(color online) (a) A unit cell of the phononic crystal based on an LiNbO slab with periodically etched cylindrical holes; r and a are the radius of the holes and the lattice constant, respectively. (b) The irreducible Brillouin zone for the hexagonal lattices. (c) The band structure of the phononic crystal slab with hexagonal lattices calculated by COMSOL with the parameters of m, m, and m. The shaded part indicates the complete phononic band gap obtained in the structure.