Abstract
Analysis of two-wave coupling in cubic crystal classes 23 and 4¯3m is presented for arbitrary orientation of the grating vector and the applied electric field. We show that the most efficient two-wave coupling is achieved when both the grating vector and the external electric field are parallel to the 〈1̅11〉 axis in the (110) crystal cut. However, beam coupling is asymmetric in this geometry: The enhancement coefficient may differ by orders of magnitude whether interacting beams propagate in the 〈110〉 direction or backwards. Because of this asymmetry, the far-field fanning patterns of photorefractive Bi12TiO20 crystals strongly depend on the direction of the pump-beam propagation. A theoretical model is able to explain main features of the observed fanning patterns if the piezoelectric and photoelastic effects are incorporated into the system of vectorial coupled-wave equations.
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