Papers by Wolfgang Langbein
arXiv (Cornell University), Feb 5, 2020
We investigate dynamics of resonantly excited excitons in single-layers of MoSe 2 and WS 2 down t... more We investigate dynamics of resonantly excited excitons in single-layers of MoSe 2 and WS 2 down to 4.5 K. To this end, we measure the delay dependence of the heterodyne four-wave mixing (FWM) amplitude induced by three, short laser pulses. This signal depends not only on the population of optically active excitons, which affects the absorption of the probe, but also on the population of optically inactive states, by interaction-induced energy shift, influencing the refractive index experienced by the probe. As such, it offers insight into density dynamics of excitons which do not directly couple to photons. Reproducing the coherent signal detected in amplitude and phase, the FWM delay dependence is modeled by a coherent superposition of several exponential decay components, with characteristic time constants from 0.1 picosecond up to 1 nanosecond. With increasing excitation intensity and/or temperature, we observe strong interference effects in the FWM field amplitude, resulting in progressively more complex and nonintuitive signal dynamics. We attribute this behaviour to increasingly populated exciton dark states, which change the FWM field phase by the relative effect on absorption and refractive index. We observe that exciton recombination occurs on a significantly longer timescale in WS 2 with respect to MoSe 2 , which is attributed to the dark character of exciton ground state in the former and the bright in the latter.
Technical Digest. Summaries of Papers Presented at the International Quantum Electronics Conference. Conference Edition. 1998 Technical Digest Series, Vol.7 (IEEE Cat. No.98CH36236)
CLEO '97., Summaries of Papers Presented at the Conference on Lasers and Electro-Optics
We investigate dynamics of resonantly excited excitons in single-layers of MoSe2 and WS2 down to ... more We investigate dynamics of resonantly excited excitons in single-layers of MoSe2 and WS2 down to 4.5K. To this end, we measure the delay dependence of the heterodyne four-wave mixing (F M) amplitude induced by three, short laser pulses. This signal depends not only on the population of optically active excitons, which affects the absorption of the probe, but also on the population of optically inactive states, by interaction-induced energy shift, influencing the refractive index experienced by the probe. As such, it offers insight into density dynamics of excitons which do not directly couple to photons. Reproducing the coherent signal detected in amplitude and phase, the FWM delay dependence is modeled by a coherent superposition of several exponential decay components, with characteristic time constants from 0.1 picosecond up to 1 nanosecond. With increasing excitation intensity and/or temperature, we observe strong interference effects in the FWM field amplitude, resulting in pro...
arXiv: Quantum Gases, 2019
Coherent bosonic ensembles offer the promise of harnessing quantum effects in photonic and quantu... more Coherent bosonic ensembles offer the promise of harnessing quantum effects in photonic and quantum circuits. In the dynamic equilibrium regime, the application of polariton condensates is hindered by exciton-polariton scattering induced de-coherence in the presence of a dark exciton reservoir. By spatially separating the condensate from the reservoir, we drive the system into the weak interaction regime, where the ensemble coherence time exceeds the individual particle lifetime by nearly three orders of magnitude. The observed nanosecond coherence provides an upper limit for polariton self-interactions. In contrast to conventional photon lasers, we observe an increased contribution from the super-Poissonian component of the condensate to the overall particle number fluctuations. Coupled with the recent emergence of a quantum regime in polaritonics, coherence times extended to several nanoseconds favour the realization of quantum information protocols.
Physical Review B, 1996
We report on the influence of the interface corrugation in ͑113͒-grown GaAs/AlAs superlattices on... more We report on the influence of the interface corrugation in ͑113͒-grown GaAs/AlAs superlattices on their band-edge optical properties both in theory and experiment. We calculate the subband dispersions and the optical anisotropies in a multiband k•p formalism. The dominating contribution to the optical anisotropies is found to be due to the intrinsic properties of the valence-band structure. The corrugation modifies the density of states only slightly, giving no evidence of a quantum-wire behavior. By comparing the calculation with the experimental optical anisotropy, we can estimate the corrugation height to be at most 2 monolayers. The experiments show that deviations from the regular corrugation lead to an anisotropic interface disorder. This gives rise to an enhanced anisotropy of the band-edge states, which was so far attributed to the corrugation itself. The luminescence of the localized type-I states at the band-edge show an enhanced optical anisotropy in comparison to the luminescence of the extended states, revealing the anisotropic nature of their localization sites. In type-II samples, deeply localized, isolated type-I states (⌫ quantum boxes͒ dominate the luminescence at short delays after pulsed excitation and at higher lattice temperatures or excitation densities, due to their strong radiative decay compared to the type-II states. These quantum boxes are observed individually by high spatial and spectral resolution. ͓S0163-1829͑96͒06039-0͔
Physical Review B, 2015
Polaritons of defined momentum and energy are excited resonantly on the lower polariton branch of... more Polaritons of defined momentum and energy are excited resonantly on the lower polariton branch of a planar semiconductor microcavity in the strong coupling regime, and the spectrally and momentum resolved emission is analyzed. We observe ghost branches from scattering within the lower polariton branch, as well as from scattering to the middle polariton branch, showing the non-linear mixing between different branches. Extending the theoretical treatment of spontaneous parametric luminescence developed in Ciuti et al., Phys. Rev. B 63, 041303 (2001), the eigenmodes of the driven polariton system and its photoluminescence are modeled.
Physical Review B, 1996
We report on the observation of isolated, anisotropically confined electronic states of the type-... more We report on the observation of isolated, anisotropically confined electronic states of the type-I transition in ͑113͒-grown, type-II GaAs/AlAs superlattices. They are formed by fluctuations of the periodic interface corrugation. Electronic states related to the X conduction-band minimum act, at elevated temperatures, as a transport channel to these ⌫ quantum boxes. The extremely narrow emission lines of the quantum boxes show an enhanced optical anisotropy compared to the luminescence of the extended states, revealing the anisotropic nature of the localization sites. This is confirmed by a comparison of the experimental luminescence energy and anisotropy with the results of a multiband k-p calculation for a quantum wire model system. ͓S0163-1829͑96͒06823-3͔
Physical Review Letters, 2014
We investigate the propagation and scattering of polaritons in a planar GaAs microcavity in the l... more We investigate the propagation and scattering of polaritons in a planar GaAs microcavity in the linear regime under resonant excitation. The propagation of the coherent polariton wave across an extended defect creates phase and intensity patterns with identical qualitative features previously attributed to dark and half-dark solitons of polaritons. We demonstrate that these features are observed for negligible nonlinearity (i.e., polariton-polariton interaction) and are, therefore, not sufficient to identify dark and half-dark solitons. A linear model based on the Maxwell equations is shown to reproduce the experimental observations.
Applied Physics Letters, 2014
The investigation of intrinsic interactions in polariton condensates is currently limited by the ... more The investigation of intrinsic interactions in polariton condensates is currently limited by the photonic disorder of semiconductor microcavity structures. Here, we use a strain compensated planar GaAs/AlAs0.98P0.02 microcavity with embedded InGaAs quantum wells having a reduced cross-hatch disorder to overcome this issue. Using real and reciprocal space spectroscopic imaging under non-resonant optical excitation, we observe polariton condensation and a second threshold marking the onset of photon lasing, i.e., the transition from the strong to the weak-coupling regime. Condensation in a structure with suppressed photonic disorder is a necessary step towards the implementation of periodic lattices of interacting condensates, providing a platform for on chip quantum simulations.
2011 Conference on Lasers and Electro-Optics Europe and 12th European Quantum Electronics Conference (CLEO EUROPE/EQEC), 2011
Physical Review Letters, 2005
We present measurements and simulations of coherent control and readout of the polarization in in... more We present measurements and simulations of coherent control and readout of the polarization in individual exciton states. The readout is accomplished by transient four-wave mixing detected by heterodyne spectral interferometry. We observe Rabi oscillations in the polarization, which show half the period of the Rabi oscillations in the population. A decrease of the oscillation amplitude with pulse area is observed, which is not accompanied by a change in the dephasing time. This suggests the transfer of the excitation to other states as the origin of the Rabi-oscillation damping. Detuning of the excitation enables the control of the polarization in phase and amplitude and is in qualitative agreement with simulations for a two-level system. Additionally, simultaneous Rabi flopping of several spatially and energetically close exciton states is demonstrated.
Physical Review B, 2004
A moving speckle pattern in time-momentum space of the resonantly excited Rayleigh scattering fro... more A moving speckle pattern in time-momentum space of the resonantly excited Rayleigh scattering from excitons subjected to a spatial potential gradient is observed. The spatial gradient of the exciton energy was created by an uniaxial mechanical stress applied in the plane of a semiconductor quantum well. The measured energy shift and the increase in the heavy-light hole splitting is well described by k•p theory with a strain field calculated by linear continuum elasticity theory. A model based on localized excitons in a center-of-mass potential with a systematic spatial variation explains the observed properties of the speckle pattern, showing in particular that the velocity of the speckles in momentum space is given just by the local gradient of the potential.
Physical Review B, 2007
The emission from a microcavity in the strong coupling regime excited resonantly by linearly pola... more The emission from a microcavity in the strong coupling regime excited resonantly by linearly polarized light has been measured with spatial, directional, and temporal resolution. We observe ballistic propagation of the excited polaritons as a ring spreading in real space. The linear polarization degree P of the emission at the ring is found to oscillate as a function of the azimuthal angle and of time t according to P = sin͑2͒sin͑⍀ LT t /2͒, where ⍀ LT is the longitudinal-transverse splitting of the exciton-polariton modes. This dependence holds for all investigated excitation energies corresponding to different values of ⍀ LT. The theoretical model assuming ballistic propagation of exciton polaritons in the specific cavity structure yields a detailed agreement with the experimental data. The observed polarization beats have the same nature as the recently predicted optical spin Hall effect.
Physical Review B, 2012
We report on polariton states bound to defects in planar GaAs/AlAs microcavities grown by molecul... more We report on polariton states bound to defects in planar GaAs/AlAs microcavities grown by molecular beam epitaxy. The defect types relevant for the spatial polariton dynamics in these structures are cross-hatch misfit dislocations, and point-like defects extended over several micrometers. We attribute the latter defects to Ga droplets emitted occasionally by the Ga cell during the growth. These defects, also known as oval defects, result in a dome-like local modulation of surface, which is translated into the cavity structure and leads to a lateral modulation of the cavity polariton energy of up to 15 meV. The resulting spatially localized potential landscape for the in-plane polariton motion creates a series of bound states. These states were characterized by spectrally resolved transmission imaging in real and reciprocal space, and reveal the spatial potential created by the defects. Interestingly, the defect states exhibit long lifetimes in the 10 ps range, which we attribute to a spatially smooth confinement potential.
Physical Review B, 2009
Periodic structures resonantly coupled to excitonic media allow the existence of extra intragap m... more Periodic structures resonantly coupled to excitonic media allow the existence of extra intragap modes ('Braggoritons'), due to the coupling between Bragg photon modes and 3D bulk excitons. This induces unique and unexplored dispersive features, which can be tailored by properly designing the photonic bandgap around the exciton resonance. We report that 1D Braggoritons realized with semiconductor gratings have the ability to mimic the dispersion of quantum-well microcavity polaritons. This will allow the observation of new nonlinear phenomena, such as slow-light-enhanced nonlinear propagation and an efficient parametric scattering at two 'magic frequencies'.
physica status solidi (b), 2006
We measure the dephasing time of the first optically‐active excited‐state excitonic transition in... more We measure the dephasing time of the first optically‐active excited‐state excitonic transition in strongly confined InGaAs quantum dots using transient four‐wave mixing. The optically‐driven excited‐state polarization shows a bi‐exponential decay with a significant fraction of the probed excited states exhibiting a very long dephasing time, in the nanosecond range at 10 K. The full time‐dependent four‐wave mixing polarization is microscopically calculated by taking into account both virtual and real acoustic phonon‐assisted transitions. The bi‐exponential decay is qualitatively explained by a model of two‐bright excitonic excited states non‐degenerate in the absence of in‐plane cylindrical symmetry. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
physica status solidi (b), 2003
The frequency-resolved secondary emission from excitons in a single 8 nm-wide quantum well is inv... more The frequency-resolved secondary emission from excitons in a single 8 nm-wide quantum well is investigated using speckle analysis. We model these experiments starting with a Hamiltonian in the basis of disorder eigenstates of excitons, interacting both with light and acoustical phonons. The distinction between incoherent and coherent secondary emission is intimately related with the decomposition of a two-time exciton density-matrix into a diagonal, incoherent part and a product of polarizations. The latter gives rise to speckling (intensity variations over observation angle), and is called resonant Rayleigh scattering. The results of our simulation agree pretty well with the experimental data and allow a determination of the coherent and incoherent exciton distributions.
physica status solidi (a), 2000
Users may download and print one copy of any publication from the public portal for the purpose... more Users may download and print one copy of any publication from the public portal for the purpose of private study or research. You may not further distribute the material or use it for any profit-making activity or commercial gain You may freely distribute the URL identifying the publication in the public portal If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.
New Journal of Physics, 2013
Interaction between light and matter generates optical nonlinearities, which are particularly pro... more Interaction between light and matter generates optical nonlinearities, which are particularly pronounced in the quantum strong coupling regime. When a single bosonic mode couples to a single fermionic mode, a Jaynes-Cummings (JC) ladder is formed, which we realize here using cavity photons and quantum dot excitons. We measure and model the coherent anharmonic response of this strongly coupled exciton-cavity system at resonance. Injecting two photons into the cavity, we demonstrate a √ 2 larger polariton splitting with respect to the vacuum Rabi splitting. This is achieved using coherent nonlinear spectroscopy, specifically four-wave mixing, where the coherence between the ground state and the first (second) rung of the JC ladder can be interrogated for positive
Uploads
Papers by Wolfgang Langbein