Dynamics and spin relaxation of excitons in GaAs/AlxGa1-xAs quantum wells

We present a study of the optical spin orientation and optical alignment relaxation of (e1:hh1)1S excitons in GaAs/Al0.33Ga0.67As narrow quantum wells at low temperatures. The photoluminescence was excited by ps pulses within the (e1:hh1) exciton band and its decay, as well as that of the degrees of the circular and linear polarizations [Pcir(t) and Plin(t)] were measured. The observed decay curves are analyzed by a model based on the in-plane wave-vector (K) dependence of the energy (τa), spin (τcir), and alignment (τlin) relaxation times of delocalized excitons. These were found to vary in the range of 120<τa<230 ps, 50>τcir>30 ps, and 15>τlin>5 ps for 0.019 A-1<K<0.032 A-1. The model fitting is not sensitive to the relaxation times of the K=0 state, which are very long: 1000>τlin0, τcir0>300 ps. We further assume that the hole spin-relaxation rate is much faster than that of the electron. This allows us to estimate the relative contribution of each of these particles' spin-relaxation rate to τcir and τlin.