Abstract

Previous measurements in a n-i-p-i structure with InAs/GaAs superlattice quantum wells in the i regions seemed to indicate that the field-screening nonlinearity saturates at low intensities I ~ 5 W/cm². We present evidence here that carriers that are "stuck" in the wells near this intensity can escape beyond I ~ 15 W/cm² so that field screening turns on again. We used a sample grown by migration enhanced epitaxy and having 12 periods of n-i-p-i doped GaAs with three InAs/GaAs quantum wells in each i region. The quantum wells were periods of a superlattice grown to be (InAs)₂(GaAs)₅. The spectrum of the nonlinear transmission, ΔT/T, was measured by using 50-s pulses from a cw Ti-sapphire laser tuned near the exciton absorption peak at 970 nm. For I = 0.1 W/cm² to I = 103 W/ cm² the exciton absorption peak has a blue shift, compatible with photo-carrier field screening. The magnitude of ∆T/T increases with light intensity in a nonlinear way, with plateaus near I = 5 W/cm² and I = 150 W/ cm². We will show that this intensity dependence reflects the change in the ability of carriers to escape the wells as the built-in field decreases with increasing intensity.

© 1992 Optical Society of America