Phase-Type gratings induced by electro-Optic effect of the space charge fluctuation in an ionic conductor

Abstract

The strong light scattering bands perpendicular to the c-axis and wide in angular extension observed in a z-cut specimen of the ionic conducting pyroelectric crystal α-LiIO₃ under the action of at do potential are theoretically explained. The space charge distribution left by the flow of ionic current is analysed into its Fourier components. The linear electro-optic effect of the relevant internal field induces a spatial fluctuation of the nondiagonal elements of the di-electric tensor ε 23 and ε31, which in turn produces a system of phase-type volume gratings. Their wave vectors are normal to the c-axis. The angular intensity distributions of the anisotropic modes (o→e and e→o) and those of the isotropic modes (o→o and e→e) are derived by taking account of the Bragg equation and evaluating the diffraction efficiency. The results are in good agreement with the experimental observations including the asymmetry in intensity distribution of both the o→e and e→o diffraction bands, the existence of I_min = O on the weaker side and many other details. It is pointed out that the fluctuation of the space charge must be closely related to a certain kind of linear defects along the c-axis of the crystal and that similar diffraction phenomena can be found only in crystals without a center of symmetry.