Acoustic metasurfaces are at the frontier of acoustic functional material research owing to their advanced capabilities of wave manipulation at an acoustically vanishing size. Despite significant progress in the last decade, conventional acoustic metasurfaces are still fundamentally limited by their underlying physics and design principles. First, conventional metasurfaces assume that unit cells are decoupled and therefore treat them individually during the design process. Owing to diffraction, however, the non-locality of the wave field could strongly affect the efficiency and even alter the behavior of acoustic metasurfaces. Additionally, conventional acoustic metasurfaces operate by modulating the phase and are typically treated as lossless systems. Due to the narrow regions in acoustic metasurfaces’ subwavelength unit cells, however, losses are naturally present and could compromise the performance of acoustic metasurfaces. While the conventional wisdom is to minimize these effects, a counter-intuitive way of thinking has emerged, which is to harness the non-locality as well as loss for enhanced acoustic metasurface functionality. This has led to a new generation of acoustic metasurface design paradigm that is empowered by non-locality and non-Hermicity, providing new routes for controlling sound using the acoustic version of 2D materials. This review details the progress of non-local and non-Hermitian acoustic metasurfaces, providing an overview of the recent acoustic metasurface designs and discussing the critical role of non-locality and loss in acoustic metasurfaces. We further outline the synergy between non-locality and non-Hermiticity, and delineate the potential of using non-local and non-Hermitian acoustic metasurfaces as a new platform for investigating exceptional points, the hallmark of non-Hermitian physics. Finally, the current challenges and future outlook for this burgeoning field are discussed.

中文翻译：

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非局域和非厄米声学超表面


声学超表面因其在声学消失的尺寸下具有先进的波操纵能力而处于声学功能材料研究的前沿。尽管在过去十年中取得了重大进展，但传统的声学超表面仍然从根本上受到其基础物理和设计原理的限制。首先，传统的超表面假设单位单元是解耦的，因此在设计过程中单独处理它们。然而，由于衍射，波场的非局域性可能会强烈影响效率，甚至改变声学超表面的行为。此外，传统的声学超表面通过调制相位来工作，通常被视为无损系统。然而，由于声学超表面的亚波长晶胞区域较窄，损耗自然存在，并且可能会损害声学超表面的性能。虽然传统观点是尽量减少这些影响，但出现了一种反直觉的思维方式，即利用非局域性和损失来增强声学超表面功能。这催生了新一代声学超表面设计范式，该范式由非局域性和非气密性赋能，为使用声学版本的 2D 材料控制声音提供了新途径。本综述详细介绍了非局域和非厄米特声学超表面的进展，概述了最新的声学超表面设计，并讨论了非局域性和损耗在声学超表面中的关键作用。 我们进一步概述了非局域性和非厄米性之间的协同作用，并描述了使用非局域和非厄米声学超表面作为研究异常点（非厄米物理学的标志）的新平台的潜力。最后，讨论了这个新兴领域当前的挑战和未来的前景。