Temperature dependence of hypersound attenuation in silica films via picosecond acoustics

We report picosecond acoustic measurements of longitudinal sound dispersion and attenuation in an amorphous SiO2 layer at temperatures from 20 to 300 K over frequencies ranging from about 40 to 200 GHz. The sample is a radio frequency cathodic sputtered silica layer grown on a sapphire substrate with an aluminum film transducer deposited on top. Acoustic attenuation is evaluated from the simultaneous analysis of three successive echoes using transfer matrix calculation. Results are found to follow rather well a model combining coupling to thermally activated relaxations of structural defects and interactions with thermal vibrations. This leads to a nontrivial variation of the attenuation coefficient with frequency and temperature. The number density of relaxing defects in the SiO2 layer is found to be slightly higher than that in bulk v-SiO2. In contrast, similar anharmonic contribution to acoustic absorption is observed in both systems. The velocity variations are also measured and are compared to the dynamical velocity changes deduced from the sound attenuation.