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Applied Physics Letters

Reflected degenerate four-wave mixing on GaAs single quantum wells

A. Honold, Lothar Schultheis, J. Kühl, Charles W. Tu

Published Jun 20, 1988• 73 citations
Publisher

Abstract

Time-resolved degenerate four-wave mixing (DFWM) on two-dimensional (2D) excitons in GaAs single quantum wells is studied. We observe backward coherent emission (DFWM signal) from the 2D excitons comparable in efficiency to that in forward direction. This reflection or backward DFWM configuration is extremely advantageous and useful in exploring optically thin semiconductor layers.

DOI
10.1063/1.99549

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Time-resolved degenerate four-wave mixing (DFWM) on two-dimensional (2D) excitons in GaAs single quantum wells is studied. We observe backward coherent emission (DFWM signal) from the 2D excitons comparable in efficiency to that in forward direction. This reflection or backward DFWM configuration is extremely advantageous and useful in exploring optically thin semiconductor layers.
Room temperature bulk GaAs and GaAs/A1GaAs multiple quantum well (MQW) samples are studied using backward degenerate four wave mixing (DFWM) at moderately high pump intensities (4 kW/cm2). The DFWM reflectivity is measured in the band gap region as a function of laser wavelength and pump intensity. When the MQW is pumped at intensities sufficient to saturate the excitonic resonance, a dispersive DFWM spectrum consistent with conduction band contributions is observed. Preliminary experiments in bulk GaAs indicate that the phase conjugate signal strength is the same as that for the MQW at approximately the same pump intensity.
We perform two beam time integrated (TI) degenerate four wave mixing (DFWM) measurements on 8~nm GaAs quantum wells (QW). Pulses from a Ti-Sapphire laser resonantly excite only 1s heavy hole excitons. The power density of the leading pulse (I1) is increased, keeping that of the delayed pulse fixed and relatively small. We measure the TI-DFWM signal at different delays as well as the DFWM spectra at a fixed delay, as I1 is increased. We find that the dephasing rate, after an initial increase with I1, becomes independent of I1 for large I1. The spectrally integrated DFWM signal, which probes the coherent polarization of the resonantly excited QWs, initially increases with I1, reaches a peak and then decreases as 1/(root)I1 at large I1. The results are compared with theoretical models of exciton coherence.
In degenerate four-wave-mixing (DFWM) experiments the polarization of the incident laser pulses can have a strong influence on the measured signal. For GaAs quantum wells a number of DFWM experiments were performed by using either parallel or perpendicular polarization of the incident laser pulses. Experimentally, it turned out that some characteristic features of the signal differ appreciably for the two configurations. In particular, different dephasing times, phase shifts of the quantum beats, different time-resolved behavior (echoes versus free-induction decay), and intensity dependence of the measured signal have been reported.1–3