We present the results of a theoretical and experimental investigation of the photorefractive effect under a revolving external electric field in BSO and BTO crystals. We show that the revolving field enhances 90°-shifted with respect to the interference pattern space-charge field, which does not depend on the recorded grating orientation. We analyze in detail the effect of the external applied field on the two-wave mixing and show that the direction of maximum momentary two-wave mixing gain changes synchronously with the revolving applied field. We propose a configuration that demonstrates an isotropic polar pattern of the time average two-wave mixing gain.
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