Toroidal dipole (TD) with weak coupling to the electromagnetic fields offers tremendous potential for advanced design of photonic devices. However, the excitation of high quality ( Q ) factor TD resonances in these devices is challenging. Here, we investigate ultrahigh- Q factor TD resonances at terahertz frequencies arising from a distortion of symmetry-protected bound states in the continuum (BIC) in all-dielectric metasurface consisting of an array of high-index tetramer clusters. By elaborately arranging the cylinders forming an asymmetric cluster, two distinct TD resonances governed by BIC are excited and identified. One is distinguished as intracluster TD mode that occurs in the interior of tetramer cluster, and the other one is intercluster TD mode that arises from the two neighboring clusters. Such TD resonances can be turned into ultrahigh- Q leaky resonances by controlling the asymmetry of cluster. The low-loss TD resonances with extremely narrow linewidth are very sensitive to the change in the refractive index of the surrounding media, achieving ultrahigh sensitivity level of 489 GHz/RIU. These findings will open up an avenue to develop ultrasensitive photonic sensor in the terahertz regime.

中文翻译：

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全介电超表面中连续态的束缚态控制具有高Q环形偶极子共振的超灵敏太赫兹感测

与电磁场的弱耦合的环形偶极子（TD）为光子器件的高级设计提供了巨大的潜力。但是，在这些设备中激发高质量（Q）因子TD谐振具有挑战性。在这里，我们研究由介电常数高的四聚体簇组成的全介电超表面的连续谱（BIC）中受对称保护的结合态的畸变，从而在太赫兹频率上产生了超高Q因子TD共振。通过精心安排形成不对称簇的圆柱，可以激发并识别出由BIC控制的两个截然不同的TD共振。一种是在四聚体簇内部发生的簇内TD模式，另一种是由两个相邻簇产生的簇间TD模式。通过控制簇的不对称性，可以将此类TD共振转变为超高Q泄漏共振。具有极窄线宽的低损耗TD谐振对周围介质的折射率变化非常敏感，实现了489 GHz / RIU的超高灵敏度。这些发现将为在太赫兹状态下开发超灵敏光子传感器开辟一条道路。