Ultrafast nonlinear optical properties of semiconductors in low dimension

点击翻译按钮获取中文摘要

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 nonthermalized 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. From the ratio of nonthermalized to the total carriers, the thermalization time was estimated to be 20-40 fs.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.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 Frohlich interaction within an order of magnitude.Resonant coherent lattice vibrations in a BiI_3 layered crystal are observed around the indirect absorption edge by femtosecond pump and probe measurements. Coherent optical phonons that are impulsively excited by femtosecond pump pulses modulate the phase of probe pulses. The corresponding shift of the probe spectrum swings with the period of phonon oscillation, which continues for more than 100 periods. We speculate that the coherent phonon-assisted indirect exciton transitions have also contributed to the data.