Schaefer-Bergmann diffraction pattern due to the abnormal Bragg reflection in birefringent media

A new type of the Schaefer-Bergmann diffraction patterns originated from the abnormal Bragg reflection phenomenon in anisotropic media has been demonstrated using α quartz and monoclinic Pb2MoO5crystals. The wave-vector direction of the diffracted light has been expressed in analytical equations for the general cases in uniaxial crystals as a function of those of the incident light beam and the acoustic wave, on the basis of the simple pseudo momentum conservation relation. The shape of the abnormal patterns can be determined using the equations and the known data on the acoustic and the optical wave-vector surfaces, and calculated patterns for α quartz are in good agreement with the observed ones. The equations predict that a small difference appears in the shape of the patterns between two cases where the polarization planes of the incident optical beams are orthogonal with each other, and this fact has been well confirmed by the present experiment.