具有几何锥形超颖表面的固体中声导波的反常折射
Anomalous refraction of acoustic guided waves in solids with geometrically tapered metasurfaces
摘要：
超颖表面的概念最近开辟了新的令人兴奋的方向，工程师在光学和声学介质的折射性能。超颖表面通常通过组装亚波长各向异性散射体的阵列来设计，所述亚波长各向异性散射体能够以相当非常规的方式来塑造入射波前。超颖表面的概念首先出现在光子学中，后来扩展到声学，而其在固体中弹性波传播中的应用相对未被探索。我们研究的声学超颖表面的设计，以控制弹性导波薄壁结构元件。这些工程不连续性使得导波模式的异常折射与广义斯涅耳定律一致。超颖表面由局部共振的环面状锥体制成，使得入射波能够精确相移，这最终影响折射特性。我们表明，异常折射可以实现对透射反对称模式（$A_0$）时，无论是使用对称（$S_0$）或反对称（$A_0$）入射波，前者显然涉及模式转换。相同的超颖表面设计还允许实现用于非近轴传播的嵌入平面聚焦透镜和相位掩模的结构。
英文摘要：
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.
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