Separation of optical Kerr and free-carrier nonlinear responses with femtosecond light pulses in LiNbO3 crystals

Time-resolved four-wave pump-probe experiments with femtosecond light pulses, including readout of the fundamental K grating and also of the 2K grating, allow to separate the free-carrier nonlinearity caused by two-photon excitation from the instantaneous optical Kerr nonlinearity. The method is applicable to wide-gap optical materials. For lithium niobate crystals, we show, using 200 fs pulses, that the buildup of the free-carrier grating adiabatically follows the absorbed energy. This finding and the results of modeling lead us to the conclusion that the relaxation (thermalization) time of the photoexcited band carriers is shorter than 100 fs.