Optical Modes of Vibration in an Ionic Crystal Slab Including Retardation. II. Radiative Region

The optical modes of an ionic crystal slab are examined in the radiative region starting from Maxwell's equations including retardation. Since any excitation of the crystal in this region will radiate its energy, it is argued that the modes which characterize the system are virtual modes, which have oscillatory fields outside the slab and correspond to energy transport out of the slab. The determining equations for the virtual modes are obtained and are solved for LiF considering both $S$ and $P$ polarizations and a wide range of slab thicknesses. Characteristics of the modes of particular importance in determining the optical properties of a slab are emphasized. The effect of lattice anharmonicities on the properties of the virtual modes is discussed.