from C.Z. Tan, Optical interference in overtones and combination bands in [itex]\alpha[/itex]-quartz, Journal of Physics and Chemistry of Solids 64 (2003) 121–125.In solid state theory, there are three modes for each elastic wave in crystals, one of longitudinal polarization and two of transverse polarization [1,2]. In the transverse optic (TO) mode the induced polarization is perpendicular to the wavevector, whereas in the longitudinal optic (LO) mode the electric polarization is parallel to the wavevector . The Brillouin and the Raman scatterings are induced by acoustic and optical phonons in solids. Because of the random distribution of phases, the acoustic and optical phonons do not produce the regular diffraction gratings for the incident light whose frequency is quite different from that of the optic modes in the infrared region . The transverse wave of the incident light couples with the TO modes near resonance frequencies, producing a polariton in crystals . Interaction or coupling of the incident light with the TO modes is enhanced at the frequencies of overtones and combination bands of the fundamental lattice vibrations [2,4]
 M. Born, K. Huang, Dynamical Theory of Crystal Lattices, Clarendon Press, Oxford, 1988.
 C. Kittel, Introduction to Solid State Physics, Wiley, New York, 1996.
 H. Tanaka, T. Sonehara, S. Takagi, Phys. Rev. Lett. 5 (1997) 881.
 C.Z. Tan, J. Arndt, J. Chem. Phys. 112 (2000) 5970.
Relevant to some work I am doing on thermal conductivity. :grumpy: