Isotropic diffraction induced by concentration gratings in alpha-LiIO3 : Fe

Iron doped lithium iodate (alpha -LiIO3:Fe) of hexagonal modification is in vestigated by holographic experiments in a geometry where an electrooptic c ontribution to the diagonal elements of the change of the dielectric permit tivity tensor is forbidden by symmetry. Anisotropic diffraction (electroopt ically allowed) which we expected to observe in that geometry did not occur . Instead, isotropic diffraction (electrooptically forbidden) from light-in duced gratings was found. To clarify its origin the properties of these gra tings are studied as well as photo-induced transparency occurring in the ul traviolet, The experiments show that two types of gratings exist, one of th em being an absorption grating. The second grating is attributed to origina te from concentration changes which arise from the transport of ions trigge red by nonuniform illumination. (C) 2001 Elsevier Science B.V. All rights r eserved.