Femtosecond laser interaction with photo-refractive crystals: High frequency field and internal field contributions to the refractive index changes

A femtosecond-laser interaction experiment with a high laser intensity of TW/cm² that is close to the optical breakdown threshold for dielectrics is performed.The high intensity results in fast excitation of large number of free carriers (10²⁰-10²¹cm^-3) followed by the diffusion, recombination and re-trapping of charges. These effects are responsible for the short time of the build-up of the electric field of charge separation and for the high rate of refractive index change. The amplitude of laser electric field at TW/cm² intensity constitutes 27.46 MV/cm that is two orders of magnitude larger than the photovoltaic field or the field of spontaneous polarisation in ferroelectrics. The amplitude of the electrons oscillations in such a field is ~2-3 times larger then shift of Li (0.9 A) and Nb (0.5 A) responsible for spontaneous polarisation in LiNbO₃. Therefore the predominance of the photovoltaic effect and even survival of the ferroelectric state under such perturbation is questionable and is the subject of discussion.