Abstract
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.
Highlights
Zero-angle refraction of elastic waves in metamaterials has attracted attention for its extraordinary wave collimation properties
We show that by inhomogeneously changing the ellipse orientation in a triangular lattice, an elastic near-zero refractive index metamaterial (NZIM) can be obtained that possesses a double Dirac cone with fourfold degeneracy at the center of the Brillouin zone
The evolution of the unit cell when varying C is depicted in the inset of Fig. 1a, where pattern I corresponds to the radical configuration (C = 0°) with the nontrivial phase and pattern ш corresponds to the azimuthal configuration (C = 90°) with the trivial phase
Summary
Zero-angle refraction of elastic waves in metamaterials has attracted attention for its extraordinary wave collimation properties. Owing to the topological effects of the edge modes, this form of refraction possesses the abilities to immunize against defects and is independent of the input field These performance superiorities open up a new way to study topological zero refraction in PCs. To date, intensive research effort has been devoted to achieve topological positive and negative refraction in the scalar (longitudinal wave) system of fluid airborne acoustic[42,43,44,45,46], whose practical role is largely limited. Our research provides a robust way to manipulate the refraction characteristics of longitudinal and transverse waves simultaneously and has potential applications in elastic wave collimation and underwater communications
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