CONTINUUM MODEL OF THE OPTICAL MODES OF VIBRATION OF AN IONIC-CRYSTAL SLAB

Optical vibrations of a thin ionic slab are described by a macroscopic theory involving the hybridization of LO, TO, and interface-polariton modes. The resultant modes are triple hybrids, which satisfy both elastic and electromagnetic boundary conditions at the two surfaces. Analytic expressions are derived for the relative ionic displacements and related electric fields, and for the dispersion relations, assuming elastic isotropy and neglecting retardation effects. Comparison of mode patterns with those obtained by Fuchs and Kliewer, who used electromagnetic boundary conditions only, show that Fuchs-Kliewer modes are a good approximation to the system of the triple-hybrid modes our theory describes. © 1994 The American Physical Society.