Photoinduced Alignment and Multi-Processes of Refractive-Index Gratings in Pre-Irradiated Films of an Azobenzene-Containing Liquid-Crystalline Polymer

Abstract

After being pre-treated upon irradiation of unpolarized light at 366 nm, an azobenzene-containing liquid-crystalline polymer film was photoinduced alignment by one linearly polarized laser beam at 488 nm, leading to a large birefringence of 0.21. Then holographic gratings were recorded in the pre-irradiated polymer films by interference of two coherent laser beams. Little surface relief was obtained by the holographic recording process, indicating that a refractive-index grating paid most of contributions to the diffraction efficiency. Because the cooperative motion of the azobenzene mesogens were eliminated in the cis-azobenzene-rich isotropic film by the pre-irradiation, multi-processes of the refractive-index gratings were successfully achieved. The obtained grating structures were clearly shown in the polarizing optical microscopic pictures, which were also verified by their diffraction patterns. The azobenzene-containing liquid-crystalline polymer used in this study has potential application in high-density information storage.