Picosecond optical nonlinearity in lead chalcogenide semiconductors.

Optical nonlinearities of PbSe are studied in picosecond pump-probe experiments at wavelengths from 4 to 8 micrometer. Excitation of an electron-hole plasma leads to a shift of the absorption edge to higher energies and to large changes of the refractive index by up to -0.1. The optical nonlinearities result from band filling by both electrons and holes. The time-dependent absorption change shows a partial recovery within 100 ps, giving evidence of enhanced recombination rates. Theoretical calculations of the absorption change are in agreement witht the experimental data.