Picosecond Nonlinear Optical Responses in Photorefractive Crystals

Picosecond-pulse laser excitation was used to establish and probe refractive index gratings in KNbO3, KTaO3, and mixed crystals of KTa1-xNbxO3. The purpose of this work was first to identify the physical processes contributing to fast nonlinear optical responses in these samples, and second to determine how these processes evolve with time into the space-charge field photorefractive effect observed with continuous wave excitation. The techniques of pulse-probe degenerate four-wave mixing and nondegenerate four-wave mixing with a continuous wave probe laser were both employed to obtain information about the photorefractive processes that take place after fast pulse excitation. The measurements were made as a function of laser fluence, crossing angle of the write beams, sample orientation with respect to the grating wavevector, polarization directions of the write and probe beams, and write beam wavelength. Mixed crystals with the niobium concentration parameter x varying from 0 to 1.0 were investigated along with KNbO3 samples containing a variety of dopant ions.