Hypersound damping in vitreous silica measured by picosecond acoustics

The attenuation of longitudinal acoustic phonons up to frequencies nearing 2500.3em0exGHz is measured in vitreous silica with a picosecond optical technique. By taking advantage of interferences on the probe beam, difficulties encountered in early pioneering experiments are alleviated. Sound damping at 2500.3em0exGHz and room temperature is consistent with relaxation dominated by anharmonic interactions with the thermal bath, extending optical Brillouin scattering data. Our result is at variance with claims of a recent deep-UV experiment which reported a rapid damping increase beyond 1000.3em0exGHz. A comprehensive picture of the frequency dependence of sound attenuation in v-SiO₂ can be proposed.