Polaritons in molecular crystals

Abstract

The simultaneous propagation of lattice phonons and photons in molecular crystals is discussed. The equation of propagation of the modes involving molecular displacements and an electric field is derived based on the concept of dynamic multipoles and taking into account retardation effects. The equation is analyzed mainly in a long wave limit in order to study the optical properties of lattice phonons. The elastic and piezoelectric constants are then obtained in terms of molecular parameters of the model. Finally, as an example, the polariton spectrum of crystalline HCN is analyzed.