Optical Responses

Abstract

Abstract Important information about the excitation spectra of semiconductors and insulators can be obtained by studying the frequency dependence of the absorption of light passing through the material. This chapter first introduces the matrix element for optical absorption spectra. At low temperatures, transitions take place only between the occupied valence band states and the unoccupied conduction band states. An expression is developed for the transition probability per unit time between two such states, wherein a matrix element — involving the perturbation Hamiltonian in Chapter 24 — connects the initial (valence band) state and the final (conduction band) state. The remainder of the chapter discusses the singular points in the optical absorption; the frequency-dependent susceptibility; some analytic properties of the susceptibility; the Kramers–Kronig relations; and some examples of optical spectra. Sample problems are also provided at the end of the chapter.