Plasmon-polariton coupling in anisotropic metals in the non-local regime

Abstract

It is predicted that the polariton and plasmon modes of a solid state plasma which are simultaneously excited when light is incident on a metal at an oblique angle can be strongly coupled by an optical lattice anisotropy stemming from interband transitions. A theoretical study of the dispersion relations of the coupled plasmon-polariton modes is presented. It is shown that the simple hydrodynamic (or near-local) model for the description of non-local effects in metals gives an accurate description of the coupling around the longitudinal plasma edge. It is demonstrated that the polariton dispersion relation is strongly modified at the longitudinal plasma edge by the Coulomb field of the plasmon. The optically interesting situation stemming from the fact that one of the principal axes of the conductivity tensor is fixed by the scattering plane whereas the principal axes of the dielectric tensor of the lattice can be rotated by rotating the crystal is investigated. Numerical results based on free-electron data for hexagonal Zn are obtained. It is proposed that the polariton-plasmon coupling is used for measurements of optical lattice anisotropies at frequencies around the plasmon plasma edge.