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Journal of the Optical Society of America B

Mechanism of transverse instability of counterpropagation in photorefractive media

Boris I. Sturman, Alexander I. Chernykh

Published Jul 1, 1995• 12 citations
Publisher

Abstract

It is shown that counterpropagation of two light beams in a photorefractive medium can be unstable against spontaneous emission of light waves of the same frequency and polarization traveling at small angles. The mechanism of the instability is based on the buildup of a set of spatial reflection gratings. The threshold thickness and the corresponding emission angle are found. Results of the theory are applied to interpretation of existing experimental data.

DOI
10.1364/josab.12.001384

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It is shown that counterpropagation of two light beams in a photorefractive medium can be unstable against spontaneous emission of light waves of the same frequency and polarization traveling at small angles. The mechanism of the instability is based on the buildup of a set of spatial reflection gratings. The threshold thickness and the corresponding emission angle are found. Results of the theory are applied to interpretation of existing experimental data.
We derive the threshold conditions for the instability of counterpropagating waves in a nonlinear medium with local photorefractive response against the excitation of transverse small-angle structures. These conditions allow for all the important types of diffraction from refractive-index reflection gratings and are not limited to the case of strict frequency degeneracy of the waves. We study the dependence of the crystal-thickness threshold and the secondary wave emission angle on the crystal parameters and the pump conditions. We show that when the pump wave intensities differ considerably, excitation of standing light structures is replaced by excitation of traveling structures. Finally, we discuss the applications of the theory to experiments with the photorefractive crystals LiNbO 3 and LiTaO 3 .
We describe one of the possible ways of development of a spatial instability of counterpropagating strong light beams in resonant media. The interplay between self-focusing and saturation effects results in a formation of counterpropagating stable filaments. The nondegenerate four-photon scattering in these waveguiding structures leads to a far-field multiconical emission with the cones' frequencies shifted to both red and blue sides of the resonant transition. \textcopyright{} 1996 The American Physical Society.