Femtosecond optical nonlinearities under resonant excitation of excitons in CdSe

Femtosecond pump-and-probe spectroscopy was utilized to study the optical nonlinearities of excitons in CdSe. 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 a rather unique observation 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. 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 emitting LO phonons. The observed time constant agrees with the calculated scattering time based on the Frohlich interaction within an order of magnitude.