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水下声学在水下通讯、水下定位和导航等方面具有广泛的应用。借助拓扑物理的概念，研究水基声子晶体中的拓扑态为水下声波的创新性调控提供了一种全新的手段，既有基础研究价值，又有重要的应用前景。本文设计了一种一维的双层铁栅水基声子晶体，通过把层间的相对横向平移量等效成一个合成维度，实现了合成二维狄拉克点。相对平移量的改变导致二重简并能带打开带隙。伴随着带隙的打开、闭合以及再次打开，能带发生翻转，也就是发生拓扑相变，从一种能谷相变化到另一种能谷相。在不同能谷相声子晶体构成的界面处，能谷陈数保证界面态的确定性存在。数值仿真与实验测量结果吻合良好，都展示了不同能谷相声子晶体的体能带以及它们之间的界面态色散。本文提出的水基声子晶体结构简单，借助合成维度的概念，为在低维实空间体系中研究高维体系拓扑特性提供了一种有效的途经，有望为拓扑功能性水声器件的设计提供新思路。此外，我们可以把实空间体系拓展到二维甚至是三维，并引入更多的合成维度，用于研究更高维度体系的拓扑态及其输运特性。Underwater acoustics has wide applications in underwater communications, underwater positioning, underwater navigation, and so on. Inspired by the concept of topological physics, the study of topological states in waterborne phononic crystals provides a brand-new way for innovative control of underwater waves, which has both basic research value and important application prospects. In this work, we design a one-dimensional bilayer iron grid waterborne phononic crystal to realize a synthetic two-dimensional Dirac point by considering the relative lateral translation between the two layers as a synthetic dimension. Through changing the relative lateral translation, the double degenerate band opens a gap, which is characterized by the valley Chern number. As the band gap opens, closes and reopens, the bulk band undergoes a band inversion, that is, a topological phase transition from one valley topological phase to another. At the interface formed by two phononic crystals with distinct valley topological phases, the valley Chen number ensures the deterministic existence of the interface state. Experimental measurements are in good agreement with numerical simulations, both showing the bulk bands of waterborne phononic crystals at different valley topological phases and the interface state dispersion between them. The waterborne phononic crystal proposed in this work hosts a simple structure. With the help of the concept of synthetic dimension, it provides an effective way to study the topological properties of high-dimensional systems in low-dimensional real space systems, and offers new ideas for the design of topological functional underwater acoustic devices. In addition, we can expand the real space system to two or even three dimensions, and introduce more synthetic dimensions to study the topological states and associated transport characteristics of higher-dimensional systems.
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Keywords:
- waterborne phononic crystal /
- synthetic dimension /
- Dirac point /
- interface state
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