Long-lived polarized photoluminescence from separately localized e-h pairs in GaAs/AlxGa1-xAs quantum wells

We study the polarization of the long-lived component of the band-edge photoluminescence in undoped GaAs/AlxGa1-xAs multiple quantum wells at low temperatures. A high degree of both linear and circular polarizations (Plin and Pcir) persists over more than 100 nsec and is attributed to separately localized e-h pairs. We show that the long-lived Plin 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 Plin and Pcir on excitation energy indicates that during energy relaxation the e-h pairs preserve their spin orientation but their linear polarization is destroyed.