Surface acoustic wave band gaps in a diamond-based two-dimensional locally resonant phononic crystal for high frequency applications

Abstract

We report on the theoretical investigation of a locally resonant phononic crystal operating in a hypersonic regime. Through the computation of the band structure and the acoustic displacement field of a two-dimensional array composed of aluminum stubs squarely arranged on a diamond semi-infinite substrate, we show the propagation of guided surface waves in the nonradiative region of diamond substrate (sound cone), as limited by the slowest bulk acoustic wave velocity. Owing to its highest acoustic velocities among all materials, diamond offers a very large nonradiative region allowing band gaps opening for surface acoustic waves. We show that hypersonic band gaps are opened in the nonradiative region as a result of local resonances of aluminum stubs which interact with the guided surface modes propagating on the diamond surface. In addition to the computation of band structure of surface acoustic waves, here, using local resonance mechanism in aluminum/diamond semi-infinite structure, we show that we can confine and guide only one surface defect mode in a waveguide which is unusual compared to the phononic crystals Bragg-based mechanism. This property can be very useful for filtering and demultiplexing applications, notably in hypersonic regime. The creation and the behavior of local resonance bandgap as well as the physics basis are discussed as function of geometrical parameters and involved localized mode natures. Numerical simulations are made making use of the finite element method combined to the supercell technique.