Orientation and doping dependence of two-photon absorption in semiconductors

Abstract

Considerable interest has been shown in measurements of nonlinear optical effects in semiconductors using picosecond pulses. Carrier diffusion, photorefractive effects, nonlinear pulse propagation, and transient energy transfer, for example, have all been investigated on these time scales. Such studies often involve semiconductor samples having deep (subbandgap) defect states, whether introduced intentionally, e.g., by doping, or otherwise, which are resonant or near-resonant with the photon energy of the exciting pulses. Such studies may also require the use of a specific crystal orientation. An accurate knowledge of two-photon absorption coefficient β is needed to describe and model these interactions. However, several other nonlinear absorptive processes are possible in these circumstances, in particular saturation of the deep level absorption via single photon excitation and stepwise two-photon absorption via the defect levels. We describe single-beam nonlinear transmission measurements in a variety of semiconductor samples; GaAs:EL2, GaAs:Cr, GaAs:Si, CdTe:V, InP:Fe, and undoped GaAs and CdTe in several orientations. By using 5-ps pulses at 960 nm of fluence up to 2 mJ cm−2, provided by a styryl 13 synchronous dye amplifier system, we work in a regime where the virtual two-photon absorption process dominates. Values of β are thus extracted unambiguously. Several of these samples have demonstrated photorefractive beam coupling at this wavelength and pulse duration. The β-values are thus of both fundamental interest and important in analyzing those experiments.