Résumé
Sound attenuation in glasses exhibits a rich variety of phenomena which depend on the frequency and temperature. Typically above 10 K, the thermally activated relaxations of defects which can be modelled by two-well potentials are the major source of attenuation. At higher temperatures, the anharmonic interaction of sound waves with the thermal phonon bath becomes important. This initially varies with the square of the frequency, making it the dominant source of attenuation at high frequencies, typically above 10 GHz in the case of vitreous silica. How the attenuation evolves at still higher frequencies remains currently debated. Picosecond optical techniques now offer a possible approach to access sub-THz sound properties. We report recent accurate measurements of attenuation of longitudinal acoustic phonons up to frequencies near 300 GHz in vitreous silica. At very high frequencies, in the THz range, sound changes in nature, loosing its wavelike character. This Ioffe-Regel crossover nearly coincides with the excess of vibrational modes manifested by the boson peak.