Investigation of surface acoustic wave propagation in composite pillar based phononic crystals within both local resonance and Bragg scattering mechanism regimes

We investigate the propagation of surface acoustic wave through phononic crystals (PCs) that consist of composite pillars, comprising of a cap metallic pillar and a bottom epoxy pillar on the semi-infinite substrate. The computation of band structures, together with measured transmission spectra, shows the important role of the bottom pillar in engineering the band structures for its Young's modulus one order of magnitude smaller than that of the cap pillar. We also discuss the conditions of ignoring the influences of the bottom pillar in PCs featuring extremely high cap pillars. We finally modify the height of the bottom pillar to engineer the band structures of PCs, leading to, in both the simulation and experiment, Fano resonance-like transmission in the long wavelength regime.