Uskova O. Light-induced Orientation of Liquid Crystal Doped with Photosensitive Dye on the Polymer Surface.

Light-induced orientation of doped liquid crystal (LC) - 5CB was studied. We found the effect of hidden photoalignment of 5CB doped with azo-dye Methyl Red (MR) on non-photosensitive polymer surface (PVCN-F) after polarized irradiation of the cell in the isotropic phase. The direction of a light-induced easy axis on the polymer can be either parallel or perpendicular to the polarization of the incident light depending on the light intensity. We attribute this behavior to two mechanisms of photoalignment: light-induced adsorption of MR molecules on the substrate and desorption from previously adsorbed dye-layer. The experimental results on photoalignment of LC on thin dye film confirm our model. It was shown the crucial role of LC orientational ordering in mesophase and the nature of the aligning surface in the effect of the light-induced anchoring. It was found that the light-induced easy axis was formed on the background of the already existing anisotropic "dark" dye layer and the final anchoring is d etermined by cumulative effect of "dark" and light-induced contributions to anchoring. The effect of the formation of permanent light-induced spatially modulated textures in a planar LC cell filled with 5CB+MR was observed. The patterns appeared under the irradiation of the tested surface through the liquid crystal layer with laser gauss beam in MR molecules absorption band. It was shown that the effect is associated with spatial modulation of the light polarization over tested surface, due to conformation nonlinearity of the mixture that leads to appropriate modulation of easy orientation axis on photoadsorbing surface. .