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Journal of the Optical Society of America B

Dephasing of interacting heavy-hole and light-hole excitons in GaAs quantum wells

K. Bott, E. J. Mayer, G. O. Smith, V. Heuckeroth, Matthias Hübner, J. Kühl, T. Meier, A. Schulze, Markus Lindberg, S. W. Koch, P. Thomas, R. Hey, Klaus Ploog

Published May 1, 1996• 29 citations
Publisher

Abstract

We describe a novel three-pulse degenerate four-wave mixing configuration that permits the direct observation of the coherence between the σ+ and the σ− exciton states in a GaAs quantum well excited in a two-photon process. It is found that the phase coherence between the two single heavy-hole exciton states decays with a time constant that is considerably longer than the dephasing time of the coherence between these states and the ground state (interband coherence). All the experimental data are well described by numerical solutions of the optical Bloch equations for a phenomenological multilevel model.

DOI
10.1364/josab.13.001026

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Abstract We investigate coherences between σ+ and σ− exciton states in a 10 nm thick ZnSe/ZnMgSe single quantum well by spectrally resolved three‐pulse degenerate four‐wave mixing using circularly polarized 100 fs pulses. Intensity dependent measurements are performed to study the influence of exciton–exciton scattering on their dephasing rates. The experiments reveal nearly equal zero‐density dephasing rates for coherences between σ+ and σ− heavy‐ and/or light‐hole exciton states as compared to the dephasing rate of the interband coherence between these states and the ground state. The measurements also demonstrate that exciton‐exciton scattering destroys the coherence between heavy‐ and light‐hole excitons about three times more effectively than the coherence between σ+ and σ− heavy‐hole excitons. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

physica status solidi (b)•Feb 10, 2004•1 citation
We describe a novel three-pulse degenerate four-wave mixing configuration that permits the direct observation of the coherence between the σ+ and the σ− exciton states in a GaAs quantum well excited in a two-photon process. It is found that the phase coherence between the two single heavy-hole exciton states decays with a time constant that is considerably longer than the dephasing time of the coherence between these states and the ground state (interband coherence). All the experimental data are well described by numerical solutions of the optical Bloch equations for a phenomenological multilevel model.
We investigate interband coherences as well as coherences between ${\ensuremath{\sigma}}^{+}$ and ${\ensuremath{\sigma}}^{\ensuremath{-}}$ exciton states in a 10 nm thick ZnSe/ZnMgSe single quantum well by spectrally resolved three-pulse degenerate four-wave mixing using circularly polarized 100 fs pulses. Polarization and intensity dependent measurements are performed to identify the coherent processes responsible for the nonlinear signal into the ${\mathbf{k}}_{3}+{\mathbf{k}}_{2}\ensuremath{-}{\mathbf{k}}_{1}$ direction and to study the influence of Coulomb scattering on the dephasing rates. The experiments reveal nearly equal zero-density dephasing rates for coherences between ${\ensuremath{\sigma}}^{+}$ and ${\ensuremath{\sigma}}^{\ensuremath{-}}$ heavy- and/or light-hole exciton states as compared to the dephasing rate of the interband coherence between these states and the ground state. The measurements also demonstrate that exciton--exciton scattering destroys the phase coherence between heavy- and light-hole excitons about three times more effectively than the coherence between ${\ensuremath{\sigma}}^{+}$ and ${\ensuremath{\sigma}}^{\ensuremath{-}}$ heavy-hole excitons. The characteristic features of the experimental results are explained and reproduced by numerical calculations of the optical Bloch equations for a ten-level model including excitation induced dephasing.
Three-beam time-integrated transient four-wave mixing (TFWM) is used to probe both light-hole (LH) and heavy-hole (HH) exciton coherences in weakly disordered GaAs quantum wells. The period of the quantum beats between the LH and the HH exciton coherences changes as a function of first-second pulse delay. A theoretical model for the TFWM signal, including exciton dephasing rates that vary with frequency across the inhomogeneous linewidth, reproduces the observations. The inclusion of inhomogeneous dephasing also improves the correspondence between theory and experiment when data as a function of both first-second and second-third pulse delays are simultaneously examined.
Spectrally resolved and time-integrated degenerate four-wave mixing (FWM) experiments with ultra-short pulses have been used to investigate the relaxation times and coherent interactions of excitons in the Boltzmann and in the quantum kinetic regime in ZnSe single quantum wells.Three-pulse FWM experiments using 100 fs pulses in a 10 nm thick ZnSe/ZnMgSe single quantum well reveal nearly equal zero-density dephasing rates for coherences between + σ and - σ heavy-and/or light-hole exciton states as compared to the dephasing rate of the interband coherence between these states and the ground state.The measurements also demonstrate that exciton-exciton scattering destroys the phase coherence between heavy-and light-hole excitons about three times more effectively than the coherence between + σ and + σ heavy-hole excitons.The different coherent nonlinear signals obtained in a three-beam FWM experiment are explained and reproduced by numerical calculations of the optical Bloch equations for a ten-level model including excitation induced dephasing (EID).Two-beam FWM experiments using 30 fs pulses on a 10 nm ZnSe quantum well reveal marked quantum beats and a pronounced non-exponential decay for delay times smaller ~500 fs.The pronounced peak at pulse overlap is attributed to rapid destructive interference of the created polarization and to spin dependent quantum kinetic effects caused by the transient formation of an electron-hole (eh) pair grating.The three-beam FWM trace shows a bi-exponential decay which is attributed to a nonlinear polarization that is caused by the interaction of polarization 3 k with both an exciton grating and an eh-pair grating.Model calculations using the OBEs of a two level system that includes ∑ -+
Secondary emission from GaAs quantum wells following resonant pulsed excitation is studied in the time domain. Selective excitation of heavy- and light-hole excitons is accomplished in a two-pulse excitation experiment which preserves sub-ps time resolution. After the initial rise the emission decays exponentially at a fast rate and then with another much slower exponential decay at longer times. The variations in the initial decay times with excitation density and sample temperature are similar to the variation in dephasing rates measured in four-wave mixing experiments. The decay in the visibility of heavy- and light-hole exciton beating is consistent with the decay rates measured for both species separately.
We demonstrate the presence of strong exciton-exciton correlations in GaAs-AlGaAs multiple quantum wells by using the polarization selection rules for four-wave mixing and a test that was originally designed to distinguish between quantum beats and polarization interference. We show that, when the four-wave-mixing signal is produced by two pulses that have the same circular polarization, dynamic beating behavior is observed at the heavy-hole-light-hole frequency that provides evidence of Coulomb-induced correlations that go beyond those expected from the semiconductor Bloch equations in the Hartree-Fock approximation and beyond the local field corrections used in simple phenomenological models. Moreover, this behavior cannot be explained by invoking biexcitonic effects. We demonstrate that this dynamic excitonic coupling can be explained in terms of a competition between quantum-beat-like and polarization-interference-like behavior, and we show that such behavior can be used to place quantitative limits on the coupling strength. In addition, we use a simple phenomenological model to show that excitation-induced-dephasing can produce such correlations.
Abstract We investigate coherences between σ+ and σ− exciton states in a 10 nm thick ZnSe/ZnMgSe single quantum well by spectrally resolved three‐pulse degenerate four‐wave mixing using circularly polarized 100 fs pulses. Intensity dependent measurements are performed to study the influence of exciton–exciton scattering on their dephasing rates. The experiments reveal nearly equal zero‐density dephasing rates for coherences between σ+ and σ− heavy‐ and/or light‐hole exciton states as compared to the dephasing rate of the interband coherence between these states and the ground state. The measurements also demonstrate that exciton‐exciton scattering destroys the coherence between heavy‐ and light‐hole excitons about three times more effectively than the coherence between σ+ and σ− heavy‐hole excitons. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)