We report enhancement of the contrast of holographic grating with respect to modulation of interference pattern. This phenomenon is associated with the contribution of a strong photogalvanic (photovoltaic) current into a charge transport mechanism in photorefractive KNb03. High contrast of the interference pattern as well as a large photogalvanic current in the crystal result in the shift of the grating with respect to the modulation pattern. This fact is beyond the frames of traditional models for media endowed with strong photogalvanic effect.
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