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 GaAs/AlAs type-II multiple quantum-well (MQW) structure. Due to the spatial separation of electrons from the holes, we observe a significant increase in the gain lifetime (≳150 ps) in type-II MQWs, as compared to type -I MQWs (∼50 ps). In addition, we investigate the effect of a longitudinal electric field on Γ-X energy splitting in type-II structures. Finally, at early times we observe an ultrafast nonlinear optical response in the gain/absorption spectra which we attributed to electron-hole scattering without carrier loss.
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