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International audience; We report picosecond acoustic measurements in silica-glass films grown by wet thermal oxidation on a (111) silicon substrate. The longitudinal acoustic phonons are observed over the range from 150 to 300 GHz using an infrared pump and a second harmonic blue probe. The transducer is an aluminum thin film deposited on top. Multiple interference effects are analyzed and fully taken into account. They lead to a signal presenting rapid oscillations as a function of the sample thicknesses. The latter are determined by separate interferometry. Our remarkably precise acoustic attenuation results are found to follow rather well a model combining thermally activated relaxations and anharmonicity. New values for the optical absorption of silicon in the 400-to-500-nm region of the spectrum are obtained as a by-product.

International audience; There exists a special interest in investigating acoustical and optical vibrations of glasses in the one THz frequency region. The main experimental tools that are used to this effect are briefly discussed. The current understanding of the boson-peak vibrations and of their resonance with acoustic branches are explained. Specific new results on the origin of the boson peak and on the end of acoustic branches are presented for borates.