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 InGaAs/GaAs multiple quantum well structures with growth directions oriented along the [100] and [111] crystal axes by measuring both the steady-state and time-resolved differential transmission spectra. As expected, the spectra for the [100] sample are indicative of excitonic bleaching at all times and for all excitation levels, and a carrier recombination time of 0.8 ns and a nonlinear cross section (change in absorption coefficient per carrier pair) of approximately 8 X 10(-14) CM2 are extracted for the [100] sample. By comparison, for low excitation levels, the spectra for the [111] sample are consistent with a blueshift of the exciton, indicating a screening of the strain-induced piezoelectric field. At higher excitation levels, the spectra are dominated by excitonic bleaching. Under identical 1 ps pulsed excitation conditions, 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 in the absorption coefficient caused by screening in the [111] sample are an order of magnitude larger than the changes caused by bleaching in the [100] sample. It was demonstrated that the larger steady-state response for the [111] sample is caused by carrier accumulation over the longer (density-dependent) lifetime for that sample and that it is not the result of a larger nonlinear cross section. The slow, nonexponential, density-dependent recombination rates measured for the [111] sample 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.