Exciton Coherence in GaAs Quantum Wells Probed by Four Wave Mixing Measurements at High Excitation Intensities

摘要

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 (I_1) 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 I_1 is increased. We find that the dephasing rate, after an initial increase with I_1, becomes independent of I_1 for large I_1. The spectrally integrated DFWM signal, which probes the coherent polarization of the resonantly excited QWs, initially increases with I_1, reaches a peak and then decreases as 1/I_1~(1/2) at large I_1. The results are compared with theoretical models of exciton coherence.