标题： 直接观察二阶拓扑光子晶体板的角态 显示英文标题

标题： Direct observation of corner states in second-order topological photonic crystal slabs

摘要： 近年来，一种不遵循通常体-边对应原理的高阶拓扑相在电子绝缘体中被引入，并被带入经典系统中，其特点是具有间隙内的角/铰链态。迄今为止，二阶拓扑绝缘体已在机械超材料、微波电路、拓扑电学电路和声学超材料中实现。在这里，我们使用近场扫描测量，展示了在由完美电导体（PEC）上的周期性介质杆组成的二阶拓扑光子晶体（PC）板中角态的直接观察。基于广义的二维（2D）Su-Schrieffer-Heeger（SSH）模型，我们表明角态的出现源于非零的边缘偶极极化，而不是非零的体四极极化。我们通过调节两个相邻杆之间的胞内距离，展示了PC板二维Zak相的拓扑转变。我们还在微波范围内直接观察到了间隙内的1D边态和0D角态。我们的工作表明，PC板是一个强大的平台，可以直接观察拓扑态，并为研究高阶光子拓扑绝缘体铺平了道路。 显示英文摘要

摘要： Recently, higher-order topological phases that do not obey the usual bulk-edge correspondence principle have been introduced in electronic insulators and brought into classical systems, featuring with in-gap corner/hinge states. So far, second-order topological insulators have been realized in mechanical metamaterials, microwave circuit, topolectrical circuit and acoustic metamaterials. Here, using near-field scanning measurements, we show the direct observation of corner states in second-order topological photonic crystal (PC) slabs consisting of periodic dielectric rods on a perfect electric conductor (PEC). Based on the generalized two-dimensional (2D) Su-Schrieffer-Heeger (SSH) model, we show that the emergence of corner states roots in the nonzero edge dipolar polarization instead of the nonzero bulk quadrupole polarization. We demonstrate the topological transition of 2D Zak phases of PC slabs by tuning intra-cell distances between two neighboring rods. We also directly observe in-gap 1D edge states and 0D corner states in the microwave regime. Our work presents that the PC slab is a powerful platform to directly observe topological states, and paves the way to study higher-order photonic topological insulators.