Raman Scattering from Polar Phonons

Abstract

Raman scattering from long-wavelength polar phonons is considered from an alternative formulation emphasizing lattice-dynamical as opposed to crystal-optic aspects. The modifications of the Raman frequencies and intensities due to the mode coupling through macroscopic electric field components are discussed quantitatively. In particular, we show in a natural way why it is possible to observe Raman-"inactive" polar modes in materials where other Raman-active polar modes are present. Our predictions are verified by a study of the dependence on the propagation direction of the frequencies and intensities of the "forbidden" $A_2$-like modes in $\alpha$-quartz.