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physica status solidi (b)

Homogeneous linewidth of quantum well excitons from resonance fluorescence spectra

Daniel Schwedt, Ch. Nacke, H. Stolz, Soheyla Eshlaghi, D. Reuter, Andreas Dirk Wieck

Published Oct 9, 2003• 2 citations
Publisher

Abstract

Abstract We report high resolution measurements of resonance fluorescence spectra of high quality GaAs quantum wells after small bandwidth cw excitation. The resulting spectra can be well described with means of light scattering from a statistically distributed ensemble of three‐level‐systems. Using this model we are able to separate the homogeneous linewidth from the inhomogeneous ensemble response. Thus, we present a simple linear tool for sample characterization. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

DOI
10.1002/pssb.200301888

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Homogeneous linewidth of quantum well excitonsfrom resonance fluorescence spectra

Daniel Schwedt, Ch. Nacke, H. Stolz +3 more

Abstract Editor's Choice of this issue is the article by D. Schwedt et al. [1] on high resolution measurements of resonance fluorescence spectra from GaAs quantum wells. In the cover picture, the authors point out the connection between the experimental data and the model of disordered excitons. The lower left part shows a series of polarized photoluminescence spectra, fitted with very good agreement by a model of statistically distributed three‐level‐systems. The upper right part images the excitation mechanism. Only those excitons of an inhomogeneous distribution (indicated by the blue line) contribute to the resonance fluorescence (red line) which overlap spectrally with the monochromatic laser. It could be verified experimentally that the resonance fluorescence exhibits two parts: delta‐like Rayleigh scattering and Lorentzian luminescence. Daniel Schwedt is working in the DFG research group 'Quantum Optics in Semiconductor Nanostructures' and concentrates on quantum optical experiments on III–V semiconductor quantum well structures.

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Summary form only given. After the first observation of resonance scattering from excitons in quantum wells about 1980, this topic has received much interest and has given rise to a hot debate on the origin of this scattering process. Recently, the determination of the temporal behaviour and the coherence properties of this scattering has initiated revival of interferometric techniques in semiconductor optics. Finally, the observation of speckles in the angular distribution proved that this scattering indeed originates from exciton states localized by disorder. However, complementary information on the nature of the excitons and the interplay between scattering and luminescence can be obtained from high resolution cw spectroscopy, having the advantage that only a small fraction of the states within the homogeneous linewidth is excited. However yhe polarization behaviour is quite different for the coherent Rayleigh component and the luminescence, depending on the orientation, splitting and relaxation of the localized states. The experimental results for a 20 nm wide GaAs quantum well clearly confirm this prediction.

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Excitons in Semiconductor Quantum Wells Studied Using Two-Dimensional Coherent Spectroscopy

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Correlated electron-hole pairs, or excitons, in semiconductor nanostructures have been studied extensively over the past few decades. The optical response of excitons is complicated due to inhomogeneous broadening, presence of multiple states, and exciton-exciton interactions. In this work we bring new perspectives to exciton physics in semiconductor quantum wells (QWs) through two-dimensional coherent spectroscopy (2DCS).\nThe effect of QW growth direction on the optical properties of excitons is explored by studying (110)-oriented GaAs QWs. The homogeneous and inhomogeneous linewidths of the heavy-hole exciton resonance are measured. By probing the optical nonlinear response for polarization along the in-plane crystal axes [1-10] and [001], we measure different homogeneous linewidths for the two orthogonal directions. This difference is found to be due to anisotropic excitation-induced dephasing, caused by a crystal-axis-dependent absorption coefficient. The extrapolated zero-excitation density homogeneous linewidth exhibits an activation-like temperature dependence.\nSpectral diffusion of excitons in (001)-oriented QWs has been studied. We show that the spectral diffusion characteristics depend strongly on the sample temperature. Spectral diffusion is generally assumed to follow the strong-redistribution approximation, partly because of lack of any evidence to the contrary. We find that this assumption is violated at low sample temperatures for excitons in QWs; high-energy excitons preferentially relax due to a negligible phonon population at low temperatures. The frequency-frequency correlation function is measured through a numerical fitting procedure to quantify spectral diffusion for sample temperatures $>20$ K.\nExciton-exciton interactions affect the light-matter interactions in QWs significantly. We present an intuitive and simple model for these interactions by treating excitons as interacting bosons. We show that the polarization-dependent exciton dephasing rate in GaAs quantum wells is due to the bosonic character of excitons. We fit slices from simulated spectra to those from the experimentally measured spectra and show that interference between two different quantum mechanical pathways results in a slower dephasing rate for co-circular and co-linear polarization of optical excitation pulses. This interference does not exist for cross-linearly polarized excitation pulses resulting in a faster dephasing rate. Additionally, we were able to separately quantify inter- and intra-mode interactions between excitons through exciton-density-dependent measurements.

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Abstract We use heterodyne spectral interferometry (HSI) to investigate the transient four‐wave mixing field (FWM) from an individual localized excitonic transition. FWM intensity and spectrum as well as the spatial resolution in FWM hyperspectral imaging are measured as function of excitation intensity. A third‐order non‐linear response of the FWM followed by saturation and the onset of a Rabi‐flop are observed with increasing intensity. The intensity dependence of the spectral response depends on the excitation pulse duration. For short pulses (∼0.2 ps), having a broad spectrum, a weak broadening of the emission line, similar to excitation induced dephasing (EID), is found. For long pulses (∼0.5 ps) instead, no signatures of EID are found, but individual multi‐excitonic transitions are observed. When using the FWM intensity as image contrast by spatially scanning a region of the sample surface, a spatial resolution beyond the diffraction limit is measured in the third‐order regime, while for higher intensities the FWM saturation deteriorates the spatial resolution. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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High-resolution nonlinear laser spectroscopy of heavy-hole excitons in GaAs/AlGaAs quantum-well structures: A direct measure of the exciton line shape

J. T. Remillard, H. Wang, Duncan G. Steel +3 more

We report high-resolution nonlinear laser-spectroscopy studies of the n=1 heavy-hole exciton in GaAs multiple quantum wells. Using a new method of cw frequency-domain four-wave mixing, we first show that the excitation relaxation consists of both fast and slow components. We then show that the contribution of well-width fluctuations to inhomogeneous broadening can be elimiated enabling us to obtain a direct measure of the intrinsic exciton line shape. Under certain conditions, this line shape is non-Lorentzian and cannot be explained on the basis of simple excited-state relaxation dynamics.

Physical Review Letters•Jun 12, 1989•24 citations
Abstract We report high resolution measurements of resonance fluorescence spectra of high quality GaAs quantum wells after small bandwidth cw excitation. The resulting spectra can be well described with means of light scattering from a statistically distributed ensemble of three‐level‐systems. Using this model we are able to separate the homogeneous linewidth from the inhomogeneous ensemble response. Thus, we present a simple linear tool for sample characterization. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Abstract Editor's Choice of this issue is the article by D. Schwedt et al. [1] on high resolution measurements of resonance fluorescence spectra from GaAs quantum wells. In the cover picture, the authors point out the connection between the experimental data and the model of disordered excitons. The lower left part shows a series of polarized photoluminescence spectra, fitted with very good agreement by a model of statistically distributed three‐level‐systems. The upper right part images the excitation mechanism. Only those excitons of an inhomogeneous distribution (indicated by the blue line) contribute to the resonance fluorescence (red line) which overlap spectrally with the monochromatic laser. It could be verified experimentally that the resonance fluorescence exhibits two parts: delta‐like Rayleigh scattering and Lorentzian luminescence. Daniel Schwedt is working in the DFG research group 'Quantum Optics in Semiconductor Nanostructures' and concentrates on quantum optical experiments on III–V semiconductor quantum well structures.
Summary form only given. After the first observation of resonance scattering from excitons in quantum wells about 1980, this topic has received much interest and has given rise to a hot debate on the origin of this scattering process. Recently, the determination of the temporal behaviour and the coherence properties of this scattering has initiated revival of interferometric techniques in semiconductor optics. Finally, the observation of speckles in the angular distribution proved that this scattering indeed originates from exciton states localized by disorder. However, complementary information on the nature of the excitons and the interplay between scattering and luminescence can be obtained from high resolution cw spectroscopy, having the advantage that only a small fraction of the states within the homogeneous linewidth is excited. However yhe polarization behaviour is quite different for the coherent Rayleigh component and the luminescence, depending on the orientation, splitting and relaxation of the localized states. The experimental results for a 20 nm wide GaAs quantum well clearly confirm this prediction.
Correlated electron-hole pairs, or excitons, in semiconductor nanostructures have been studied extensively over the past few decades. The optical response of excitons is complicated due to inhomogeneous broadening, presence of multiple states, and exciton-exciton interactions. In this work we bring new perspectives to exciton physics in semiconductor quantum wells (QWs) through two-dimensional coherent spectroscopy (2DCS).\nThe effect of QW growth direction on the optical properties of excitons is explored by studying (110)-oriented GaAs QWs. The homogeneous and inhomogeneous linewidths of the heavy-hole exciton resonance are measured. By probing the optical nonlinear response for polarization along the in-plane crystal axes [1-10] and [001], we measure different homogeneous linewidths for the two orthogonal directions. This difference is found to be due to anisotropic excitation-induced dephasing, caused by a crystal-axis-dependent absorption coefficient. The extrapolated zero-excitation density homogeneous linewidth exhibits an activation-like temperature dependence.\nSpectral diffusion of excitons in (001)-oriented QWs has been studied. We show that the spectral diffusion characteristics depend strongly on the sample temperature. Spectral diffusion is generally assumed to follow the strong-redistribution approximation, partly because of lack of any evidence to the contrary. We find that this assumption is violated at low sample temperatures for excitons in QWs; high-energy excitons preferentially relax due to a negligible phonon population at low temperatures. The frequency-frequency correlation function is measured through a numerical fitting procedure to quantify spectral diffusion for sample temperatures >20>20>20 K.\nExciton-exciton interactions affect the light-matter interactions in QWs significantly. We present an intuitive and simple model for these interactions by treating excitons as interacting bosons. We show that the polarization-dependent exciton dephasing rate in GaAs quantum wells is due to the bosonic character of excitons. We fit slices from simulated spectra to those from the experimentally measured spectra and show that interference between two different quantum mechanical pathways results in a slower dephasing rate for co-circular and co-linear polarization of optical excitation pulses. This interference does not exist for cross-linearly polarized excitation pulses resulting in a faster dephasing rate. Additionally, we were able to separately quantify inter- and intra-mode interactions between excitons through exciton-density-dependent measurements.
Abstract We use heterodyne spectral interferometry (HSI) to investigate the transient four‐wave mixing field (FWM) from an individual localized excitonic transition. FWM intensity and spectrum as well as the spatial resolution in FWM hyperspectral imaging are measured as function of excitation intensity. A third‐order non‐linear response of the FWM followed by saturation and the onset of a Rabi‐flop are observed with increasing intensity. The intensity dependence of the spectral response depends on the excitation pulse duration. For short pulses (∼0.2 ps), having a broad spectrum, a weak broadening of the emission line, similar to excitation induced dephasing (EID), is found. For long pulses (∼0.5 ps) instead, no signatures of EID are found, but individual multi‐excitonic transitions are observed. When using the FWM intensity as image contrast by spatially scanning a region of the sample surface, a spatial resolution beyond the diffraction limit is measured in the third‐order regime, while for higher intensities the FWM saturation deteriorates the spatial resolution. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
We report high-resolution nonlinear laser-spectroscopy studies of the n=1 heavy-hole exciton in GaAs multiple quantum wells. Using a new method of cw frequency-domain four-wave mixing, we first show that the excitation relaxation consists of both fast and slow components. We then show that the contribution of well-width fluctuations to inhomogeneous broadening can be elimiated enabling us to obtain a direct measure of the intrinsic exciton line shape. Under certain conditions, this line shape is non-Lorentzian and cannot be explained on the basis of simple excited-state relaxation dynamics.