Polariton Spectrum ofα-Quartz

Abstract

Mixed electromagnetic-mechanical excitations known as "polaritons" are observed in Raman forward-scattering experiments on $\alpha$-quartz using an argon laser. Dispersion relations of the polariton frequencies are extended from the case of simple diatomic lattices to a more complex system, namely, $\alpha$-quartz, which has nine atoms per unit cell. The dependence of frequency upon scattering angle observed for these mixed excitations in quartz is shown to be in very good agreement with that predicted. Polaritons associated with the transverse phonons at 1072, 797, and 450 ${\mathrm{cm}}^{-1\mathrm{}}$ in $\alpha$-quartz are discussed in the present work. The polariton associated with the 1072-${\mathrm{cm}}^{-1\mathrm{}}$ phonon mode is observed to range in frequency from 1072 to 822 ${\mathrm{cm}}^{-1\mathrm{}}$; the polariton associated with the 797-${\mathrm{cm}}^{-1\mathrm{}}$ phonon mode is observed at frequencies from 797 to 700 ${\mathrm{cm}}^{-1\mathrm{}}$; and that associated with the 450-${\mathrm{cm}}^{-1\mathrm{}}$ phonon mode is observed from 450 to 407 ${\mathrm{cm}}^{-1\mathrm{}}$.