Photo-induced orientation behaviors of azobenzene liquid crystal copolymers for photonic crystals

Abstract

A certain wavelength of incident light is forbidden based on the optical periodicity related to the intervals and refractive indices of the comprising materials. For multi-bilayered films with periodicity, on–off switching of reflection is possible due to the presence or absence of a refractive index difference between two consisting films. Changes in the refractive index can be induced by stimuli-responsive orientation behaviors of liquid crystal (LC) molecules with birefringence. Azobenzene (Az) groups are employed for photo response and copolymerized with non-photo-responsive LC mesogen groups for cooperative orientation to improve light penetration by the dilution of Az concentration. LC mesogen groups are designed to have a high refractive index to improve the reflection contrast. The refractive indices and the orientation behaviors of the copolymer films are investigated. On–off switching of reflection for multi-bilayered films consisting of alternate Az-containing LC copolymers and poly vinyl alcohol is conducted using ultraviolet and visible light irradiation, and their reflection intensity and response speed are compared between the different LC mesogen types and molar ratios.