Photochemical Modulation of Alignment in Liquid Crystals

Liquid crystals (LCs) show an anisotropic nature in various aspects of optical and electronic properties. This anisotropy enables LC materials to be used as active media in display devices. Birefringence, an anisotropy in optical properties of LCs, is especially useful from the viewpoint of changing refractive indices of LC materials. If alignment of LCs can be changed by light, a large change in refractive index can be induced by light. This is the principle of LC photonics, in which optical signals are processed with the aid of LC materials. In this article, various approaches are described to change the alignment of the LC molecules: 1) photochemically induced phase transition of LCs by photochromic reactions is an order to disorder transition and can be brought about in a time scale of micro- to nanosecond; 2) reorientation of LC molecules by means of linearly polarized light is a change in alignment of LC molecules that occurs within the LC phases and induces a large change in refractive index, but needs more time. Applications of LC systems as photonic materials are also discussed.