ABSTRACT
This chapter describes the way in which defects can be used to make two different sorts of solid state laser. In addition, only relatively low concentrations of defects can be introduced into the crystals. This means that to obtain useful changes in optical density rather thick crystals must be used, which increases the amount of material needed to store each bit of information. The diffusion coefficients of both anion and cation vacancies is very low at room temperature, but the colour centres spread considerable distances into the crystals; a movement easily measured with an optical microscope. Non-stoichiometry can have a profound effect on the optical properties of solids, especially colour. This can come about because of the presence of ‘coloured’ impurities as when the presence of cobalt ions turns normally clear glass blue. Electrochromic materials change colour when subjected to an electric field. The chapter looks at how to use defects in tungsten trioxide to make electrochromic displays.
