Abstract

We report the first time-resolved circular dichroism (CD) studies of nonthermal carrier distributions in GaAs quantum wells. Specifically, we excite a nonthermal carrier distribution at small (30 meV) excess energy using circularly polarized 100 fs light pulses, and measure the difference between the absorption spectrum obtained for right and left hand circular polarization, as a function of time. Previously, it has been shown that when nonthermal distribution are excited in quantum wells, the screening nonlinearities of the two-dimensional (2D) exciton states are accompanied by a phase-space filling (PSF) component that evolves as the carriers thermalize.^[1] The PSF component measures the distribution of occupied momentum states, so that its separate measurement is important for an understanding of the dynamics of thermalization. We show that the circular dichroism response to a screening interaction is zero, whereas the response to phase space filling is large, making separation of these two effects possible. Carriers thermalize in 2D quantum well states within 100 fs, inducing PSF nonlinearities in the excitons as they scatter to the bandedge; zero dimensional (0D) states, however, remain in a nonthermal distribution for 100 picoseconds, demonstrating for the first time the dramatic slowing of the thermalization process in a 0D system.

© 1992 IQEC