Direct observation of a hypersonic band gap in two-dimensional single crystalline phononic structures

Brillouin light scattering is employed to record the phonon dispersion relation of two-dimensional (2D) hypersonic phononic crystals with square lattice plane group p4mm symmetry. The samples are single crystalline arrays of cylindrical holes with a lattice constant of 750 nm and 35% porosity patterned in epoxy using interference lithography. The dispersion relation reveals the presence of a phononic band gap between 1.21 and 1.57 GHz at the edge of the first Brillouin zone for elastic waves propagating along the [10] direction and conclusively demonstrates a band gap in a single crystalline 2D polymer based phononic structure at hypersonic frequencies.