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Journal of Experimental and Theoretical Physics

Critical enhancement of nonlinear response in fast photorefractive crystals

M. V. Gorkunov, Evgeniy V. Podivilov, Boris I. Sturman

Published Mar 1, 2002• 2 citations

Abstract

It is shown that when approaching the spatial subharmonics generation threshold in fast photorefractive crystals (the sillenites, CdTe), a practically unlimited (singular) amplification of the nonlinear photorefractive response is possible, which results in a drastic increase in the spatial amplification of weak signals. A theory of critical spatial amplification is developed. This theory takes into account real attributes of fast photorefractive crystals such as the vectorial nature of wave coupling and nonuniform broadening of resonances owing to light absorption. The theory is applied to the analysis of the observable characteristics of critical enhancement and to optimization of the conditions of experiments aimed at the detection and investigation of this phenomenon.

DOI: 10.1134/1.1469145

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It is shown that when approaching the spatial subharmonics generation threshold in fast photorefractive crystals (the sillenites, CdTe), a practically unlimited (singular) amplification of the nonlinear photorefractive response is possible, which results in a drastic increase in the spatial amplification of weak signals. A theory of critical spatial amplification is developed. This theory takes into account real attributes of fast photorefractive crystals such as the vectorial nature of wave coupling and nonuniform broadening of resonances owing to light absorption. The theory is applied to the analysis of the observable characteristics of critical enhancement and to optimization of the conditions of experiments aimed at the detection and investigation of this phenomenon.

We propose a theoretical model for the recently reported anomalously long decay of a space-charge field that was resonantly excited in a photorefractive sillenite crystal [Phys. Rev. Lett. 79, 67 (1997)]. The model is based on the conventional concept of charge transfer and does not require excessively large values of the quality factor for space-charge waves. The effect in question is qualified as a critical slowing of the relaxation process as the threshold of optical oscillation is approached.

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Photorefractive backward-wave four-wave mixing may be configured for the amplification of the phase conjugate output and the attenuation of the transmitted output. The nondegenerate-in-frequency interaction is studied in such a configuration, realized in a semiconductor CdTe crystal. The spectra measured at different outputs show the gain and attenuation resonances. The preliminary study of optical pulse propagation demonstrates that, due to different nonlinear dispersions at different outputs of the selected wave mixing configuration, a slowing down of light is observed at the phase conjugate output, while an acceleration of light is detected at the transmitted output for the same input pulse at the same interaction.

It is shown that, in photorefractive crystals with a dominating local nonlinear response, small temporal fluctuations of the space-charge field can induce strong noisy (stochastic) backscattering. For sufficiently large interaction lengths a small nonlocal component of the photorefractive response can suppress the fluctuation of the scattered-light intensity and can result in nearly complete and regular nonlinear reflection. Stochastic stimulated backscattering is detected and investigated in LiNbO3:Fe crystals. The scattering manifests itself in the optical configuration, where noiseless spatial amplification of backward waves is impossible. The main characteristics of the observed backscattering are in good qualitative agreement with the calculated ones.

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