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 crystal such as 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.
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