$q$ Dependence of Low-Frequency Raman Scattering in Silica Glass

Accurate measurements of the dependence of low-frequency Raman scattering on the scattering angle were performed in two silica glasses. By a comparison of spectra measured at a large scattering angle (close to back scattering) and a small one (close to forward scattering), we for the first time observed a $\mathbf{q}$ dependence of the low-frequency light scattering in glasses. From the magnitude of the effect, the vibration correlation length is estimated and compared with results from the picosecond optical technique; a reasonable agreement is found.