Fundamental Optical Properties of Materials II

This chapter focuses on fundamental optical properties with emphasis on optical absorption. The important features in the absorption coefficient versus electromagnetic radiation or photon energy behavior as the photon energy increases can be classified in the following types of absorptions: reststrahlen or lattice absorption, in which the radiation is absorbed by vibrations of the crystal ions; free carrier absorption due to the presence of free electrons and holes, an effect that decreases with increasing photon energy; an impurity absorption band (usually narrow) due the various dopants; exciton absorption peaks that are usually observed at low temperatures and are close to the fundamental absorption edge; and band-to-band or fundamental absorption of photons, which excites an electron from the valence to the conduction band. The chapter describes characteristics of some of these absorptions. It also deals with the effective medium approximation to describe the optical properties of heterogeneous media, that is, mixtures of different phases.