Surface waves in highly ordered poly-graphite and gold micro-layers studied by picosecond ultrasonic technique

Surface waves propagation in micro-layers is very useful for the evaluation of mechanical properties in anisotropic materials. In the present work, they are detected without any transducer by means of beam distortion detection technique in gold and highly ordered poly-graphite (HOPG) micro-layers. The surface wave velocity in HOPG is evaluated as 20 km/s, value close to Rayleigh wave velocity in diamond. The typical frequency (~10 GHz) of surface wave in HOPG is extremely high. Fast diffusion of electrons plays an important role in the process of sound excitation in gold. For this reason, a two-temperature model has been applied to describe the three-dimensional thermoelastic generation of bulk and surface acoustic waves. Good coincidence between experimental and calculated wavefronts is obtained.