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

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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.