Reduction of light-induced refractive-index changes by decreased modulation of light patterns in photorefractive crystals

In holographic experiments many photorefractive crystals show refractive-index changes much smaller than one would expect from the known electro-optic coefficients and space-charge fields. We show that degradation of the interference pattern is the origin of this effect. The degree of modulation of a light grating in photorefractive crystals is measured by three methods and compared with the modulation of the grating of the incident light. All methods, measurement of the amplitudes of fundamental and second-order gratings, of the grating amplitudes in a crystal with and without another crystal in front of it, and of the drift currents through an inhomogeneously illuminated sample, yield consistent results: In lithium niobate there is almost no degradation of the light pattern. However, in our barium titanate and potassium tantalate–niobate samples the degree of modulation is smaller, and the reduction factor is 0.55 and 0.60, respectively. Inhomogeneities of refractive-index changes are shown to be the origin of the effect.