Abstract
Abstract 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.
Highlights
Terahertz (THz) technology has made great achievements in a large number of applications including biomedical diagnosis [1], security screening [2], and wireless communication [3] in recent years
We demonstrate the existence of Toroidal dipole (TD) resonances in our asymmetric all-dielectric metasurface structure for y-polarization
As symmetric breaking is introduced to this metasurface structure (a = 0.4 μm), as shown in Figure 2B, besides the magnetic dipole (MD) resonance previously discussed, two additional ultrasharp resonances located at 2.25 and 2.41 THz that originate from TD modes appear in transmission spectrum, which are marked as TD1 and TD2, respectively
Summary
Terahertz (THz) technology has made great achievements in a large number of applications including biomedical diagnosis [1], security screening [2], and wireless communication [3] in recent years. A high-index microtube-based all-dielectric metasurface structure was proposed, and a low-loss TD resonance could be obtained with a high-Q factor of 118 as well as a strong local field enhancement in the deep subwavelength scale, offering application potentials such as sensing, energy harvesting, and nonlinear optical effects [36]. It was revealed that all-dielectric metasurface with symmetry breaking of unit cell can support high-Q factor Fano resonances [41, 43] or TD resonances [44, 45] arising from the distortion of symmetryprotected BIC Such a BIC-inspired mechanism makes it possible to access extremely high-Q resonances and giant enhancement of electromagnetic fields [47], realizing many useful functionalities including lasing [48], polarization [49], and sensing [50]. Our findings present an effective approach for realizing ultrasensitive sensing at THz frequencies
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