All-angle Negative Reflection with An Ultrathin Acoustic Gradient Metasurface: Floquet-Bloch Modes Perspective and Experimental Verification

Bingyi Liu,Wenyu Zhao,Yongyuan Jiang,Jiajun Zhao,Xiaodong Xu

doi:10.1038/s41598-017-14387-9

Abstract

Metasurface with gradient phase response offers new alternative for steering the propagation of waves. Conventional Snell’s law has been revised by taking the contribution of local phase gradient into account. However, the requirement of momentum matching along the metasurface sets its nontrivial beam manipulation functionality within a limited-angle incidence. In this work, we theoretically and experimentally demonstrate that the acoustic gradient metasurface supports the negative reflection for all-angle incidence. The mode expansion theory is developed to help understand how the gradient metasurface tailors the incident beams, and the all-angle negative reflection occurs when the first negative order Floquet-Bloch mode dominates inside the metasurface slab. The coiling-up space structures are utilized to build desired acoustic gradient metasurface, and the all-angle negative reflections have been perfectly verified by experimental measurements. Our work offers the Floquet-Bloch modes perspective for qualitatively understanding the reflection behaviors of the acoustic gradient metasurface, and the all-angle negative reflection characteristic possessed by acoustic gradient metasurface could enable a new degree of the acoustic wave manipulating and be applied in the functional diffractive acoustic elements, such as the all-angle acoustic back reflector.

Highlights

We will introduce the perspective of Floquet-Bloch mode to help understand how the gradient metasurface interacts with the incident field, and how the acoustic energy trapped by the metasurface slab is tunneled into the −1 order diffraction for all-angle incidence
The acoustic gradient metasurface(AGM) we study here is modeled as an anisotropic waveguide sandwiched between the free space(air) and the rigid wall(sound hard boundary)
The all-angle negative reflection characteristic possessed by the acoustic gradient metasurface has the potential application in composing ultrathin compact acoustic insulator operating in reflection mode[34], for the incident acoustic wave would be reflected back into the incident half space and thereby avoid the cross talk between the acoustic waves coming from opposite directions

Summary

Introduction

Notwithstanding that, recently, several research articles have reported that the apparent free-space propagating scattered field, which is featured as negative reflection[34,35] and negative refraction[12,36,37], can be observed even when the beam illuminates beyond the critical angle. These intriguing phenomena indicate that the underlying physical mechanism of the beam manipulation functionality of the acoustic gradient metasurface is still needed to be studied. The perspective of Floquet-Bloch mode for all-angle negative reflection has been well validated by both the theoretical calculations and experimental measurement results, and the apparent negative reflection exists beyond the critical angle incidence

Methods

Results

Conclusion