Excitation and detection of surface acoustic phonon modes in Au/Al₂O₃ multilayers

A time-resolved pump-probe technique was used to observe the surface acoustic modes of Au/Al₂O₃ multilayers. As the thickness of the Au layers was increased from 2 to 5 nm, a change in the excitability of particular surface phonon modes with frequencies up to 200 GHz was observed. We compare our results with transfer-matrix theoretical calculations. In addition, we model the superlattice in a finite element simulation environment and use the data to evaluate the excitability of the surface modes. The model predictions correlate well with the experimental data. Simultaneous excitation of the surface and normal (propagating) phonon modes is observed in the Au(5 nm)/Al₂O₃(45 nm) superlattice. The normal modes allow us to calculate the effective sound velocity of the superlattice, which agrees well with the theory calculation.