Polarization gaps and negative group velocity in chiral phononic crystals: Layer multiple scattering method

We designed chiral phononic crystals and studied their properties by using the layer multiple scattering method. The transmittance curves and the corresponding band structures show that this kind of structure possesses significant polarization gaps. The chiral structures break the symmetry so that the degenerate transverse modes split into a pair of right-hand polarized mode and left-hand polarized mode. The polarization splitting in the low-frequency range is enhanced in systems of large filling ratios. We also demonstrate that chiral structures containing strongly resonant units can induce negative group velocity in elastic waves, and stronger resonance brings a wider band of negative group velocity.