Long-lived polarized photoluminescence from separately localized e-h pairs in GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As quantum wells

We study the polarization of the long-lived component of the band-edge photoluminescence in undoped GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As multiple quantum wells at low temperatures. A high degree of both linear and circular polarizations (${\mathit{P}}_{\mathrm{lin}}$ and ${\mathit{P}}_{\mathrm{cir}}$) persists over more than 100 nsec and is attributed to separately localized e-h pairs. We show that the long-lived ${\mathit{P}}_{\mathrm{lin}}$ is not due to optical alignment but to resonant excitation and subsequent emission of e-h pairs oriented parallel to the light polarization. The spectral dependence of ${\mathit{P}}_{\mathrm{lin}}$ and ${\mathit{P}}_{\mathrm{cir}}$ on excitation energy indicates that during energy relaxation the e-h pairs preserve their spin orientation but their linear polarization is destroyed.