Laser Beam Modulation Freezing on a Liquid Crystal Surface

We report the first observation of the formation of permanent light-induced annular patterns in a planar cell filled with dye-doped liquid crystal. The sample is placed between one rubbed surface, giving unidirectional planar alignment, and one isotropic surface, providing degenerated planar alignment. The patterns appear under the irradiation of the isotropic surface through the liquid crystal layer. The experimental results are explained in terms of spatial modulation of the light polarization over the isotropic surface, due to conformation nonlinearity of the mixture. The light-induced adsorption of azo-dye on the initially isotropic surface leads to "freezing" of this modulation and the appearance of a twisted director structure.