三维空气声学拓扑态研究

Topology is a brand-new degree of freedom to describe the propagation of wave or particle, distinguished from the other classical degrees of freedom such as energy, momentum, spin and phase. In past decades, the concept of topology has been playing a very important role in condensed matter physics. A lot of new electronic and photonic topological states have been theoretically proposed and experimentally observed, such as quantum anomalous Hall effect，topological insulator and Weyl semimetal. The topology describes the global behavior of matter with "step" and "quantum" characters. The hallmark of topological states is their backscattering-immune propagation or spin-momentum locking, which have great potential in applications. From 2015, the researchers find that the longitudinal airborne sound can also be used to realize acoustic topological states. From then on, topological acoustics is becoming one of emerging and booming research fields. However, Up to date, the experimentally realized acoustic topological states mainly focus on tow-dimensional systems, which are relative large size and far from practical application. And, how to construct and control pseudospin of longitudinal acoustic wave need to be further investigated. In this proposal, we focus on three-dimensional topological states of airborne sound. Such extra dimension can bring us more types of acoustic topological states, even deeper fundamental physics. In particular, our project includes three parts: 1) to theoretically and experimental investigate three-dimensional acoustic topological insulators; 2) to study other types of three-dimensional acoustic topological states, such as Dirac and nodal-line acoustic topological "semimetals" and corner states; 3) from the point of application view, to manipulate the acoustic pseudospins, try to realize three-dimensional acoustic topological propagation. We believe we can find and realize some novel and unique acoustic topological effect and phenomena via this project, which can definitely deepen our understanding of the topological physics, and may explore some new acoustic topological functional prototype devices.

拓扑，作为除频率、波矢、自旋、相位外全新的全局自由度，在近几十年凝聚态物理的研究中发挥了巨大的作用，其背散射抑制和自旋-轨道锁定输运等特性有望获得巨大应用。自2015年起，人们发现空气纵声波也可实现拓扑态，并可通过声学人工微结构方便地制备和调控。声学拓扑态是一个新兴的，快速发展的领域。然而，目前的实现体系主要集中在二维体系，且尺寸较大不易应用；对构造和应用纵声波赝自旋的研究任显不够。在本项目中，我们拟着重关注三维空气声波体系的拓扑态问题。维度的增加将带来更多的拓扑类型，以及更丰富、更深刻的物理。我们拟从以下几个方面着手，1）理论探索并实验实现三维空气声拓扑绝缘体；2）研究多类型的空气声拓扑态；3）调控声赝自旋态，探索空气声三维拓扑传输。我们相信利用三维空气声拓扑模型有望实现电子系统中难以实现或调控的拓扑效应和现象，从而加深对凝聚态物理中拓扑本质的认识，也更接近现实应用。

本项目关注三维空气声波体系的拓扑态问题，主要研究人工微结构声学材料的拓扑物态设计、制备和调控。经过四年的研究，团队圆满完成了计划书中所约定的内容，在三维声学拓扑绝缘体和拓扑半金属方面取得了一系列成果，建立了系统的相关理论和技术路线。实现了带宽大于20%，对比度大于20dB的拓扑声传输。基于弹性声波系统，研制出了高Q值声学拓扑微腔，展示了高效吸声、上行、下载以及拓扑慢声等功能。代表性工作包括：1）率先研制了一种全新的半狄拉克型三维声拓扑绝缘体，构建了具有方向性色散的杂化声学表面态：一个方向为线性，而另一个方向为二次型。2）首次实现了三维声拓扑绝缘体以及高阶拓扑绝缘体，通过对称性重构和破缺方式构建了无能隙的二维一阶狄拉克型表面态和一维二阶棱态，拓展了声学拓扑材料的适用范围和操控维度。3）实验证实了拓扑狄拉克半金属中的体-棱对应关系，发现无论何种棱型，一维棱态均可稳定存在并直接连接一对狄拉克点，互补形貌（如锯齿和胡须状）的棱态位于互补的布里渊区中。4）研制了弹性声拓扑波导-谐振器件原型，在临界耦合时，实现了能量只进不出且无背散射这一独特功能。目前，本项目已发表高水平论文10余篇（含接收），包括: Phys. Rev. Lett.文章2篇、Nat. Commun.文章2篇、Natl. Sci. Rev.文章1篇，多项工作受到国内外广泛关注。在人才培养方面，已毕业硕士生4名，指导在读硕士研究生5人、博士研究生3人，专职科研和博士后人员各1人，何程获2020年国家自然科学基金优青项目资助。总体而言，项目按计划顺利开展，已全部完成研究任务，并达到了预期目标，在多个具体方面的工作属首次实现，如三维声拓扑绝缘体、拓扑弹性声临界耦合等，在三维声拓扑物态及调控方面研究起到了显著的推动作用。