Laser induced refractive index change in nematic liquid crystals

Abstract

The liquid crystalline state of matter has the optical properties of solids and the flow characteristics of liquids. In particular, liquid crystals (LC) are fluid states with spontaneous anisotropy. One of the optical properties of liquid crystals is birefringence (Δn=n_e - n_o) and phase retardation of polarized light. There are various theories explaining the behavior of liquid crystals under the influence of external perturbations. It has been shown (both theoretically and experimentally) that thermotropic (temperature-sensitive) liquid crystal molecules align their long molecular axis along the direction of perturbation. These perturbations may be in the form of magnetic, electric or optical field. The director is the unit vector that describes the preferred orientation of a liquid crystal system. Thus, liquid crystal molecule reorientation due to an applied field is equivalent (to a good degree of approximation) to director reorientation. The importance of parameters (index of refraction, viscosity, elasticity) involving macro level (bulk) response can be clearly seen when it is used in industrial applications, the most common of which are the Liquid Crystal Displays (LCD). Since different types of LCD applications require different parameters (e.g. LCD for real time displays need faster switching time; LCD for portable displays need low power consumption) it follows that in order to produce a good liquid crystal material, certain control over these parameters must be imposed. This study aims to quantitatively demonstrate some mechanisms involved when a liquid crystal is subjected to optical perturbation. The physical proof of Freedericksz transition will likewise be presented. The equations used to model our results are limited to nematic liquid crystals since other types of LCs have different state equations.