Effects of domains on crystal-optics properties and intensity of Raman scattering in a 45° Y-cut KH_2PO_4 crystal

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Abstract
In the course of a light-scattering experiment with a 45° Y-cut KH2PO4 crystal, some new crystal-optics phenomena were observed in the ferroelectric phase when light was incident 45° to the polar axis. (i) A bright oblique line appears in the crystal, (ii) two mutually orthogonally polarized beams, of which images consist of many stripes, emerge from the crystal perpendicular to the incident beams, and (iii) the irradiances of these beams as well as that of the Raman scattering exhibit oscillatory changes with temperature and tend to constant values at Tc - T> 10°-15°. These effects are explained in terms of reflection and interference of light by domains. It is found that domain walls in KH2PO4 act as reflecting planes, that many beams generated by the reflection at least partially interfere, and that the domain structure can drastically change the polarization state of light as it passes through the crystal. This provides a new optical method to obtain knowledge of domain structures in ferroelectrics.