Résumé
Recently, an alternative way of measuring hypersound attenuation in silica thin films using ultrafast acoustics was proposed. The attenuation of longitudinal acoustic phonons around 250 GHz has been measured in vitreous silica deposited on a silicon substrate. This method is based on the emission of a short-acoustic pulse from a thin-metallic layer on top of the sample. The high frequency content of the pulse is optically detected in the silicon substrate after propagation in silica. By comparing the signal obtained for various film thicknesses, we can precisely extract sound attenuation in silica in the sub-terahertz range. In order to reach higher and higher frequencies, 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. In this work, the high frequency content of the acoustic emission was improved by studying the influence of the chemical nature, the thickness, and the deposition parameters of the transducer on the detection of ultra-high frequencies in the substrate. As a result of this optimization, it is expected that the hypersound attenuation can be measured in silica up to 350 GHz. A second conclusion of this work is an alternative way of emitting the acoustic pulse based on a resonance effect.