DYNAMIC CONTRIBUTIONS TO THE OPTICAL STARK-EFFECT IN SEMICONDUCTORS

We Present femtosecond time-resolved optical pump-probe measurements o f the coherent excitonic nonlinear optical response in GaAs quantum we lls. The data exhibit features that cannot be explained by the convent ional description of the optical Stark effect as a rigid blueshift of the exciton absorption spectrum when the pump field is present. Using the semiconductor Bloch equations we develop a model that permits the pump-induced changes in the probe transmission to be calculated for ar bitrary laser pulse profiles, both chirped and unchirped. The results demonstrate that the amplitude and phase spectral structure of the pro be strongly influence the detection signal when the pulses are ultrafa st (i.e., faster than the polarization decay time), in marked contrast to the normal assumptions made in transient optical spectroscopy. We examine these dynamic contributions in terms of four-wave mixing of di fferent frequencies in the probe spectrum, and show that in addition t o the optical Stark shift normally measured, there is an additional co mponent arising from dynamic changes in the refractive index due to re sonantly enhanced cross-phase modulation.