Optical Properties of Halogen-Bridged Mixed-Valent Metal Complexes: Excitons and Nonlinear Excitations

In this chapter, a brief review of theoretical and experimental studies on the optical properties of halogen-bridged mixed-valent metal complexes is given, with special emphases on the following two topics. One is the nature of the charge transfer band, and the other is the intragap absorptions due to nonlinear excitations such as solitons or polarons. In relation to the first topics, we introduce a theory of spectral shapes of optical absorptions in various charge-density-wave states. In that theory, two important effects beyond the mean field theory are taken into account: the electron-hole correlation on the metal atoms and the classical fluctuation of the halogen-sublattice coordinates. It is found that the spectral shapes both of the linear absorption and of that in an electric field, i.e., the electroabsorption agree well with the experimental results if we assume that the electron-hole attraction is rather strong in real systems. Moreover, the intensity of the third-harmonic generation is also calculated, using the same assumption. In relation to the second topics, on the other hand, we show some examples of theoretical calculations in which the modifications of the absorption spectra due to a soliton or a polaron is obtained. It is clarified that the intragap-absorption bands also are drastically renormalized by the exciton effect.