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physica status solidi (b)

Non‐markovian polarization decay in semiconductor quantum wells studied by three‐beam four‐wave mixing

F. Löser, D. Klatt, K.‐H. Pantke, K. Leo, Huib J. Bakker, Jagdeep Shah, Klaus Köhler

Published Mar 1, 1995• 5 citations
Publisher

Abstract

Abstract The polarization and population dynamics of resonantly generated excitons in GaAs/AlGaAs quantum well structures are investigated using transient three‐pulse four‐wave mixing. The decay dynamics show strongly nonexponential behavior for both variation of the delay between the first two pulses reflecting the polarization dynamics and variation of the delay between the second two pulses, which reflects the population dynamics. A comparison with a theoretical model shows that the dynamics of both polarization and population are mainly determined by frequency fluctuations and not by scattering processes or by recombination. The characteristic time of the fluctuations TC has to be energy‐dependent to fully describe the results.

DOI
10.1002/pssb.2221880141

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Recent transient four-wave-mixing experiments in the two-pulse photon-echo configuration performed on GaAs quantum wells have yielded the surprising result that in some samples the T2{\mathit{T}}_{2}T2​ time obtained from the polarization decay can be shorter for cross-polarized input fields than for parallel input fields. A concomitant change from photon-echo to free-induction-decay behavior has also been observed. A phenomenological model is presented which explains these observations by means of a disorder-induced coupling of the ${\mathrm{\ensuremath{\sigma}}}^{+}$ and ${\mathrm{\ensuremath{\sigma}}}^{\mathrm{\ensuremath{-}}}$ exciton transitions which is inhomogeneous in coupling strength. Such coupling also leads to a remarkable change in the light-hole--heavy-hole exciton quantum beat phase as a function of pulse delay for a critical relative orientation of the polarization of the incident pulses. Experimental observations of this effect are presented to support the validity of the model.
Abstract We perform 3‐pulse transient four‐wave‐mixing on a GaAs/AlGaAs multiple quantum well. This allows us to measure both the decay of the Raman coherence between the heavy‐hole and light‐hole excitons and the decay of the respective optical coherences. We observe that the decay rate of the Raman coherence is greater than the sum of decay rates of the optical coherences. This cannot happen within a Markov approximation for the scattering processes, as verified by simulations. The simulations show that such behavior can occur only in the presence of anti‐correlated scattering processes in a non‐Markovian regime.
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We study the coherent response of an optically dense exciton system in GaAs quantum wells (QWs). For this, we resonantly excite 1s1s1s heavy-hole excitons in the QWs at 8 K with subpicosecond laser pulses. The excitonic polarization is probed using time integrated (TI) degenerate four wave mixing (DFWM) measurements in self-diffraction geometry. The average power density (I1){(I}_{1})(I1​) of the leading laser pulse is systematically increased to large values, keeping that of the delayed pulse small and fixed. The dephasing rate of the excitonic coherent polarization (determined by the rate of decrease of the TI-DFWM signal with delay between the exciting pulses) attains saturation at large I1{I}_{1}I1​ after an initial rise at small I1.{I}_{1}.I1​. In comparison, the coherent polarization (as given by the DFWM signal at a fixed delay) increases initially with I1,{I}_{1},I1​, reaches a peak and then decreases at large I1.{I}_{1}.I1​.