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Applied Physics Letters

Femtosecond time-resolved free-induction decay of room-temperature excitons in GaAs quantum wells

Mary‐Ann Mycek, Shimon Weiss, J.‐Y. Bigot, Stefan Schmitt-Rink, D. S. Chemla, Wilfried Schaefer

Published May 25, 1992• 33 citations
Publisher

Abstract

We directly resolve the femtosecond free-induction decay of excitons in GaAs quantum wells at room temperature by upconverting the diffracted signal from a transient four-wave-mixing experiment. At high laser intensities, we find significant deviations from the temporal evolution of the time-integrated and peak amplitude signals, which we attribute to many-body Coulomb effects.

DOI
10.1063/1.106889

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We directly resolve the femtosecond free-induction decay of excitons in GaAs quantum wells at room temperature by upconverting the diffracted signal from a transient four-wave-mixing experiment. At high laser intensities, we find significant deviations from the temporal evolution of the time-integrated and peak amplitude signals, which we attribute to many-body Coulomb effects.
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In time-resolved four-wave mixing (FWM) experiments, two ultrashort pulses delayed by Δt and with wavevectors k1 and k2 interfere in a sample to produce a diffracted signal (DS) in the direction ks = 2 k2 − k1. In the case of a homogeneously broadened system, the DS is emitted immediately after the second pulse corresponding to the "free- induction decay" of the nonlinear polarization. Because of the short dephasing times T2 in solids, the DS is usually not time-resolved, but is measured as a function of Δt using a slow detector. Valuable information contained in the temporal profile of the DS is thus lost Here, we report the first measurement of the temporal evolution of the DS on a fs time scale for the case of selective excitation of semiconductors.
The coherent emission from excitons corresponding to different subband transitions in a 500 Åbroad GaAs/AlGaAs quantum well is studied by spectrally resolved transient four-wave mixing (FWM). For the simultaneous excitation of several subband transitions, the dephasing times of the lowest exciton and higher- order exciton transitions are determined from the width of the resonances in the FWM spectrum. The decay of the spectrally integrated FWM signal as a function of the time delay between the excitation laser pulses does not reflect the dephasing times but is determined by the laser pulse width for multi-subband excitation. This is indicative of quantum interference between exciton and continuum transitions. Many-body Coulomb effects have to be included in a theoretical treatment of these experimental observations.