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

Abstract

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