We show that a constant component of the holographic current may emerge in photorefractive crystals even in the absence of an applied electric field if the recording pattern is oscillating in a definite law. We derive conditions on the oscillations for the effect to appear. For instance, a fringe pattern with an intensity I ~ 1 + m cos[kx + Δcos ωt + δ cos(2ωt + φ)] is fitted (here x is a linear coordinate, t is time, and m, k, ω, and φ are constants). In this case the dc component is j0 ~ −sin φ. One can detect the relative phases of the harmonics by this means.
© 1993 Optical Society of America
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