Nonlinear Optical Response of Piezoelectric Multiple Quantum Wells Under Bias

Strained multiple quantum wells (MQWs) that are grown on [111]-oriented substrates have huge intrinsic piezoelectric fields along the growth directions and are attractive for a number of novel nonlinear optical, electronic and electro-optical device applications (e.g. blue shifting SEEDs1). Unfortunately, until recently, very little was known about the nonlinear properties of these piezoelectric MQWs. The existence2 of piezoelectric fields in strained [111]-oriented InGaAs/GaAs QWs and the screening3 of these fields by photoexcited carriers had been demonstrated. In addition, the nonlinear responses of piezoelectric MQW structures to steady-state4 and pulsed5 optical excitations had been reported. However, prior to our recent work, the shot-to-shot accumulation of carriers (which are long-lived because they are spatially separated) had prevented the time resolution of the nonlinear optical and carrier dynamics in piezoelectric MQWs. In the last few months, by placing piezoelectric MQWs in p-i-n structures that were carefully designed to reduce the carrier lifetime, we have finally begun to temporally resolve the nonlinear response. For example, we have been able to definitively demonstrate that the bandstructure of our samples are mechanically clamped, not free; that the nonlinear response is dominated by long range screening of the in-well piezoelectric field by carriers that have escaped the wells and not by in-well screening as originally proposed,6 and that the percarrier nonlinear response is more than an order of magnitude smaller than that in [100]-oriented structures, not an order of magnitude larger as originally purported.4