CONTINUUM MODEL OF THE OPTICAL MODES OF VIBRATION OF AN IONIC-CRYSTALSLAB

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 ela stic and electromagnetic boundary conditions at the two surfaces. Anal ytic expressions are derived for the relative ionic displacements and related electric fields, and for the dispersion relations, assuming el astic isotropy and neglecting retardation effects. Comparison of mode patterns with those obtained by Fuchs and Kliewer, who used electromag netic boundary conditions only, show that Fuchs-Kliewer modes are a go od approximation to the system of the triple-hybrid modes our theory d escribes.