Interface and surface polaritons of semiinfinite biaxial crystals

Interface and surface polaritons of an arbitrary semiinfinite crystal cut of biaxial symmetry are investigated. It is shown that regular interface modes can occur only for certain orientations of the propagation vector or special crystal cuts. From energy flow considerations it is found that ordinary interface modes can exist only if the two‐dimensional propagation vector (kx, ky) is along a principal axis of the dielectric tensor. In uniaxial crystals two different types of interface polaritons are found to occur. The first is a regular and real interface mode whose dispersion relation depends neither on the crystal cut nor on the direction of propagation. The second is a generalized interface mode which may have real or virtual character.