Résumé:

Recently we proposed an alternative way of measuring hypersound attenuation in silica thin films using ultrafast acoustics [1-2]. This way, the attenuation of longitudinal acoustic phonons up to 300 GHz has been measured in vitreous silica [3]. Our method is based on the emission of a short acoustic pulse from the absorption of an ultrashort optical pulse in a thin metallic layer deposited on top of the sample. The high frequency content of the pulse is detected in the substrate after propagation in silica using another laser pulse. By comparing the signal obtained for various film thicknesses, we can extract sound attenuation in silica in the sub-terahertz range. Multiple interference effects in the multilayer are analyzed and fully taken into account[3]. Our accurate acoustic attenuation results are found to follow rather well a model combining thermally activated relaxations (TAR) and anharmonicity. In the present work, we first show how to reach higher and higher frequencies. For that it is needed to improve both acoustic emission and detection. Concerning emission, it is governed by the thin metallic layer deposited on top which converts the ultrashort optical pulse in an acoustic pulse. Usually Al is preferred due to its efficiency in light-sound conversion. We will also report the results obtained using the same experimental scheme but at low temperatures. On cooling, the TAR contribution is expected to first increase, and this should be rather easily observable. As shown, an important difficulty to overcome is the thermal heating induced with the laser technique. We will present different experimental evidences which attest that the sample is at the expected low tempetature and then some hypersound attenuation measurements.