EXCITON QUANTUM BEATS IN TYPE-II GAAS/ALAS SUPERLATTICES IN LONGITUDINAL AND IN-PLANE MAGNETIC FIELDS

We report on the observation of quantum beats in the time-resolved exciton photoluminescence of type-II GaAs/AlAs superlattices in a magnetic field. The fine structure of excitons is obtained from the period of the quantum beats between the radiative states. At zero field, the degeneracy of the heavy exciton radiative doublet is lifted owing to the interface-induced mixing of heavy- and light-hole states. In a field parallel to the growth direction an increase of the splitting energy of the doublet is observed. The exciton g factor is determined. In an in-plane field the four heavy-exciton sublevels can be coherently excited. Depending on the incident polarization and field value, one or several periods of the quantum beats are observed. The experimental results are analyzed within the framework of the density-matrix formalism. The electron transverse g factor and the splitting energy of the nonradiative states are obtained. Exciton quantum beats are observed not only with direct exciton creation but also with band-to-band excitation. In the latter case we discuss the conditions of observation of the quantum beats with respect to the spin-relaxation times of the initially created electron-hole pairs.