Nonlinear absorption of laser radiation in InP and in GaAs

Nonlinear absorption of light in semiconductor materials with wide forbidden band is an important factor determining the feasibility of light emitting semiconductor devices based on two photon excitation. From this standpoint, an experimental study was made of nonlinear absorption in weakly laser irradiated InP and GaAs. An evaluation of the results, with interpretation based on theoretical relations for optical density and depthwise distribution of photon density, reveal that the former increases linearly in GaAs and not quite linearly in InP with increasing excitation intensity while the reflection coefficient remains constant. There is also a linear relation between the reciprocal of transmitted photon density and the reciprocal of incident photon density. With the light intensity decreasing penetration depth, there appears a linear component of absorption characteristic of one photon absorption as well as a nonlinear component. The latter is evidently attributable to other mechanisms, the most likely one being two photon absorption with attendant absorption by free charge carriers or through impurity levels of various depths.