In this work, we numerically and experimentally demonstrate that all-angle negative refraction can be obtained with the acoustic gradient metasurface of subwavelength thickness. The coiling labyrinthine structures are utilized to build the desired gradient metasurface, and the apparent negative refraction occurring beyond the critical incident angle has been validated by simulations and experimental measurements, which agrees well with the theoretical predictions given by the revised generalized law of refraction while taking the contribution of the Bragg scattering into account. This work provides the solution to manipulate the acoustic waves and shows good promise in building functional diffractive acoustic elements.
In this work, we present a type of binary metasurface (BM) to generate an acoustic Airy beam in air. Two coding bits, a rectangular cavity (bit “0”) and a waveguide with seven Helmholtz resonators (bit “1”), are adopted to construct the acoustic structure, which offers degrees of freedom to manipulate the transmitted field. The operating band is capable of customizing in an ultrabroadband of 3000–15 000 Hz owing to the linear-like phase shift and high transmittance of the coding bits. To verify the feasibility of the design, a BM with a certain parameter (w = 5) is fabricated with photosensitive resin via stereolithography, and the working band is customized as 4000–5500 Hz. The experiment results show that the apparent self-bending beam is able to be generated in a broadband, which agree well with the numerical simulation. In addition, we further demonstrate that self-focusing can be realized by taking advantage of two symmetrical BMs conveniently, which improve the functionality of the coding bits. These results may provide potential application in biomedical ultrasound and nondestructive testing.
In this work, we numerically and experimentally demonstrate that broadband acoustic focusing can be realized using a sub-wavelength binary metasurface. Rectangular cavities and Helmholtz resonators are utilized to construct a coding system, which brings the desired transmittance and phase difference in a wide range of wavelengths. The apparent acoustic focusing is validated in a bandwidth of 0.8f0–1.6f0 by experimental measurements, which agrees well with the numerical simulations and offers a degree of freedom to manipulate the focal length actively. This work provides a solution to design a sub-wavelength planar lens with broadband and robustness properties, which may have promising applications in numerous acoustic engineering procedures, including biomedical diagnosis and non-destructive testing.
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