  Surface acoustic waves in two-dimensional phononic crystals consisting of a
square array of shallow, two to three micron deep cylindrical void inclusions
are studied computationally via the finite element method. For the [110]
propagation direction on a (001) GaAs half-space, the conventional Rayleigh
wave modes, the layered substrate-associated Sezawa and Lamb modes, the high
frequency longitudinal surface waves and bulk waves exhibit hybridization and
modal interaction. The longitudinal, vertical shear and horizontal shear bulk
wave dispersions are observed to be significant thresholds for surface acoustic
waves on a shallow phononic crystal. This results in dramatic changes in the
attenuation and surface boundedness properties that enable supersonic modes
with diminished attenuation in otherwise largely bulk wave-radiative dispersion
branch continua of modes.
