ASYMMETRY OF 2-WAVE COUPLING IN CUBIC PHOTOREFRACTIVE CRYSTALS

Analysis of two-wave coupling in cubic crystal classes 23 and (4) over bar 3m is presented for arbitrary orientation of the grating vector a nd the applied electric field. We show that the most efficient two-wav e coupling is achieved when both the grating vector and the external e lectric field are parallel to the [(1) over bar 11] axis in the (110) crystal cut. However, beam coupling is asymmetric in this geometry: Th e enhancement coefficient may differ by orders of magnitude whether in teracting 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 p ropagation. A theoretical model is able to explain main features of th e observed fanning patterns if the piezoelectric and photoelastic effe cts are incorporated into the system of vectorial coupled-wave equatio ns. (C) 1997 Optical Society of America [S0740-3224(97)01612-3].