Abstract

We describe the femtosecond optical gain nonlinearities for the unusual case of electrons which are distributed between the direct-GaAs and indirect-AlAs layers in a type-II GaAs/AlAs multiple quantum-well (MQW) structure. We are able to vary the magnitude of the optical gain using a longitudinal static electric field to control the distribution of electrons bewteen the GaAs and AlAs layers. In type-II MQWs electrons initially optically excited in the GaAs layers transfer on a subpicosecond timescale to the lower lying energy level in the AlAs layers,¹ resulting in electrons and holes separated in both real and reciprocal spaces. By applying an electric field in the growth direction of the MQWs we decrease the difference between the indirect- and direct-electron energy levels. The presence of the electric field redistributes the electrons toward the GaAs layer, and consequently, increases the magnitude of the optical gain. Near the indirect-to-direct crossover region, we observe ultrafast optical gain and absorption nonlinearities that replicate those observed for type-I GaAs/Al_0.45Ga_0.55As QWs,² whereas in flat band (no applied field; strictly type-II) we observe the recently reported femtosecond optical nonlinearities for type-II MQWs.³

© 1991 Optical Society of America