Collaborative Research: Engineering Exceptional Points for Sound Control with Non-Hermitian Acoustic Metasurfaces

该项目致力于创建一类新的非厄米绕射声学超曲面。声波变形表面是一种薄的人造材料，可以控制声波。这些二维材料被赋予了对反射和传输声音的非凡控制能力。这项研究将在非厄米量子力学和声学之间建立类比，导致声学超表面设计策略的范式转变。该项目将通过发现新的机制来设计具有新功能的材料，从而广泛地推进声学超表面领域，这些材料将在噪音控制、建筑声学和通信中得到应用。这项研究的结果还将鼓励未来的基础研究，在这些研究中，声学可以作为实现量子力学概念的平台。研究活动将涉及本科生以及来自代表性不足群体的学生，并将促进创新教育活动。非厄米系统在宇称时间对称性破缺阈值以下具有完全实值的能谱，这被称为它的例外点。在这里，系统经历了戏剧性的相变，两个或更多的特征值及其对应的特征向量合并。虽然从物理上来说，这意味着该系统将展示奇异的波功能，但要达到一个特异点，需要对系统的假想部分进行明智的裁剪。这里的研究团队将非厄米物理学与声学超表面结合起来，寻求开发利用超表面微结构中无处不在的耗散的根本新设计。该项目将研究非厄米绕射变表面的散射矩阵，以便建立框架，将一定数量和顺序的例外点与其相应的散射矩阵联系起来。我们将从理论上研究以可控的方式剪裁变形表面积木中的损耗的方法，并将通过经典的声学理论来研究整个绕射行为背后的机制。最后，该团队将设计、制造并实验演示利用固有损耗的新型声学设备，如单向声扩散器。 这一奖项反映了国家科学基金会的法定使命，并通过使用基金会的智力价值和更广泛的影响审查标准进行评估，被认为值得支持。

This project focuses on creating a new class of non-Hermitian diffractive acoustic metasurfaces. Acoustic metasurfaces are thin artificial materials that can control sound waves. These two-dimensional materials are empowered with extraordinary control over the reflected and transmitted sound. This research will establish analogies between non-Hermitian quantum mechanics and acoustics, resulting in a paradigm shift in the design strategies for acoustic metasurfaces. The project will broadly advance the field of acoustic metasurfaces through discovering new mechanisms for designing materials with novel functionalities, which will have applications in noise control, architectural acoustics, and communication. The outcome of this research will also encourage future fundamental research where acoustics could serve as a platform to materialize quantum mechanics concepts. The research activities will involve undergraduate students, as well as students from under-represented groups, and will enable innovative educational activities. Non-Hermitian systems possess entirely real-valued energy spectra below a parity-time symmetry breaking threshold, which is referred to as its exceptional point. Here, the system undergoes a dramatic phase change with two or more eigenvalues and their corresponding eigenvectors coalescing. While physically this implies that the system would exhibit exotic wave functionalities, attaining an exceptional point requires the judicious tailoring of the system’s imaginary parts. The research team here, marries non-Hermitian physics with acoustic metasurfaces and seeks to develop fundamentally new designs that exploit the ubiquitous dissipation in the metasurface micro-structure. The project will examine the scattering matrices of non-Hermitian diffractive metasurfaces in order to establish frameworks that link exceptional points of a certain number and order to their corresponding scattering matrices. Methods for tailoring the loss in metasurface building blocks in a controllable manner will be theoretically investigated and mechanisms behind the overall diffraction behavior will be studied via classical acoustic theories. At last, the team will design, fabricate and experimentally demonstrate new acoustic devices that harness intrinsic losses, such as unidirectional sound diffusers.  This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.