Femtosecond time-resolved Z-scan investigations of optical nonlinearities in ZnSe

Degenerate (single-color) and nondegenerate (two-color) time-resolved Z-scan techniques, with femtosecond time resolution, have been implemented to measure the magnitude and dynamical processes of the optical nonlinearities of ZnSe. In the time-resolved Z-scan spectra a reversal of sign in the refractive-index change Δn as a function of time delay was observed. A bound electronic contribution with a resolution limited response (∼200 fs) gave rise to a positive Δn at zero time delay, whereas a free-carrier contribution resulted in a negative Δn at longer time delays. The free-carrier contribution to Δn has two initial rapid relaxation components with time constants of a few picoseconds and a few tens of picoseconds, respectively. The long-lived carriers also contributed to a slow relaxation component in Δn on a nanosecond time scale.