Microwave modulation of circularly polarized exciton photonluminescence in GaAs/AlAs multiple quantum wells

This study describes microwave (mw) modulated circularly polarized photoluminescence (MPL) of the inhomogeneously broadened (e1:hh1)1S exciton, in undoped GaAs/AlAs multiple quantum wells, at T=1.4 K. A circularly polarized excitation creates spin-oriented excitons and free carriers, and the latter are heated by the absorption of mw radiation. The degree of PL circular polarization (${\mathit{P}}_{\mathrm{cir}}$) and of the MPL signal (Δ${\mathit{P}}_{\mathrm{cir}}$) is measured under a magnetic field, applied normal to the quantum well plane. This experimental procedure enables us to investigate the influence of the spin-oriented mw-heated free electrons on the exciton transfer among the localized exciton sites. The modulated spectrum resolves the inhomogeneously broadened PL band into a fine structure corresponding to different exciton localizations: delocalized, localized, and impurity-bound excitons. Each resolved MPL band has a distinct intensity dependence on the mw power and on the magnetic-field strength, which is due to the different exciton trapping rate and the restricted electron motion (induced by a magnetic field). We studied the effect of the magnetic field on the degree of MPL polarization (Δ${\mathit{P}}_{\mathrm{cir}}$) of the various excitons, and found the MPL of delocalized excitons to be completely unpolarized, that of localized excitons partially polarized, and of impurity-bound excitons completely polarized. The observation of Δ${\mathit{P}}_{\mathrm{cir}}$∼1 for strongly localized excitons is explained by a preferred exciton localization on acceptors that were neutralized by spin-oriented holes. The other observed Δ${\mathit{P}}_{\mathrm{cir}}$ values indicate that the change in the exciton spin orientation, induced by the hot-carrier–exciton interaction, depends on their mutual spin orientation. © 1996 The American Physical Society.