Femtosecond time-resolved refractive index changes in CdS0.75Se0.25 and CdS

A pump-probe, light by light deflection technique employing 120 fs pulses at λ=620 nm has been used to measure the temporal and irradiance (I) dependence of the refractive index (n) in 100-μm-thick single crystals of CdS0.75Se0.25 and CdS at 295 K. For I<3 GW/cm2 only instantaneous negative changes in n are observed and are attributed to Stark and two-photon resonance effects while for higher I negative changes with onset times of approximately 2 ps are attributed to the cooling of two-photon generated carriers. For I≳30 GW/cm2 a partial recovery of n occurs within 10 ps and is attributed to possible threshold dependent recombination processes. The variation of probe-beam deflection with pump irradiance has been determined for different probe delays and related to two photon absorption of the pump beam.