Experimental realization of all-angle negative refraction in acoustic gradient metasurface

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.