Observation of Interband Two-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 saturation behavior with a saturation intensity of 65 GW/c${\mathrm{m}}^2$. This observation is possible because the optical pulse duration is shorter than the 350 fs thermalization time of the photocreated carriers in their bands that we measured. From the thermalization time an averaged carrier–LO-phonon interaction duration is deduced which is of the order of 25 fs. Examination of the band structure and the electronic state density of CdS shows that this saturation is due to the electron population rather than to the hole population.