Refractive Index of Optical Materials

Abstract

This chapter deals with the use of methods for measuring the refractive index of optical materials. It contains five sections: The first section recalls some bases of the electromagnetic theory of light leading to the main characteristics of the index of refraction, and their consequences in geometrical optics (Snell–Descartes laws), in the spectral transmission and absorption of optical media, or the polarization of light beams at interfaces between optical media. The second section describes the more or less classical panel of methods that have been devised to measure refractive indices of bulk materials: these are essentially based upon either the refraction or reflection of light inside prisms (minimum deviation angle, Littrow methods,…) polarizing properties of optical boundaries (ellipsometric, Brewster configurations). While the previous section consists of refractive index characterization at a given temperature, the third section is dedicated to the dependence of the refractive index upon the temperature: the normalized thermo-optic coefficient (NTOC) is defined here and an experimental set-up specially designed for this purpose by one of the authors is described in detail. The last section is concerned with the fact that most optical components are thin film coated in order to improve their performance in transmission, reflection or absorption. Since spectrophotometry is extensively used to characterize these coatings, the operating principle of spectrophotometers is recalled, as well as the main parameters of these deposited films that one can expect to extract by using this technology from spectrophotometric measurements. Various spectrophotometric procedures are described to determine the optical constants of optical ‘‘systems'' (bulk and thin film compounds) in the case of homogeneous or inhomogeneous films, slightly absorbing or not.