ULTRAFAST OPTICAL NONLINEARITIES OF TYPE-II ALXGA1-XAS/ALAS MULTIPLE QUANTUM-WELLS

We present experimental studies of the transient optical nonlinearities near the direct band gap of type II AlxGa1-xAs/AlAs multiple quantum wells. The response of the optical nonlinearity at the band-gap excitonic transition is critically dependent on laser photon energy. A complete recovery of the bleached absorption in the lower part of the excitonic resonance with a picosecond time constant at room temperature occurs due to an enhanced collision broadening of the excitonic resonance after the fast spatial Γ-X transfer. Our results demonstrate that the intrinsic response of optical devices based on exciton nonlinearities can be much faster in type II structures as compared to type I structures.