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

这个项目的重点是创建一类新的非厄米衍射声学超表面。声学超表面是一种薄的人造材料，可以控制声波。这些二维材料对反射和传播的声音具有非凡的控制能力。这项研究将建立非厄米量子力学和声学之间的类比，从而导致声学超表面设计策略的范式转变。该项目将通过发现具有新功能的设计材料的新机制，广泛推进声学超表面领域，这将在噪声控制，建筑声学和通信方面得到应用。这项研究的结果也将鼓励未来的基础研究，其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，并将使创新的教育活动成为可能。非厄米系统在奇偶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，系统经历了一个戏剧性的相变与两个或多个特征值和它们相应的特征向量合并。虽然从物理上讲，这意味着系统将表现出奇异的波功能，但要达到一个特殊点，需要对系统的虚部进行明智的剪裁。这里的研究团队将非厄米物理与声学超表面结合起来，并寻求开发新的设计，利用超表面微观结构中无处不在的耗散。该项目将检查非厄米衍射超表面的散射矩阵，以便建立将一定数量和顺序的异常点与其相应的散射矩阵联系起来的框架。将从理论上研究以可控的方式剪裁超表面构建块中的损失的方法，并通过经典声学理论研究整体衍射行为背后的机制。最后，该团队将设计、制造和实验展示利用固有损耗的新型声学设备，如单向扩音器。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识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.