Acoustic edge mode in spiral-based metamaterials at subwavelength scale

Recently, the quantum effect of condensed matter physics is introduced into the acoustic field, which lays a new pathway to manipulate the acoustic wave. However, the acoustic topological insulator based on Bragg scattering requires their lattice constant to be comparable with the wavelength. In this paper, a novel subwavelength spiral element on the basis of the Archimedean spiral is proposed. Thanks to the central resonator with a slender curved channels scheme, the subwavelength Dirac cone forms in the band structure. The eigenfrequency of the element can be changed by the spiral geometric parameters, which triggers the topological phase transition characterized by opposite valley Chern numbers. The backscattering-immune unidirectional transmission edge state exists at different topological boundaries, suggesting its great robustness even at the sharp bends. Also the topological edge modes along the Z-shaped interfaces are verified by the experiments and the calculation, which provides an effective structure to control the low frequency acoustic wave transmission.