拓扑声学中新颖物理效应的研究

Topological acoustics or topological sonic crystal is a new concept proposed in recent two years, and this concept soon attracts significant attention of physical scientists, especially those experts in condensed matter physics. The unique advantage of topological sonic crystals is that due to the protection of nontrivial topological properties of frequency bands, there are robust one-way edge states immune to backscattering existing in topological sonic crystals, and these one-way edge states can propagate around arbitrary defects on the boundary without any losses. This unique property of robust one-way edge states can be utilized to design novel acoustic waveguides, beneficial to the future design of integrated acoustic devices. However, compared with the highly developed research fields of traditional sonic crystals, acoustic metamaterials, topological electronics, and topological photonics, the research on topological acoustics has just started, leaving many unresolved questions for scientists to explore. Based on the stated significance of topological acoustics, in this project we will study the physical effects and mechanism of topological sonic crystals from theory, simulation and experiment simultaneously. Firstly, we will realize topological sonic crystals with large Chern numbers, containing multiple gapless edge states in one single bandgap, by modulating the frequencies of Dirac degeneracies and quadratic degeneracies. Secondly, we will realize topological Anderson sonic crystal, which can recover topologically protected one-way edge states by introducing disorder into a topological sonic crystal with simultaneously broken time-reversal symmetry and space-inversion symmetry. Thirdly, we will realize Floquet topological sonic crystal by introducing broken z-reversal symmetry into the designed structure, where z is along the third dimension in space. By executing this project, we will have a systematic and deep understanding of topological acoustics, paving the theoretical foundation for its applications in important regimes of ultrasonic medicine, architectural acoustics, nondestructive detection, hydroacoustics, etc..

拓扑声学或拓扑声子晶体作为最近两年才提出的新概念，一出现就引起了研究人员尤其是凝聚态物理学家的高度重视，其特殊的优点在于：受能带的非平庸拓扑性质保护，拓扑声子晶体中存在能够抑制背散射的鲁棒的单向边界态，这种单向边界态能够绕过缺陷进行无损耗地传播，可以用来设计新型声波导，有利于未来集成声学器件的设计。然而，与已经高度发展的传统声子晶体、声学超构材料以及拓扑电子学、拓扑光子学相比，拓扑声子晶体的研究才刚刚开始，还有很多尚未解答的问题等待研究人员去探究。有鉴于此，申请人把拓扑声子晶体作为研究对象，从理论、模拟、实验三个方面系统深入研究其物理效应与内在机制，并探索三种不同性质的拓扑声子晶体：拥有大陈数的拓扑声子晶体、拓扑安德森声子晶体以及Floquet拓扑声子晶体。通过本项目的研究，争取对拓扑声子晶体有一个系统而深入的认识，为其在超声医学、建筑声学、无损检测、水声学等重要领域的应用奠定理论基础。

拓扑声子晶体是当前凝聚态物理研究领域的前沿热点课题之一，由于能带非平庸拓扑性质的保护，拓扑声子晶体拥有能够抑制背散射的鲁棒的单向声学边界态，采用这种新型声波导有利于未来集成声学器件的设计。因此，深入系统地研究拓扑声子晶体的物理原理或内在机制，并将相关领域（例如传统声子晶体、声学超构材料）中已有的知识与拓扑电子学或拓扑光子学的最新进展结合起来，会在拓扑声学这一新领域碰撞出新的想法并会产生新的物理效应，在超声医学、建筑声学、无损检测、水声学等重要领域都有非常实际的应用前景。. 本课题将拓扑声子晶体作为研究对象，从理论、模拟、实验三个方面系统深入研究其物理效应与内在机制，研究如何利用拓扑性质调控声波的传播，取得以下成果：1.设计了具有大陈数的时间反演对称性破缺的拓扑声子晶体，在带隙中存在多条受能带拓扑性质保护的单向传输无能隙边界态；2.在具有手性层间耦合的三维声子晶体中实验观测到了声学外尔点与表面态费米弧，验证了表面态的无障碍传播；3.利用声学表面倏逝波的干涉构造了声学斯格明子晶格，晶格单元内部的声质点振速矢量呈现出类似于磁性斯格明子的拓扑自旋构型；4.发展了矢量声学探测技术。. 本项目在拓扑声学方面的研究结果拓展了声学微结构材料及物理效应的研究领域，为研究声子玻色系统的拓扑效应提供了研究平台，为声学器件，特别是声子集成芯片、声学探测、声学二极管等新兴领域的应用，开拓了新思路。