Effects of strains on optical mode properties in crystals of the rocksalt and zinc-blende structures

A microscopic, lattice-dynamical theory is presented of the effects of strains on the frequencies of $\overrightarrow{\mathrm{k}}\approx 0$ optical modes and transverse effective charges in polar crystals. The general results are applied to cubic crystals of the rocksalt and zinc-blende structures. The strain dependence of the transverse effective charge and the optical-frequency dielectric constant is incorporated into the theory together with the strain dependence of the atomic force constants associated with the short-range interatomic forces. A theory of the effects of strains on polariton dispersion curves in crystals of the rocksalt and zinc-blende structures is also presented. The predictions of the theory, calculated on the basis of simple models of crystals of the rocksalt and zinc-blende structures, are compared with existing experimental data.