Picosecond measurements of absorptive and refractive optical nonlinearities in GaP at 532 nm

Picosecond time-resolved pump-probe and Z-scan measurements are used to investigate the nonlinear refraction and absorption associated with intense picosecond excitation above the indirect band edge of GaP. The pump-probe results reveal both an instantaneous absorption feature, which is consistent with two-photon absorption, and a long-lived feature that is associated with the generation of free carriers by linear indirect absorption. Z scans conducted with pulses of 25- and 95-ps duration indicate that, for these pulse widths, the nonlinear refraction in GaP is dominated by the linear generation of free carriers and that the role of the electronic Kerr effect is negligible. The measured free-carrier refraction is compared with the value predicted from a simple model based on band filling. This model predicts that the primary contribution to the index change at the applied optical frequency arises from holes blocking direct transitions near the zone center and that similar blocking of indirect transitions has a negligible effect on the index.