OPTICAL NONLINEARITIES AND ULTRAFAST CHARGE TRANSPORT IN ALL-BINARY INAS/GAAS STRAINED HETERO N-I-P-IS

We use picosecond differential spectroscopy to temporally and spectrally resolve the formation and decay of nonlinearities and space-charge fields in a hetero n-i-p-i that contains quantum wells in the intrinsic regions that are composed of all-binary InAs/GaAs short-period strained-layer superlattices. The evolution of the optical response is determined by competition between excitonic bleaching and the excitonic shift caused by screening of the built-in electric field of the n-i-p-i. The relative contributions of the two resulting optical nonlinearities are complicated functions of fluence, time, and wavelength, with the detailed dynamics determined by thermionic emission from the wells, picosecond charge transport over nanometer dimensions, screening, and recombination. At low fluences, excitonic bleaching is the source of an ultrafast nonlinear response that can be turned on and off in < 10 ps. This initial excitonic bleaching gives way to a blue shift of the exciton as the carriers escape the wells in approximately 3 ps and drift to screen the built-in field in < 10 ps. The blue shift persists until the 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 approximately 10 ns, the bleaching contribution disappears as the carriers within the wells recombine, leaving only the persistent blue shift.