Separation of bound and free carrier contributions to the refractive index change induced in II-VI semiconductors by femtosecond pulses

Time-resolved probe beam deflection and degenerate four-wave mixing (DFWM) have both been used to determine the temporal evolution of the refractive index changes induced in ZnSe and CdS 0.75 Se 0.25 single crystals at T=300 K by lambda =620 nm, 120 fs pulses with irradiances (I) up to 500 GW cm -2 . Negative refractive index changes which follow the pump pulse profile are attributed to virtual electronic transitions and two-photon resonances, while two-photon-generated hot carriers and their cooling rate determine the magnitude and temporal evolution of the refractive index change for probe delays up to 4 ps. No further evolution is observed up to 100 ps unless I>30 GW cm -2 , in which case the refractive index appears to recover on a 10 ps timescale following carrier relaxation: the possible role of a threshold dependent recombination process in this recovery is discussed.