TRANSVERSE INSTABILITY THRESHOLD IN COUNTERPROPAGATING LIGHT-BEAMS FOR A NONLINEAR MEDIUM WITH LOCAL PHOTOREFRACTIVE RESPONSE

We derive the threshold conditions for the instability of counterpropa gating waves in a nonlinear medium with local photorefractive response against the excitation of transverse small-angle structures. These co nditions allow for all the important types of diffraction from refract ive-index reflection gratings and are not limited to the case of stric t frequency degeneracy of the waves. We study the dependence of the cr ystal-thickness threshold and the secondary wave emission angle on the crystal parameters and the pump conditions. We show that when the pum p wave intensities differ considerably, excitation of standing light s tructures is replaced by excitation of traveling structures. Finally, we discuss the applications of the theory to experiments with the phot orefractive crystals LiNbO3 and LiTaO3. (C) 1997 American Institute of Physics.