Surface acoustic waves in pillars-based two-dimensional phononic structures with different lattice symmetries

The theoretical study deals with the propagation of surface acoustic waves in two-dimensional arrays of resonant elements with different symmetry lattices. The resonant elements are cylindrical pillars on the surface of a semi-infinite substrate. The obtained band structures show the interaction of the pillars acoustic resonances with the semi-infinite medium which form additional band gaps that are decoupled from Bragg gaps. Especially, the frequency position of the lowest band gap is invariant with respect to lattice symmetries. Thereby, this position is independent of the lattice pitch, which is unexpected in band gaps based on Bragg interferences. However, the role of the period remains important for defining the non-radiative region limited by the slowest bulk mode and influencing the existence of the guided modes. Numerical simulations are based on the efficient finite element method and considered silicon pillars on a silicon substrate.