Damping mechanisms of phonon polaritons, exploited by stimulated Raman gain measurements

Abstract

Using a two-beam amplifier experiment stimulated Raman gain measurements were performed for the polariton branch of the (Formula presented) (TO) optical phonon mode in ZnO-doped congruent (Formula presented) and undoped nearly stoichiometric (Formula presented) for polariton frequencies from 30 to (Formula presented) The dependence of the peak value and linewidth of the Raman gain curve on the polariton frequency provides detailed information on the frequency-dependent damping of the polariton. Spontaneous Raman spectra of the (Formula presented) phonon, with particular emphasis upon the low-frequency wing of the Raman line, complete the results. The experimental results are carefully analyzed by comparison with a theory connecting the stimulated and spontaneous Raman data to the dielectric function of (Formula presented) which contains the damping of the polaritons. As damping mechanisms the anharmonic decay of the phonon part of the polariton, the scattering at crystal defects and the coupling to low-frequency excitations are discussed. We relate three of the low-frequency excitations to (Formula presented) phonon modes of (Formula presented) and the major part to defect modes. © 1998 The American Physical Society.