-probe technique to observe the spin Hall effect of light (SHEL) in an absorbing medium. In essence, a locally confined pump-induced modification of a material's absorptivity can effectively be used as an induced aperture allowing one to detect the transversely displaced circular polarization components of an incident beam through differential transmission techniques. We consider linear absorption mechanisms such as free-carrier absorption and Pauli blocking as well as nonlinear absorption processes such as two-photon absorption. For absorption mechanisms that do not depend on light polarization, the SHEL of the probe beam is obtained directly, while polarization-dependent properties give an effective SHEL displacement that depends on the action of the SHEL on both pump and probe beams. Using 150 fs pump, 820 nm pump and probe pulses we observe SHEL effects in silicon via free-carrier absorption. SHEL effects are also observed via Pauli blocking at 820 nm and two-photon absorption at 1550 nm in GaAs using ∼150 fs pump and probe pulses. © 2010 The American Physical Society.