Attenuation of longitudinal-acoustic phonons in amorphousSiO2at frequencies up to 440 GHz

We report measurements of the attenuation of longitudinal-acoustic phonons in amorphous ${\mathrm{SiO}}_2$ for frequencies between 76 and 440 GHz, and in the temperature range 80–300 K. The samples are chemical-vapor-deposited films grown on tungsten substrates, and the phonons are generated and detected using picosecond optical techniques. The phonon attenuation is found to depend only weakly on temperature. In the upper part of the frequency range, there is some evidence that the frequency dependence is faster than quadratic, thus indicating that the attenuation cannot be explained by classical relaxation theory. A discussion of the relation between these results and the analysis of thermal-conductivity measurements is given.