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

We report on the observation of quantum beats in the time-resolved exc iton photoluminescence of type-II GaAs/AlAs superlattices in a magneti c field. The fine structure of excitons is obtained from the period of the quantum beats between the radiative states. At zero field, the de generacy 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 o f the doublet is observed. The exciton g factor is determined. In an i n-plane field the four heavy-exciton sublevels can be coherently excit ed. Depending on the incident polarization and field value, one or sev eral periods of the quantum beats are observed. The experimental resul ts are analyzed within the framework of the density-matrix formalism. The electron transverse g factor and the splitting energy of the nonra diative states are obtained. Exciton quantum beats are observed not on ly with direct exciton creation but also with band-to-band excitation. In the latter case we discuss the conditions of observation of the qu antum beats with respect to the spin-relaxation times of the initially created electron-hole pairs.