Coupling of Plasmons to Polar Phonons in Degenerate Semiconductors

Abstract

The coupling between longitudinal optical phonons and plasmons in degenerate polar semiconductors is investigated. At the appropriate carrier density (${10}^{17}$ to ${10}^{18}$ in III-V compounds) the free-carrier plasma frequency is comparable to the optical frequency, and the plasmons and phonons cease to be approximately independent excitations of the system. Then the two new normal modes are strong admixtures of phonons and plasmons. Several features of these modes are treated. In the long-wavelength limit the frequency and phonon strength are expressed as functions of the carrier density. The phonon strength is a measure of the amount of ionic motion present in a mode. The dispersion and the damping are discussed in a qualitative manner. The modes exhibit a repulsion and avoid intersection. Some experiments are considered in which these properties might be investigated. For example, the optical reflectivity gives the mode frequencies in the long-wavelength limit. The calculated shift of the frequencies by the interaction can be of the order of 5 meV at commonly used carrier concentrations.