RESUMO
We have used the ultrafast pump-probe technique known as picosecond ultrasonics to generate and detect surface acoustic waves on a structure consisting of nanoscale Al lines on SiO2 on Si. We report results from ten samples with varying pitch (1000-140â¯nm) and SiO2 film thickness (112â¯nm or 60â¯nm), and compare our results to an isotropic elastic calculation and a coarse-grained molecular dynamics simulation. In all cases we are able to detect and identify a Rayleigh-like surface acoustic wave with wavelength equal to the pitch of the lines and frequency in the range of 5-24â¯GHz. In some samples, we are able to detect additional, higher frequency surface acoustic waves or independent modes of the Al lines with frequencies close to 50â¯GHz. We also describe the effects of probe beam polarization on the measurement's sensitivity to the different surface modes.
