Lattice dynamics in two-photon-excited CdS studied by picosecond time-resolved X-ray diffraction

Lattice dynamics and radiative processes in single-crystal cadmium sulfide induced by two-photon excitation with a femtosecond laser are investigated. The development of lattice expansion is directly observed by picosecond time-resolved X-ray diffraction. The obtained lattice dynamics are explained on the basis of a thermally induced impulsive-strain model. The model calculation indicates that two- and more-photon absorption processes occur and that reflectivity rapidly increases under laser irradiation. In photoluminescence spectroscopy, the spectra for TW cm⁻² excitation are shifted to lower energy and show an additional shoulder at 2.35 eV. Furthermore, emission due to Fabry-Perot laser modes with self-formed cavities was observed under 11 TW cm⁻² excitation. The discrepancy between carrier densities deduced from the lattice expansion and the PL spectra indicates that the predominant process at a higher carrier density is not radiative recombination, but Auger recombination followed by lattice heating.