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Journal of the Optical Society of America B

Coherent exciton dynamics as a function of quantum-well number

D. Weber, Jochen Feldmann, Ernst Otto Göbel, T. Stroucken, A. Knorr, Stephan W. Koch, D. S. Citrin, Klaus Köhler

Published Jun 1, 1996• 16 citations
Publisher

Abstract

We have investigated the coherent dynamics of excitons in a single, a double, and a quintuple quantum-well structure under resonant excitation. In pump-and-probe experiments on the quintuple quantum well we observe a fast decay of the excitonic population within the initial coherent regime. This fast initial population decay is less pronounced for the double quantum well and is almost absent for the single quantum well. The energy linked to the initially lost coherent population is found in the coherent electromagnetic emission induced by the first-order polarization. More specifically, the intensity of this directional light emission increases quadratically with the number of quantum wells. Interference effects and electromagnetic coupling effects between polarizations located in distinct quantum wells probably play a crucial role in the superlinear coherent emission process.

DOI
10.1364/josab.13.001241

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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.
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The intensity dependence of pump-induced excitonic absorption changes is studied in high-quality In0.04Ga0.96As{\mathrm{In}}_{0.04}{\mathrm{Ga}}_{0.96}\mathrm{As}In0.04​Ga0.96​As quantum wells for different detunings and polarization configurations. For resonant excitation with increasing pump intensity a saturation of both the exciton bleaching and the induced absorption due to two-exciton resonances is observed, whereas for pumping below the exciton a characteristic polarization dependence of higher-order contributions to the optical Stark effect is found. Microscopic calculations including Coulomb correlations show that the experimental observations can be described by the coherent dynamics of single- and two-exciton resonances up to fifth order in the optical field.