Polariton‐ and directional dispersion of polar modes in orthorhombic KNbO 3 investigated by light scattering

Experimental data on polaritons of type A 1 (TO), B 1 (TO), and B 2 (TO) in orthorhombic KNbO 3 with wave vectors propagating in the xy‐, yz ‐ and xz ‐planes, respectively, have been compared with dispersion curves calculated on the basis of the phonon data. Data on mixed mode polaritons to type B 1 + B 2 , A 1 , + B 2 , and A 1 + B 1 in the xy‐, yz ‐and xz ‐planes are shown to fit numerically calculated corresponding dispersion curves. Finally all directional dispersion branches of polaritons in the phonon limit ( k → ∞) are calculated for the three symmetry planes. Experimental data on the 45°‐phonons are presented in order to check the theoretical curves.