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Quantum Electronics

Influence of the maximum space-charge field on the rate of self-bending of the path of a speckle beam in photorefractive crystals

O V Lyubomudrov, Vladimir V. Shkunov

Published Sep 30, 1994• 6 citations
Publisher

Abstract

A self-similar solution of the problem of nonlinear propagation of a speckle beam was obtained for photorefractive crystals with a diffusion-type response. The solution allows for the finite value of the maximum space-charge field. It is shown that this factor modifies the profile of the angular spectrum of the self-similar solution and increases the radius of curvature of the path compared with the case of an unrestricted field of a diffusion grating.

DOI
10.1070/qe1994v024n09abeh000246

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A self-similar solution of the problem of nonlinear propagation of a speckle beam was obtained for photorefractive crystals with a diffusion-type response. The solution allows for the finite value of the maximum space-charge field. It is shown that this factor modifies the profile of the angular spectrum of the self-similar solution and increases the radius of curvature of the path compared with the case of an unrestricted field of a diffusion grating.
The propagation of a light beam in a photorefractive crystal in an alternating meander electric field is studied. A nonlinear equation is obtained for a spatial charge field formed in the crystal upon modulation of the light intensity synchronously with the applied electric field. It is established theoretically that the spatial charge field has two components, which make contributions to different mechanisms of a photoresponse providing self-focusing and the appearance of spatial solitons, as well as the trajectory self-bending. The conditions are found at which the soliton regime is realised and parameters affecting this regime in a greater extent are determined.
We formulate an anisotropic nonlocal theory of the space charge field induced by the coherent counterpropagating beams in biased photorefractive crystals. We establish that the competition between the drift and diffusion terms has to be taken into account when the crystal ĉ axis is tilted with respect to the propagation direction of the beams. We demonstrate that this configuration combines the features of both spatial soliton formation without energy exchange and two-wave mixing with energy exchange leading to pattern formation.
Stimulated photorefractive phase conjugators often exhibit well-defined curved beam paths that cannot be explained by simple beam fanning. We propose a model of these apparently curved paths as a series of straight-line segments, with beams propagating in both directions along these segments. These line segments spring from the amplification of scattered light between regions of the crystal already containing counterpropagating pump beams. As these line segments form, they create new interaction regions that generate new segments, thereby making the final beam path appear to be curved. Application of our model to a single-interaction-region mutually pumped phase conjugator shows that the threshold coupling strength required for the appearance of these new segments is only slightly higher than the threshold for the phase conjugator itself.
A theoretical model is constructed for four-wave mixing in a photorefractive crystal where a transmission grating is formed by the drift–diffusion nonlinearity mechanism in the absence of an external electrostatic field and the response of the medium is nonlinear in respect of the modulation parameter. A comparison is made with a model in which the response of the medium is linear in respect of the modulation parameter. Theoretical models of four-wave and two-wave mixing are also compared with experiments.