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Visualization of refractive index modulation by optical channeling

We discuss manifestation of the channeling effect in the visualization of hexagonal patterns in KNbO3 and phase gratings in LiNbO3. We have shown that visualization of the domain patterns, discovered earlier in the subharmonic beam in the photorefractive BSO crystal, may be explained by the channeling effect. A wave-packet description allows us to explain backward movement of domains due to the specific dispersion law for the space-charge waves. An analogy is discussed with domain formation in the charge-density-waves, known in quasi-1D conductors. Non-linear phase shift of the couple's waves may lead to discomensuration.