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physica status solidi (b)

Relevance of Dephasing Processes for the Ultrafast Rise of Emission from Resonantly Created Excitons in Quantum Wells

Stefan Haacke, Gary R. Hayes, Robert Anthony Taylor, B. Deveaud, R. Zimmermann, Israel Bar-Joseph

Published Nov 1, 1997• 11 citations
Publisher

Abstract

We present a comparative study of time-integrated four-wave-mixing and femtosecond emission under resonant excitation on excitons in weakly disordered GaAs quantum wells. At highest exciton densities when dephasing dominates the spectral width (homogeneous broadening), we find that the rise time of the incoherent luminescence signal is given by TIii 2/2. At lowest densities, optical coherence times approach the exciton radiative lifetime (15 to 20 ps). This confirms our previous result that coherent resonant Rayleigh scattering is responsible for the short rise time of the excitonic emission. We also show clear evidence for dephasing due to exciton–phonon interaction, as the rise time of the emission decreases dramatically when the sample temperature is increased.

DOI
10.1002/1521-3951(199711)204:1<35::aid-pssb35>3.0.co;2-9

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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.