Generation and detection of surface acoustic waves in aluminum

We present optical studies of Rayleigh surface acoustic wave excitation and detection in aluminum. Gigahertz-frequency Rayleigh waves are generated on the surface of an Al film, by optically pumping and probing the surface through a periodic array of polymer wires. We map out the dispersion relation of Rayleigh waves in the polymer-loaded metal systems in three limits of Al thickness on Si: an Al half-space, a finite thickness Al layer, and an ultra-thin Al layer. The measured Rayleigh mode-crossover from Al to Si agrees well with molecular dynamics simulations and generally follows the trend predicted using elasticity theory. The simulated data for the Rayleigh branch agree very well with the measured modes. We are satisfied that the molecular dynamics simulation is capable of modeling acoustic waves in polymer-on-metal transducers. In particular, we show this for polymer thicknesses approaching the metal thickness. In the limit of large polymer loading, the frequency-shift of the Rayleigh mode is considerable and is not predictable using perturbation theory. We demonstrate that the molecular dynamics simulation has merit in predicting surface acoustic wave modes and vibrational modes in large Poisson ratio materials, such as polymers.