Abstract
The ability to slow down wave propagation in materials has attracted significant research interest. A successful solution will give rise to manageable enhanced wave–matter interaction, freewheeling phase engineering and spatial compression of wave signals. The existing methods are typically associated with constructing dispersive materials or structures with local resonators, thus resulting in unavoidable distortion of waveforms. Here we show that, with helical-structured acoustic metamaterials, it is now possible to implement dispersion-free sound deceleration. The helical-structured metamaterials present a non-dispersive high effective refractive index that is tunable through adjusting the helicity of structures, while the wavefront revolution plays a dominant role in reducing the group velocity. Finally, we numerically and experimentally demonstrate that the helical-structured metamaterials with designed inhomogeneous unit cells can turn a normally incident plane wave into a self-accelerating beam on the prescribed parabolic trajectory. The helical-structured metamaterials will have profound impact to applications in explorations of slow wave physics.
Highlights
The ability to slow down wave propagation in materials has attracted significant research interest
We present a type of dispersion-free helicalstructured metamaterials that are able to slow down acoustic waves at broad bandwidth, by introducing helical wave rotation and wavefront revolution to the propagating waves
Our work provides a fertile ground for acoustic wave manipulations and fundamental explorations of slow wave physics
Summary
The ability to slow down wave propagation in materials has attracted significant research interest. The phase change is decided by the helicity of the proposed metamaterials, tunable by adjusting the thread lead Such flexibility is highly desirable in phase engineering applications[32,33,34,35,36], such as designs of innovative ultrathin flat acoustic lenses, acoustic rectifiers, high efficient couplers for surface acoustic waves and self-accelerating beam generators[13,14,15,16,17,18,37,38,39,40]. From the effective medium point of view, the new helical-structured metamaterial, as a whole, is equivalent to an effective high-indexed medium without introducing extra rigid medium, providing high space utilization in the folding process and non-dispersive acoustic impedance (Supplementary Figs 1 and 2). Given the same effective refractive index or space folding ratio, the effective mass density of air in the helical-structured metamaterial is much larger than the one in the labyrinthine metamaterial[15,16,17,18], provides a highly competitive mass module candidate for the more complex spring-mass modelled resonance-based metamaterial
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