Frequency dependence of phonon-polariton damping in lithium niobate

We have measured the gain for stimulated Raman scattering from phonon polaritons of the 256 cm^-1 A₁ mode of MgO:LiNbO₃ as a function of polariton frequency. The comparison with calculations provides new information on the damping of phonon polaritons, in particular on the propagation of the polaritons out of the excitation volume, their coupling to low-frequency excitations, and the frequency dependence of the damping constant of the optical phonon mode, which is in agreement with the anharmonic decay of the optical phonon into two acoustic phonons and the scattering at crystal defects. {} 1996 The American Physical Society.