Abstract

We use differential spectroscopy with tunable psec pulses to temporally and spectrally resolve the formation and decay of optical nonlinearities and space-charge fields in a hetero n-i-p-i that contains quantum wells in the intrinsic region composed of all-binary InAs/GaAs short-period strained layer superlattices. The evolution of the resulting nonlinearity is determined by a delicate competition between excitonic bleaching and the excitonic shift caused by a screening of the builtin field. The relative contributions of the two nonlinearities are complicated functions of fluence, time and wavelength, with the detailed dynamics determined by thermionic emission from the wells, psec charge transport over nm dimensions, screening, and carrier recombinations. The distinctive spectral signatures for excitonic bleaching and shifting allow us to distinguish the two contributions to the nonlinear dynamics and to follow the charge transport and space-charge formation. At the lower fluences, the initial excitonic bleaching gives way to a blue shift of the exciton as the carriers escape the wells in ~3 psec and drift to screen the built-in field in <10 psec. This blue shift persists until the spatially separated carriers recombine nonexponentially on microsecond time scales. At higher fluences, excitonic bleaching and the blue shift are observed simultaneously, since only a fraction of the carriers are required to screen the field and the wells remain partially occupied. On the time scale of ~10 nsec, the bleaching contribution disappears as the carriers within the wells recombine, leaving only the persistent blue shift.

© 1991 Optical Society of America