MAGNITUDE, ORIGIN, AND EVOLUTION OF PIEZOELECTRIC OPTICAL NONLINEARITIES IN STRAINED [111]B INGAAS GAAS QUANTUM-WELLS

The magnitudes of excitonic nonlinearities were compared at 12 K in In GaAs/GaAs multiple quantum well structures with growth directions orie nted along the [100] and [111] crystal axes by measuring both the stea dy-state and time-resolved differential transmission spectra. As expec ted, the spectra for the [100] sample are indicative of excitonic blea ching at all times and for all excitation levels, and a carrier recomb ination time of 0.8 ns and a nonlinear cross section (change in absorp tion coefficient per carrier pair) of approximately 8 X 10(-14) CM2 ar e extracted for the [100] sample. By comparison, for low excitation le vels, the spectra for the [111] sample are consistent with a blueshift of the exciton, indicating a screening of the strain-induced piezoele ctric field. At higher excitation levels, the spectra are dominated by excitonic bleaching. Under identical 1 ps pulsed excitation condition s, the magnitudes of the changes in the absorption coefficient caused by screening in the [111] sample are comparable to those measured for bleaching in the [100] sample. By contrast, the steady-state changes i n the absorption coefficient caused by screening in the [111] sample a re an order of magnitude larger than the changes caused by bleaching i n the [100] sample. It was demonstrated that the larger steady-state r esponse for the [111] sample is caused by carrier accumulation over th e longer (density-dependent) lifetime for that sample and that it is n ot the result of a larger nonlinear cross section. The slow, nonexpone ntial, density-dependent recombination rates measured for the [111] sa mple are consistent with carrier escape and drift to screen the entire multiple quantum well structure and are not consistent with screening within the individual quantum wells.