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 CdS0.75Se0.25 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.