Ultrafast pump-and-probe spectroscopy in CdSe: Hot-carrier and exciton dynamics

An extensive study by means of femtosecond pump-and-probe spectroscopy has clarified the ultrafast dynamics of photogenerated carriers in a CdSe thin film. Under band-to-band excitation, a nonther- malized hot-carrier distribution was observed immediately upon excitation. This distribution was observed in a 200-meV broad energy range extending from the pump energy to its low-energy side. Its low-energy tail indicates that carrier-LO-phonon scattering competes with carrier-carrier scattering in the thermalization process. From the ratio of nonthermalized to the total carriers, the thermalization time was estimated to be 20–40 fs. In the wake of thermalization, cooling of photogenerated carriers was observed. With an increase of the excitation density up to 1019 cm-3, the carrier cooling rate was slower. The cooling rate observed at 4.2 K was comparable to that at room temperature. The observed cooling rate was slower than that predicted by a theoretical calculation which took into account screening of the carrier-phonon interaction. This is ascribed to hot-phonon effects of LO-phonons and TO phonons. Under resonant excitation of the A exciton, a 42-meV energy broadening of the B-exciton structure was observed. This broadening is caused by collisions between the A exciton and the B exciton. This is an example of the scattering process between different kinds of excitons. Analysis indicates that the A-exciton–B-exciton scattering time is 31 fs. This time is well explained by a simple theory based on the rigid-sphere scattering model. The broadening decreases with a time constant on the order of hundreds of picoseconds. This time constant is well explained by the lifetime of the excitons. Under resonant excitation of the B exciton, a fast recovery of the bleaching was observed at the B exciton. The recovery time of 0.9 ps is explained as the transformation time of B excitons into A excitons by LO-phonon emission. The obtained time constant agrees with the calculated scattering time based on the Fröhlich interaction within an order of magnitude