The concept of local polaritons is used to describe the optical properties of mixed crystals in the frequency region of their restrahlen band. It is shown that this concept allows for a physically transparent explanation of the presence of weak features in the spectra of so-called one-mode crystals and for one-two mode behavior. The previous models were able to explain these features only with the use of many fitting parameters. We show that under certain conditions new impurity-induced polariton modes may arise within the restrahlen of the host crystals, and study their dispersion laws and density of states. Particularly, we find that the group velocity of these excitations is proportional to the concentration of the impurities and can be thousands of times smaller than the speed of light in vacuum.

• Received 13 April 2000

DOI:https://doi.org/10.1103/PhysRevB.62.6301