OBSERVATION OF INTERBAND 2-PHOTON ABSORPTION SATURATION IN CDS

When using 100 fs light pulses with 2 eV photon energy, the degenerate two-photon absorption process in bulk wurtzite CdS exhibits a saturat ion behavior with a saturation intensity of 65 GW/cm(2). This observat ion is possible because the optical pulse duration is shorter than the 350 fs thermalization time of the photocreated carriers in their band s that we measured. From the thermalization time an averaged carrier-L O-phonon interaction duration is deduced which is of the order of 25 f s. Examination of the band structure and the electronic state density of CdS shows that this saturation is due to the electron population ra ther than to the hole population.