Jun.-Prof. Dr. Marc-Alexander Aßmann

Experimental Physics
TU Dortmund University

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  • Analysis of the Fine Structure of the D-Exciton Shell in Cuprous Oxide
    Heckötter, J. and Rommel, P. and Main, J. and Aßmann, M. and Bayer, M.
    Physica Status Solidi - Rapid Research Letters 15 (2021)
    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
    view abstract10.1002/pssr.202100335
  • Asymmetric Rydberg blockade of giant excitons in Cuprous Oxide
    Heckötter, J. and Walther, V. and Scheel, S. and Bayer, M. and Pohl, T. and Aßmann, M.
    Nature Communications 12 (2021)
    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).
    view abstract10.1038/s41467-021-23852-z
  • Coherent transfer matrix analysis of the transmission spectra of Rydberg excitons in cuprous oxide
    Stolz, H. and Schwartz, R. and Heckötter, J. and Aßmann, M. and Semkat, D. and Krüger, S.O. and Bayer, M.
    Physical Review B 104 (2021)
    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
    view abstract10.1103/PhysRevB.104.035206
  • Optical control of a dark exciton reservoir
    Kurdyubov, A.S. and Trifonov, A.V. and Gerlovin, I.Y. and Gribakin, B.F. and Grigoryev, P.S. and Mikhailov, A.V. and Ignatiev, I.V. and Efimov, Y.P. and Eliseev, S.A. and Lovtcius, V.A. and Aßmann, M. and Bayer, M. and Kavokin, A.V.
    Physical Review B 104 (2021)
    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.
    view abstract10.1103/PhysRevB.104.035414
  • Conditional spectroscopy via nonstationary optical homodyne quantum state tomography
    Thewes, J. and Lüders, C. and Aßmann, M.
    Physical Review A 101 (2020)
    Continuous variable quantum state tomography is one of the most powerful techniques to study the properties of light fields in quantum optics. However, the need for a fixed phase reference has so far prevented widespread usage in other fields such as semiconductor spectroscopy. Here, we introduce nonstationary quantum state tomography, which adapts the technique to the special requirements of ultrafast spectroscopy. In detail, we gain access to the amplitude and phase of light fields with a temporal resolution of about 100 fs without the need for a fixed phase reference. Furthermore, we show how our technique allows us to perform conditional studies of stochastic dynamics that are inaccessible experimentally by conventional means and demonstrate the capabilities experimentally by monitoring the stochastic dynamics of a thermal light field on the sub-ps scale. Finally, we discuss differences and similarities to more standard Hanbury Brown-Twiss photon correlation experiments, which may be considered as the discrete variable analogs of our technique. © 2020 American Physical Society.
    view abstract10.1103/PhysRevA.101.023824
  • Distinguishing intrinsic photon correlations from external noise with frequency-resolved homodyne detection
    Lüders, C. and Aßmann, M.
    Scientific Reports 10 (2020)
    In this work, we apply homodyne detection to investigate the frequency-resolved photon statistics of a cw light field emitted by a driven-dissipative semiconductor system in real time. We demonstrate that studying the frequency dependence of the photon number noise allows us to distinguish intrinsic noise properties of the emitter from external noise sources such as mechanical noise while maintaining a sub-picosecond temporal resolution. We further show that performing postselection on the recorded data opens up the possibility to study rare events in the dynamics of the emitter. By doing so, we demonstrate that in rare instances, additional external noise may actually result in reduced photon number noise in the emission. © 2020, The Author(s).
    view abstract10.1038/s41598-020-79686-0
  • Exciton energy oscillations induced by quantum beats
    Trifonov, A.V. and Kurdyubov, A.S. and Gerlovin, I.Ya. and Smirnov, D.S. and Kavokin, K.V. and Yugova, I.A. and Aßmann, M. and Kavokin, A.V.
    Physical Review B 102 (2020)
    In this paper, we experimentally demonstrate an oscillating energy shift of center-of-mass quantum-confined exciton levels in a semiconductor quantum well after excitation into a superposition of two quantum confined exciton states of different parity. Oscillations are observed at frequencies corresponding to the quantum beats between these states. We show that the observed effect is a manifestation of the exciton density oscillations in the real space similar to the dynamics of an electron in a Hertzian dipole. The effect is caused by the exciton-exciton exchange interaction and appears only if the excitons are in a superposition quantum state. This effect exists until the Raman coherence in the interacting excitons has decayed. This effect may be harnessed for quantum technologies requiring the quantum coherence of states. © 2020 American Physical Society.
    view abstract10.1103/PhysRevB.102.205303
  • Experimental limitation in extending the exciton series in Cu2 O towards higher principal quantum numbers
    Heckötter, J. and Janas, D. and Schwartz, R. and Aßmann, M. and Bayer, M.
    Physical Review B 101 (2020)
    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.
    view abstract10.1103/PhysRevB.101.235207
  • Formation dynamics of exciton-polariton vortices created by nonresonant annular pumping
    Berger, B. and Schmidt, D. and Ma, X. and Schumacher, S. and Schneider, C. and Höfling, S. and Aßmann, M.
    Physical Review B 101 (2020)
    We study the spontaneous formation of exciton-polariton vortices in an all-optical nonresonantly excited annular trap, which is formed by ring-shaped subpicosecond laser pulses. Since the light emitted by these vortices carries orbital angular momentum (OAM) corresponding to their topological charge, we apply a dedicated OAM spectroscopy technique to detect the OAM of the measurement signal with picosecond time resolution. This allows us to identify the formation of OAM modes and investigate the dynamics of the vortex formation process. We also study the power dependence of this process and how the ring diameter influences the formation of OAM modes. © 2020 American Physical Society.
    view abstract10.1103/PhysRevB.101.245309
  • Quantum optics with quantum dot ensembles
    Aßmann, M. and Bayer, M.
    Semiconductors and Semimetals 105 (2020)
    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.
    view abstract10.1016/bs.semsem.2020.10.003
  • Realization of all-optical vortex switching in exciton-polariton condensates
    Ma, X. and Berger, B. and Aßmann, M. and Driben, R. and Meier, T. and Schneider, C. and Höfling, S. and Schumacher, S.
    Nature Communications 11 (2020)
    Vortices are topological objects representing the circular motion of a fluid. With their additional degree of freedom, the vorticity, they have been widely investigated in many physical systems and different materials for fundamental interest and for applications in data storage and information processing. Vortices have also been observed in non-equilibrium exciton-polariton condensates in planar semiconductor microcavities. There they appear spontaneously or can be created and pinned in space using ring-shaped optical excitation profiles. However, using the vortex state for information processing not only requires creation of a vortex but also efficient control over the vortex after its creation. Here we demonstrate a simple approach to control and switch a localized polariton vortex between opposite states. In our scheme, both the optical control of vorticity and its detection through the orbital angular momentum of the emitted light are implemented in a robust and practical manner. © 2020, The Author(s).
    view abstract10.1038/s41467-020-14702-5
  • Eavesdropping attack on a trusted continuous-variable quantum random-number generator
    Thewes, J. and Lüders, C. and Aßmann, M.
    Physical Review A 100 (2019)
    Harnessing quantum processes is an efficient method to generate truly indeterministic random numbers, which are of fundamental importance for cryptographic protocols, security applications, or Monte Carlo simulations. Recently, quantum random-number generators based on continuous variables have gathered a lot of attention due to the potentially high bit rates they can deliver. Especially quadrature measurements on shot-noise-limited states have been studied in detail as they do not offer any side information to potential adversaries under ideal experimental conditions. However, they may be subject to additional classical noise beyond the quantum limit, which may become a source of side information for eavesdroppers. While such eavesdropping attacks have been investigated in theory in some detail, experimental studies are still rare. We experimentally realize a continuous-variable eavesdropping attack, based on heterodyne detection, on a trusted quantum random-number generator and discuss the limitations for secure random-number generation that arise. © 2019 American Physical Society.
    view abstract10.1103/PhysRevA.100.052318
  • Enhanced light-matter interaction in an atomically thin semiconductor coupled with dielectric nano-antennas
    Sortino, L. and Zotev, P.G. and Mignuzzi, S. and Cambiasso, J. and Schmidt, D. and Genco, A. and Aßmann, M. and Bayer, M. and Maier, S.A. and Sapienza, R. and Tartakovskii, A.I.
    Nature Communications 10 (2019)
    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).
    view abstract10.1038/s41467-019-12963-3
  • Tracking Dark Excitons with Exciton Polaritons in Semiconductor Microcavities
    Schmidt, D. and Berger, B. and Kahlert, M. and Bayer, M. and Schneider, C. and Höfling, S. and Sedov, E.S. and Kavokin, A.V. and Aßmann, M.
    Physical Review Letters 122 (2019)
    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.
    view abstract10.1103/PhysRevLett.122.047403
  • Critical Dependence of the Excitonic Absorption in Cuprous Oxide on Experimental Parameters
    Heckötter, J. and Freitag, M. and Aßmann, M. and Fröhlich, D. and Bayer, M. and Grünwald, P. and Scheel, S.
    Physics of the Solid State 60 (2018)
    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.
    view abstract10.1134/S1063783418080097
  • Dephasing of InAs quantum dot p -shell excitons studied using two-dimensional coherent spectroscopy
    Suzuki, T. and Singh, R. and Moody, G. and Aßmann, M. and Bayer, M. and Ludwig, Ar. and Wieck, A.D. and Cundiff, S.T.
    Physical Review B 98 (2018)
    The dephasing mechanisms of p-shell and s-shell excitons in an InAs self-assembled quantum dot ensemble are examined using two-dimensional coherent spectroscopy (2DCS). 2DCS provides a comprehensive picture of how the energy level structure of dots affects the exciton dephasing rates and recombination lifetimes. We find that at low temperatures, dephasing of s-shell excitons is lifetime limited, whereas p-shell excitons exhibit significant pure dephasing due to scattering between degenerate spin states. At elevated temperatures, quadratic exciton-phonon coupling plays an important role in both s-shell and p-shell exciton dephasing. We show that multiple p-shell states are also responsible for stronger phonon dephasing for these transitions. © 2018 American Physical Society.
    view abstract10.1103/PhysRevB.98.195304
  • Dissociation of excitons in Cu2 O by an electric field
    Heckötter, J. and Freitag, M. and Fröhlich, D. and Aßmann, M. and Bayer, M. and Semina, M.A. and Glazov, M.M.
    Physical Review B 98 (2018)
    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.
    view abstract10.1103/PhysRevB.98.035150
  • Influence of Magnetic Confinement on the Yellow Excitons in Cuprous Oxide Subject to an Electric Field
    Heckötter, J. and Fröhlich, D. and Aßmann, M. and Bayer, M.
    Physics of the Solid State 60 (2018)
    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.
    view abstract10.1134/S1063783418080085
  • Influence of the Wavefunction Distribution on Exciton Dissociation in Electric Field
    Heckötter, J. and Freitag, M. and Fröhlich, D. and Aßmann, M. and Bayer, M. and Semina, M.A. and Glazov, M.M.
    Physics of the Solid State 60 (2018)
    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.
    view abstract10.1134/S1063783418080115
  • Landau-Level Quantization of the Yellow Excitons in Cuprous Oxide
    Heckötter, J. and Thewes, J. and Fröhlich, D. and Aßmann, M. and Bayer, M.
    Physics of the Solid State 60 (2018)
    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.
    view abstract10.1134/S1063783418080103
  • Magneto-Stark effect of yellow excitons in cuprous oxide
    Rommel, P. and Schweiner, F. and Main, J. and Heckötter, J. and Freitag, M. and Fröhlich, D. and Lehninger, K. and Aßmann, M. and Bayer, M.
    Physical Review B 98 (2018)
    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.
    view abstract10.1103/PhysRevB.98.085206
  • Oscillations of the Degree of Circular Polarization in the Optical Spin Hall Effect
    Schmidt, D. and Berger, B. and Bayer, M. and Schneider, C. and Höfling, S. and Sedov, E. and Kavokin, A. and Aßmann, M.
    Physics of the Solid State 60 (2018)
    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.
    view abstract10.1134/S1063783418080206
  • Quantum-Optically Enhanced STORM (QUEST) for Multi-Emitter Localization
    Aßmann, M.
    Scientific Reports 8 (2018)
    Super-resolution imaging has introduced new capabilities to investigate processes at the nanometer scale by optical means. However, most super-resolution techniques require either sparse excitation of few emitters or analysis of high-order cumulants in order to identify several emitters in close vicinity. Here, we present an approach that draws upon methods from quantum optics to perform localization super-resolution imaging of densely packed emitters and determine their number automatically: Quantum-optically enhanced STORM (QUEST). By exploiting normalized photon correlations, we predict a localization precision below 30 nm or better even for closely spaced emitter up to a density of 125 emitters per μm at photon emission rates of 105 photons per second and emitter. Our technique does not require complex experimental arrangements and relies solely on spatially resolved time streams of photons and subsequent data analysis. © 2018 The Author(s).
    view abstract10.1038/s41598-018-26271-1
  • Real time g(2) monitoring with 100 kHz sampling rate
    Lüders, C. and Thewes, J. and Assmann, M.
    Optics Express 26 (2018)
    We introduce a technique to determine photon correlations of optical light fields in real time. The method is based on ultrafast phase-randomized homodyne detection and allows us to follow the temporal evolution of the second-order correlation function g(2)(0) of a light field. We demonstrate the capabilities of our approach by applying it to a laser diode operated in the threshold region. In particular, we are able to monitor the emission dynamics of the diode switching back and forth between lasing and spontaneous emission with a g(2)(0)-sampling rate of 100 kHz. © 2018 Optical Society of America.
    view abstract10.1364/OE.26.024854
  • Rydberg Excitons in the Presence of an Ultralow-Density Electron-Hole Plasma
    Heckötter, J. and Freitag, M. and Fröhlich, D. and Aßmann, M. and Bayer, M. and Grünwald, P. and Schöne, F. and Semkat, D. and Stolz, H. and Scheel, S.
    Physical Review Letters 121 (2018)
    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.
    view abstract10.1103/PhysRevLett.121.097401
  • Spectroscopy of fractional orbital angular momentum states
    Berger, B. and Kahlert, M. and Schmidt, D. and Assmann, M.
    Optics Express 26 (2018)
    We present an approach for measuring the orbital angular momentum (OAM) of light tailored towards applications in spectroscopy and non-integer OAM values. It is based on the OAM sorting method (Berkhout et al., Phys. Rev. Lett. 105, 153601 (2010)). We demonstrate that mixed OAM states and fractional OAM states can be identified using moments of the sorted output intensity distribution and OAM states with integer and non-integer topological charge can be clearly distinguished. Furthermore the difference between intrinsic OAM and total OAM for fractional OAM states is highlighted and the importance of the orientation of the fractional OAM beam is shown. All experimental results show good agreement with simulations. Finally we discuss possible applications of this method for spectroscopy of semiconductor systems such as exciton-polaritons in microcavities. © 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement.
    view abstract10.1364/OE.26.032248
  • Streak camera imaging of single photons at telecom wavelength
    Allgaier, M. and Ansari, V. and Eigner, C. and Quiring, V. and Ricken, R. and Donohue, J.M. and Czerniuk, T. and Aßmann, M. and Bayer, M. and Brecht, B. and Silberhorn, C.
    Applied Physics Letters 112 (2018)
    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).
    view abstract10.1063/1.5004110
  • Dynamics of the optical spin Hall effect
    Schmidt, D. and Berger, B. and Bayer, M. and Schneider, C. and Kamp, M. and Höfling, S. and Sedov, E. and Kavokin, A. and Aßmann, M.
    Physical Review B 96 (2017)
    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.
    view abstract10.1103/PhysRevB.96.075309
  • Giant photon bunching and quantum correlations in superradiant quantum-dot microcavity lasers
    Wiersig, J. and Foerster, A. and Leymann, H.A.M. and Jahnke, F. and Gies, C. and Aßmann, M. and Bayer, M. and Schneider, C. and Kamp, M. and Höfling, S.
    Optics InfoBase Conference Papers Part F42-CLEO_QELS 2017 (2017)
    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.
    view abstract10.1364/CLEO_QELS.2017.FTu4E.8
  • High-resolution study of the yellow excitons in Cu2 O subject to an electric field
    Heckötter, J. and Freitag, M. and Fröhlich, D. and Aßmann, M. and Bayer, M. and Semina, M.A. and Glazov, M.M.
    Physical Review B 95 (2017)
    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.
    view abstract10.1103/PhysRevB.95.035210
  • High-resolution study of the yellow excitons in Cu2O subject to an electric field
    Heckötter, J. and Freitag, M. and Fröhlich, D. and Aßmann, M. and Bayer, M. and Semina, M.A. and Glazov, M.M.
    Physical Review B 95 (2017)
    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.
    view abstract10.1103/PhysRevB.95.035210
  • Magnetoexcitons in cuprous oxide
    Schweiner, F. and Main, J. and Wunner, G. and Freitag, M. and Heckötter, J. and Uihlein, C. and Aßmann, M. and Fröhlich, D. and Bayer, M.
    Physical Review B 95 (2017)
    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.
    view abstract10.1103/PhysRevB.95.035202
  • Role of phonons in the quantum chaos of Rydberg excitons
    Freitag, M. and Heckötter, J. and Bayer, M. and Aßmann, M.
    Physical Review B 95 (2017)
    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.
    view abstract10.1103/PhysRevB.95.155204
  • Scaling laws of Rydberg excitons
    Heckötter, J. and Freitag, M. and Fröhlich, D. and Aßmann, M. and Bayer, M. and Semina, M.A. and Glazov, M.M.
    Physical Review B 96 (2017)
    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.
    view abstract10.1103/PhysRevB.96.125142
  • Coupled valence band dispersions and the quantum defect of excitons in Cu2O
    Schöne, F. and Krüger, S.-O. and Grünwald, P. and Aßmann, M. and Heckötter, J. and Thewes, J. and Stolz, H. and Fröhlich, D. and Bayer, M. and Scheel, S.
    Journal of Physics B: Atomic, Molecular and Optical Physics 49 (2016)
    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.
    view abstract10.1088/0953-4075/49/13/134003
  • Deviations of the exciton level spectrum in Cu2 O from the hydrogen series
    Schöne, F. and Krüger, S.-O. and Grünwald, P. and Stolz, H. and Scheel, S. and Aßmann, M. and Heckötter, J. and Thewes, J. and Fröhlich, D. and Bayer, M.
    Physical Review B 93 (2016)
    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.
    view abstract10.1103/PhysRevB.93.075203
  • Exciton and trion dynamics in atomically thin MoSe2 and WSe2: Effect of localization
    Godde, T. and Schmidt, D. and Schmutzler, J. and Aßmann, M. and Debus, J. and Withers, F. and Alexeev, E.M. and Del Pozo-Zamudio, O. and Skrypka, O.V. and Novoselov, K.S. and Bayer, M. and Tartakovskii, A.I.
    Physical Review B 94 (2016)
    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.
    view abstract10.1103/PhysRevB.94.165301
  • Experimental realization of a polariton beam amplifier
    Niemietz, D. and Schmutzler, J. and Lewandowski, P. and Winkler, K. and Aßmann, M. and Schumacher, S. and Brodbeck, S. and Kamp, M. and Schneider, C. and Höfling, S. and Bayer, M.
    Physical Review B 93 (2016)
    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.
    view abstract10.1103/PhysRevB.93.235301
  • Giant photon bunching, superradiant pulse emission and excitation trapping in quantum-dot nanolasers
    Jahnke, F. and Gies, C. and Aßmann, M. and Bayer, M. and Leymann, H.A.M. and Foerster, A. and Wiersig, J. and Schneider, C. and Kamp, M. and Höfling, S.
    Nature Communications 7 (2016)
    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.
    view abstract10.1038/ncomms11540
  • Magnetic field effects of Rydberg Excitons in Cu2O
    Thewes, J. and Heckötter, J. and Aßmann, M. and Fröhlich, D. and Grünwald, P. and Scheel, S. and Bayer, M.
    Proceedings of SPIE - The International Society for Optical Engineering 9749 (2016)
    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.
    view abstract10.1117/12.2218532
  • Quantum chaos and breaking of all anti-unitary symmetries in Rydberg excitons
    Aßmann, M. and Thewes, J. and Frohlich, D. and Bayer, M.
    Nature Materials 15 (2016)
    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.
    view abstract10.1038/NMAT4622
  • Signatures of Quantum Coherences in Rydberg Excitons
    Grünwald, P. and Aßmann, M. and Heckötter, J. and Fröhlich, D. and Bayer, M. and Stolz, H. and Scheel, S.
    Physical Review Letters 117 (2016)
    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.
    view abstract10.1103/PhysRevLett.117.133003
  • Spin noise of a polariton laser
    Ryzhov, I.I. and Glazov, M.M. and Kavokin, A.V. and Kozlov, G.G. and Aßmann, M. and Tsotsis, P. and Hatzopoulos, Z. and Savvidis, P.G. and Bayer, M. and Zapasskii, V.S.
    Physical Review B 93 (2016)
    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.
    view abstract10.1103/PhysRevB.93.241307
  • All-optical flow control of a polariton condensate using nonresonant excitation
    Schmutzler, J. and Lewandowski, P. and Aßmann, M. and Niemietz, D. and Schumacher, S. and Kamp, M. and Schneider, C. and Höfling, S. and Bayer, M.
    Physical Review B - Condensed Matter and Materials Physics 91 (2015)
    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.
    view abstract10.1103/PhysRevB.91.195308
  • Observation of High Angular Momentum Excitons in Cuprous Oxide
    Thewes, J. and Heckötter, J. and Kazimierczuk, T. and Aßmann, M. and Fröhlich, D. and Bayer, M. and Semina, M.A. and Glazov, M.M.
    Physical Review Letters 115 (2015)
    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.
    view abstract10.1103/PhysRevLett.115.027402
  • Photon-Statistics Excitation Spectroscopy of a Quantum-Dot Micropillar Laser
    Kazimierczuk, T. and Schmutzler, J. and Aßmann, M. and Schneider, C. and Kamp, M. and Höfling, S. and Bayer, M.
    Physical Review Letters 115 (2015)
    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.
    view abstract10.1103/PhysRevLett.115.027401
  • Stochastic pumping of a polariton fluid
    Aßmann, M. and Bayer, M.
    Physical Review A - Atomic, Molecular, and Optical Physics 91 (2015)
    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.
    view abstract10.1103/PhysRevA.91.053835
  • Influence of interactions with noncondensed particles on the coherence of a one-dimensional polariton condensate
    Schmutzler, J. and Kazimierczuk, T. and Bayraktar, O. and Aßmann, M. and Bayer, M. and Brodbeck, S. and Kamp, M. and Schneider, C. and Höfling, S.
    Physical Review B - Condensed Matter and Materials Physics 89 (2014)
    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.
    view abstract10.1103/PhysRevB.89.115119
  • Nonlinear spectroscopy of exciton-polaritons in a GaAs-based microcavity
    Schmutzler, J. and Aßmann, M. and Czerniuk, T. and Kamp, M. and Schneider, C. and Höfling, S. and Bayer, M.
    Physical Review B - Condensed Matter and Materials Physics 90 (2014)
    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.
    view abstract10.1103/PhysRevB.90.075103
  • Quantum-memory effects in the emission of quantum-dot microcavities
    Berger, C. and Huttner, U. and Mootz, M. and Kira, M. and Koch, S.W. and Tempel, J.-S. and Aßmann, M. and Bayer, M. and Mintairov, A.M. and Merz, J.L.
    Physical Review Letters 113 (2014)
    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.
    view abstract10.1103/PhysRevLett.113.093902
  • Compressive adaptive computational ghost imaging
    Aßmann, M. and Bayer, M.
    Scientific Reports 3 (2013)
    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.
    view abstract10.1038/srep01545
  • Determination of operating parameters for a GaAs-based polariton laser
    Schmutzler, J. and Veit, F. and Aßmann, M. and Tempel, J.-S. and Höfling, S. and Kamp, M. and Forchel, A. and Bayer, M.
    Applied Physics Letters 102 (2013)
    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.
    view abstract10.1063/1.4794144
  • All-optical control of quantized momenta on a polariton staircase
    Aßmann, M. and Veit, F. and Bayer, M. and Löffler, A. and Höfling, S. and Kamp, M. and Forchel, A.
    Physical Review B - Condensed Matter and Materials Physics 85 (2012)
    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.
    view abstract10.1103/PhysRevB.85.155320
  • Characterization of two-threshold behavior of the emission from a GaAs microcavity
    Tempel, J.-S. and Veit, F. and Aßmann, M. and Kreilkamp, L.E. and Rahimi-Iman, A. and Löffler, A. and Höfling, S. and Reitzenstein, S. and Worschech, L. and Forchel, A. and Bayer, M.
    Physical Review B - Condensed Matter and Materials Physics 85 (2012)
    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.
    view abstract10.1103/PhysRevB.85.075318
  • Coherence time measurements using a single detector with variable time resolution
    Assmann, M. and Bayer, M.
    Optics Letters 37 (2012)
    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.
    view abstract10.1364/OL.37.002811
  • Photon correlations in semiconductor nanostructures
    Aßmann, M. and Bayer, M.
    Quantum Optics with Semiconductor Nanostructures (2012)
    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.
    view abstract10.1533/9780857096395.2.154
  • Relaxation dynamics of optically imprinted polariton wires
    Veit, F. and Aßmann, M. and Bayer, M. and Löffler, A. and Höfling, S. and Forchel, A.
    Proceedings of SPIE - The International Society for Optical Engineering 8260 (2012)
    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).
    view abstract10.1117/12.906367
  • Spatial dynamics of stepwise homogeneously pumped polariton condensates
    Veit, F. and Aßmann, M. and Bayer, M. and Löffler, A. and Höfling, S. and Kamp, M. and Forchel, A.
    Physical Review B - Condensed Matter and Materials Physics 86 (2012)
    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.
    view abstract10.1103/PhysRevB.86.195313
  • Temperature dependence of pulsed polariton lasing in a gaas microcavity
    Tempel, J.-S. and Veit, F. and Aßmann, M. and Kreilkamp, L.E. and Ḧ ofling, S. and Kamp, M. and Forchel, A. and Bayer, M.
    New Journal of Physics 14 (2012)
    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.
    view abstract10.1088/1367-2630/14/8/083014
  • From polariton condensates to highly photonic quantum degenerate states of bosonic matter
    Aßmann, M. and Tempel, J.-S. and Veit, F. and Bayer, M. and Rahimi-Iman, A. and Löffler, A. and Höfling, S. and Reitzenstein, S. and Worschech, L. and Forchel, A.
    Proceedings of the National Academy of Sciences of the United States of America 108 (2011)
    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.
    view abstract10.1073/pnas.1009847108
  • Nonlinearity sensing via photon-statistics excitation spectroscopy
    Aßmann, M. and Bayer, M.
    Physical Review A - Atomic, Molecular, and Optical Physics 84 (2011)
    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.
    view abstract10.1103/PhysRevA.84.053806
  • Measuring the dynamics of second-order photon correlation functions inside a pulse with picosecond time resolution
    Aßmann, M. and Veit, F. and Tempel, J.-S. and Berstermann, T. and Stolz, H. and Van Der Poel, M. and Hvam, J.M. and Bayer, M.
    Optics Express 18 (2010)
    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.
    view abstract10.1364/OE.18.020229
  • Ultrafast tracking of second-order photon correlations in the emission of quantum-dot microresonator lasers
    Aßmann, M. and Veit, F. and Bayer, M. and Gies, C. and Jahnke, F. and Reitzenstein, S. and Höfling, S. and Worschech, L. and Forchel, A.
    Physical Review B - Condensed Matter and Materials Physics 81 (2010)
    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.
    view abstract10.1103/PhysRevB.81.165314
  • excitons

  • light sources

  • phonons

  • photons

  • polaritons

  • quantum theory

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