Dynamics and spin relaxation of excitons in GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As quantum wells

We present a study of the optical spin orientation and optical alignment relaxation of (e1:hh1)1S excitons in GaAs/${\mathrm{Al}}_{0.33}$${\mathrm{Ga}}_{0.67}$As narrow quantum wells at low temperatures. The photoluminescence was excited by ps pulses within the (e1:hh1) exciton band and its decay, as well as that of the degrees of the circular and linear polarizations [${\mathit{P}}_{\mathrm{cir}}$(t) and ${\mathit{P}}_{\mathrm{lin}}$(t)] were measured. The observed decay curves are analyzed by a model based on the in-plane wave-vector (${\mathbf{K}}_{\mathrm{\parallel }}$) dependence of the energy (${\mathrm{\tau }}_{\mathit{a}}$), spin (${\mathrm{\tau }}_{\mathrm{cir}}$), and alignment (${\mathrm{\tau }}_{\mathrm{lin}}$) relaxation times of delocalized excitons. These were found to vary in the range of 120<${\mathrm{\tau }}_{\mathit{a}}$<230 ps, 50>${\mathrm{\tau }}_{\mathrm{cir}}$>30 ps, and 15>${\mathrm{\tau }}_{\mathrm{lin}}$>5 ps for 0.019 A${\mathrm{̊}}^{\mathrm{-}1}$<${\mathit{K}}_{\mathrm{\parallel }}$<0.032 A${\mathrm{̊}}^{\mathrm{-}1}$. The model fitting is not sensitive to the relaxation times of the ${\mathit{K}}_{\mathrm{\parallel }}$=0 state, which are very long: 1000>${\mathrm{\tau }}_{\mathrm{lin}}^0$,${\mathrm{\tau }}_{\mathrm{cir}}^0$>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 ${\mathrm{\tau }}_{\mathrm{cir}}$ and ${\mathrm{\tau }}_{\mathrm{lin}}$.