Low Power Optical Bistability near Bound Excitons in Cadmium Sulfide

Abstract

Direct gap semiconductors exhibit some of the largest optical nonlinearities measured in condensed media. Using the resonance enhancement of the material nonlinearity near the band gap, very large nonlinear phase shifts can be accumulated over micron size distances and large changes of the absorption coefficient can also be observed at relatively low incident power. For instance, the saturation of free excitons and the associated generation of free carriers following near-band gap excitation in bulk GaAs1 and InSb2 or in GaAs/AlGaAs superlatticeslc,3 have been used to demonstrate optical bistability on the 10 mW power scale. The response time of these optical bistable devices is typically tens of nanoseconds and depends directly on the carrier recombination lifetime. Rather than using the extended states (free excitons and free carriers) of a semiconductor, as is usually done, we propose here a completely different approach for demonstrating optical bistability. We would like to use those bound states of a direct gap semiconductor which have a large oscillator strength. In particular, we have studied the saturation behavior of the I2 bound exciton in cadmium sulfide (CdS), which is an exciton bound to a neutral donor with a binding energy of about 8 meV.4