FEMTOSECOND STUDIES OF EXCITONIC OPTICAL NONLINEARITIES IN GAAS ALXGA1-XAS MULTIPLE-QUANTUM WELLS UNDER INPLANE UNIAXIAL STRAIN/

The pump-probe technique was employed in the study of excitonic optica l non-linearities at 40 K in a GaAs/Al0.1Ga0.9As multiple quantum well under thermally induced in-plane uniaxial strain. We report the first observation of an ultrafast large-angle polarization rotation of the probe light effected by the bleaching of the anisotropic absorption ne ar the heavy hole exciton peak in this material. Dynamic rotations as large as 42-degrees have been measured for a compressive strain of app roximately 0.2%. In addition, a dependence of the phase space filling produced by resonantly excited excitons on the orientation of the pump polarization vector has been observed which decays with a time consta nt of 0.5 ps. This decay is attributed to the ionization of excitons i nto free electron-hole pairs. The subsequent decline of the probe rota tion is governed by the removal of these carriers from the wells.