Nematicons: Spatial Optical Solitons in Nematic Liquid Crystals

Chapter 9: Propagation of Light Confined via Thermo-Optical Effect in Nematic Liquid Crystals

Marc Warenghem, Jean-Francois Blach, and Jean-Francois Henninot

Unité de Catalyse et de Chimie du Solide, Faculté des Sciences, Université d'Artois, Lens, France

9.1 Introduction

Every material is temperature sensitive, and its physical parameters are temperature dependent. The optical properties are not an exception; hence, the index of refraction depends on temperature. In terms of applications, this could generally be regarded as a drawback, as a temperature change affects the index, and thus the propagation and, eventually, the operation of the device. Fortunately, for most materials, such dependence is quite weak. The best way to characterize it is the thermal coefficient of refraction . For most manufactured glasses, this figure is around ±10⁻⁶ K⁻¹, a few orders of magnitude larger than that for organic liquids (5 × 10⁻⁴ K⁻¹ for acetone) and a few more for liquid crystals (10⁻² K⁻¹ for pentyl cyanobiphenyl). To give an insight of what such a figure means, let us take a textbook example: the minimum deflection of a beam by a prism. The change of that deflection is proportional to the change of the prism index of refraction. An increase of 10 K in the temperature of a prism (equilateral section) made of glass with a dn/dT = 10⁻⁶ K⁻¹ results in a variation of the beam minimum deflection ...