Abstract
A comprehensive investigation of exciton optical orientation and alignment in type-II GaAs/AlAs(001) superlattices in longitudinal and transverse magnetic fields is presented. We observe the previously predicted type-II orientation-to-alignment conversion induced by a longitudinal field due to the anisotropic exchange splitting of the localized-exciton radiative doublet. A theory of polarized exciton photoluminescence under polarized excitation is developed, that takes into account the fine structure of the excitonic quartet, the exciton spin relaxation, and the difference in the lifetimes of electric-dipole-active and inactive states. By comparing experimental and theoretical curves, the main parameters characterizing the level splitting, the recombination rates, and the spin relaxation of localized excitons are deduced. Field-induced polarization conversion provides an effective method to measure the interface preference, i.e., the difference between the fractions of excitons localized on AlAs/GaAs and GaAs/AlAs interfaces. We have also observed the field-induced transformation from [100] to [110] linear polarization, which is direct evidence of exciton cascade kinetics.
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