Topologically protected zero refraction of elastic waves in pseudospin-Hall phononic crystals

Zero-angle refraction of elastic waves in metamaterials has attracted attention for its extraordinary wave collimation properties. However, earlier implementations relied on the specific flat equifrequency curve of the phononic crystals suffer from a narrow range of incident angles or operating bandwidths, which severely hinders the exploration and design of functional devices. Here, we propose an elastic near-zero refractive index metamaterial of a triangular lattice to realize topological zero refraction with arbitrary angles of incidence and wide working frequency range. Topological robustness of the zero-angle refraction of pseudospin-Hall edge state against defects is experimentally demonstrated. Furthermore, tunable wave mode conversion associated with the zero-angle refraction is revealed and discussed. These results provide a paradigm for the simultaneous control of the refraction properties of longitudinal and transverse waves that can be employed for designing the topological elastic antennas and elastic wave collimator. Topological acoustic systems exhibit exotic properties including zero refraction, but typically only in a small region of frequency space. In this work, a zero-index acoustic metamaterial is proposed and fabricated functioning over a wide frequency regime in the presence of lattice defects.