零折射率运动介质中的非互易Willis耦合
Non-Reciprocal Willis Coupling in Zero-Index Moving Media
摘要：
机械运动可以打破声音在介质中传播的对称性，但通常只有在非常大的运动速度下才能实现显著的非互易性。在这里，我们结合移动介质与零指数声传播，产生极端的非互易性和诱导的双各向异性适度的应用速度。超材料由亥姆霍兹谐振器加载的波导阵列形成，并且它表现出由非对称威利斯耦合维持的折射率的相反符号，用于在相反方向上传播。我们利用这种响应设计了一种非互易声透镜，仅在一侧激发时聚焦，并应用于成像和超声技术。
英文摘要：
Mechanical motion can break the symmetry in which sound travels in a medium, but significant non-reciprocity is typically achieved only for very large motion speeds. Here we combine moving media with zero-index acoustic propagation, yielding extreme non-reciprocity and induced bianisotropy for modest applied speeds. The metamaterial is formed by an array of waveguides loaded by Helmholtz resonators, and it exhibits opposite signs of the refractive index sustained by asymmetric Willis coupling for propagation in opposite directions. We use this response to design a non-reciprocal acoustic lens focusing only when excitation from one side, with applications for imaging and ultrasound technology.
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