Degenerate four-wave mixing in KNbO(3): picosecond and photorefractive nanosecond response

Single 75-ps-long pulses at a wavelength of 532 nm are used to write phase gratings in KNbO(3) crystals. Two grating buildup processes are resolved. The faster and generally weaker one is instantaneous on a time scale of the pulses used and is attributed to intrinsic third-order processes. In a reduced crystal that shows preferentially electronic charge transport and that is oriented to show a space-charge-induced photorefractive effect, a higher diffraction builds up in some nanoseconds after the illumination of the crystal. The rise time of the photorefractive grating does not seem to depend on the grating period. Two tentative explanations are proposed: (i) a high electron mobility and a limited response time owing to ferroelectric relaxation processes and (ii) fast electron recombination.