Topological negative refraction of surface acoustic waves in a Weyl phononic crystal
Topological negative refraction of surface acoustic waves in a Weyl phononic crystal
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外尔声子晶体中表面声波的拓扑负折射
DOI：
10.1038/s41586-018-0367-9
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发表时间：
2018-08-02
期刊：
影响因子：
64.8
通讯作者：
Liu, Zhengyou
中科院分区：
文献类型：
--
作者：
He, Hailong;Qiu, Chunyin;Liu, Zhengyou
波的反射和折射发生在两种不同介质的界面上。这两种基本的界面波现象形成了制造各种波元件（如光学透镜）的基础。经典折射-现在称为正折射-导致透射波出现在界面法线的相对侧，与入射波相比。相反，负折射导致透射波出现在界面法线的同一侧。它已经在人造材料中观察到（1-5），遵循其理论预测（6），并刺激了许多应用，包括超分辨率成像（7）。一般来说，在折射过程中，反射是不可避免的，但这往往是不希望在设计波功能器件。在这里，我们报告负折射的拓扑表面波托管的外尔声子晶体-一个声学模拟最近发现的外尔半金属（8-12）。这种拓扑负折射发生的界面是分离晶体不同面的一维边缘。通过调整外尔声子晶体的表面终端，可以设计表面声波的恒定频率轮廓，以在某些界面处产生负折射，而在同一样品内的不同界面处实现正折射。与我们更熟悉的波在界面处的行为相反，由于恒定频率轮廓的开放性质，我们的晶体可以防止不必要的反射，这是Weyl晶体中拓扑保护表面态的标志（8-12）。
Reflection and refraction of waves occur at the interface between two different media. These two fundamental interfacial wave phenomena form the basis of fabricating various wave components, such as optical lenses. Classical refraction-now referred to as positive refraction-causes the transmitted wave to appear on the opposite side of the interface normal compared to the incident wave. By contrast, negative refraction results in the transmitted wave emerging on the same side of the interface normal. It has been observed in artificial materials(1-5), following its theoretical prediction(6), and has stimulated many applications including super-resolution imaging(7). In general, reflection is inevitable during the refraction process, but this is often undesirable in designing wave functional devices. Here we report negative refraction of topological surface waves hosted by a Weyl phononic crystal-an acoustic analogue of the recently discovered Weyl semimetals(8-12). The interfaces at which this topological negative refraction occurs are one-dimensional edges separating different facets of the crystal. By tailoring the surface terminations of the Weyl phononic crystal, constant-frequency contours of surface acoustic waves can be designed to produce negative refraction at certain interfaces, while positive refraction is realized at different interfaces within the same sample. In contrast to the more familiar behaviour of waves at interfaces, unwanted reflection can be prevented in our crystal, owing to the open nature of the constant-frequency contours, which is a hallmark of the topologically protected surface states in Weyl crystals(8-12).