Brillouin scattering measurements on optical glasses

Abstract

The room-temperature Brillouin spectra of 16 optical glasses of varying composition have been measured. Sound velocities for both transverse and longitudinal waves were determined along with the Pockels electro-optic coefficients $p_{12}$ and $p_{44}$. The damping parameter for the longitudinal phonons was obtained from linewidth measurements. The linewidths varied from 0.05 to 0.30 GHz full width at half maximum (FWHM) but averaged about 0.2 GHz, which is much larger than that in most normal crystalline materials. All spectra were recorded with a minicomputer-controlled triple-pass plane Fabry-Perot interferometer. The active alignment stabilization by computer control is discussed. Our experimental results indicate that there is at most a weak correlation between the hypersound parameters and the glass composition or other physical parameters. The frequency dependence of the damping in pure silica and the dense lanthanum flint glass LaSF-7 was measured. For these two glasses, a damping of the form $\gamma \alpha f^m$ was fit to the data, where $m\simeq 1$ for LaSF-7 and $m=2.7\mathrm{}\pm 0.2$ for pure silica. Simple phenomenological models of damping by relaxation processes indicate that the relaxation rate ${\tau }^{-1\mathrm{}}$ at room temperature (in LaSF-7) is about 1 ${\mathrm{cm}}^{-1\mathrm{}}$, whereas in pure silica ${\tau }^{-1\mathrm{}}\gg 1$ ${\mathrm{cm}}^{-1\mathrm{}}$. Also, in pure silica the frequency dependence is anomalously strong since all models predict $m<~2$.