Reversible photomodification of LiNbO₃ and LiTaO₃ by femtosecond laser pulses

We present experimental and theoretical study of refractive index modification induced by femtosecond laser pulses in photorefractive crystals. The single pulses with central wavelength of 800 nm, pulse duration of 150 fs, and energy in the range of 6-130 nJ, tightly focused into the bulk of Fe-doped LiNbO₃ and stoichiometric LiTaO₃ crystals induce refractive index change of up to about 10^-3 within the volume of about (2,0 × 2,0 × 8,0) μm³. The photomodification is independent of the polarization orientation with respect to the crystalline c-axis. The recorded region can be erased optically by a defocused low-intensity single pulse of the same laser. Recording and erasure can be repeated at the same position many times without loss of quality. These findings demonstrate the basic functionality of the ultrafast three-dimensional all-optical rewritable memory. Theoretically they are interpreted by taking into account electron photogeneration and recombination, as well as formation of a space-charge field. The presented analysis indicates dominant role of photovoltaic effect for our experimental conditions, and suggests methods for controlling various parameters of the photomodified regions. Rewritable optical memory,.