Chen Zhaoxian, Peng Yugui, Li Haoxiang, Liu Jingjing, Ding Yujiang, Liang Bin, Zhu Xue-Feng, Lu Yanqing, Cheng Jianchun, Alù Andrea
Key Laboratory of Modern Acoustics, MOE, Institute of Acoustics, Department of Physics, Nanjing University, Nanjing 210093, People's Republic of China.
College of Engineering and Applied Sciences, Nanjing University, Nanjing 210093, People's Republic of China.
Sci Adv. 2021 Nov 5;7(45):eabj1198. doi: 10.1126/sciadv.abj1198. Epub 2021 Nov 3.
In linear, lossless, time-invariant, and nonbiased acoustic systems, mode transitions are time reversible, consistent with Lorentz reciprocity and implying a strict symmetry in space-time for sound manipulation. Here, we overcome this fundamental limitation by implementing spatiotemporally modulated acoustic metamaterials that support nonreciprocal sound steering. Our mechanism relies on the coupling between an ultrathin membrane and external biasing electromagnetic fields, realizing programmable dynamic control of the acoustic impedance over a motionless and noiseless platform. The fast and flexible impedance modulation of our metamaterial imparts an effective unidirectional momentum in space-time to realize nonreciprocal transitions in -ω space between different diffraction modes. On the basis of these principles, we demonstrate efficient nonreciprocal sound steering, showcasing unidirectional evanescent wave conversion and nonreciprocal upconversion focusing. More generally, our metamaterial platform offers opportunities for generation of nonreciprocal Bloch waves and extension to other domains, such as non-Hermitian topological and parity-time symmetric acoustics.
在线性、无损、时不变且无偏的声学系统中，模式转换是时间可逆的，符合洛伦兹互易性，并意味着在时空上对声音操纵具有严格的对称性。在此，我们通过实现支持非互易声控的时空调制声学超材料来克服这一基本限制。我们的机制依赖于超薄膜与外部偏置电磁场之间的耦合，在一个静止且无噪声的平台上实现对声阻抗的可编程动态控制。我们超材料的快速且灵活的阻抗调制在时空上赋予有效单向动量，以实现不同衍射模式之间在-ω空间中的非互易转换。基于这些原理，我们展示了高效的非互易声控，呈现出单向倏逝波转换和非互易上转换聚焦。更一般地说，我们的超材料平台为产生非互易布洛赫波以及扩展到其他领域提供了机会，例如非厄米拓扑和平权时间对称声学。
