Abstract

We report on an experimental study of the origin of the nonlinearities and the size of the nonlinear index at various wavelengths close to the exciton resonance and the material band gap. It is concluded that for intensities required for optical bistability, the nonlinearity is mainly excitonic for MQWs with well sizes W of 50 and 152 Å which are either smaller or slightly larger than the exciton radius a. For 299-Å MQW, (W ≃ 2a), where the MQW behaves more like bulk but still has distinct excitonic features, the nonlinearity is due to the combination of exciton screening, band filling, and screening of the continuum states. For bulk GaAs, the effect of exciton is comparatively small and the latter two effects are responsible almost equally for the change in the refractive index. The measured nonlinear indices are rather similar for all these samples with the bulk GaAs and 299-Å MWQ exhibiting slightly larger Δn (change in index) for similar carrier densities (or for similar light intensities). The 50-Å MQW has a slightly smaller value of nonlinear index because the nonlinearity which in this sample is due to the saturation of the exciton resonance occurs at higher excitation intensities due to larger exciton binding energy.

© 1986 Optical Society of America