Anomalous refraction of acoustic guided waves in solids with geometrically tapered metasurfaces

TL;DR

This paper demonstrates how engineered acoustic metasurfaces with tapered structures can manipulate elastic guided waves in solids, enabling anomalous refraction, mode conversion, and focusing in thin-walled structures.

Contribution

It introduces a novel design of acoustic metasurfaces using torus-like tapers to control elastic wave refraction and mode conversion in solids.

Findings

Achieved anomalous refraction of guided waves consistent with generalized Snell's law.

Enabled mode conversion between symmetric and antisymmetric wave modes.

Designed metasurfaces functioning as planar lenses and phase masks for elastic waves.

Abstract

The concept of metasurface has recently opened new exciting directions to engineer the refraction properties in both optical and acoustic media. Metasurfaces are typically designed by assembling arrays of sub-wavelength anisotropic scatterers able to mold incoming wavefronts in rather unconventional ways. The concept of metasurface was pioneered in photonics and later extended to acoustics while its application to the propagation of elastic waves in solids is relatively unexplored. We investigate the design of acoustic metasurfaces to control elastic guided waves in thin-walled structural elements. These engineered discontinuities enable anomalous refraction of guided wave modes consistently with the generalized Snell's law. The metasurfaces are made out of locally-resonant torus-like tapers enabling accurate phase shift of the incoming wave which ultimately affects the refraction properties. We show that anomalous refraction can be achieved on transmitted antisymmetric modes ($A_0$) either when using a symmetric ($S_0$) or antisymmetric ($A_0$) incident wave, the former clearly involving mode conversion. The same metasurface design also allows achieving structure embedded planar focal lenses and phase masks for non-paraxial propagation.