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.

We proposed and demonstrated an ultra-high sensitive refractive index metamaterial sensor within terahertz region. The sensor consists of a gallium arsenide (GaAs) structure on the top, aluminum in the middle, and a polyimide as the bottom substrate. The simulation results show that such sensor has an absorption peak at 2.0 THz with the Q-factor up to 444. The sensor is very sensitive to the change of the refractive index (RI) of the surrounding medium. The metamaterial sensor with optimized structure has a sensitivity of 1.762 THz/RIU and a figure of merit (FoM) value of 392 RIU ⁻¹ , respectively. Meanwhile, both the absorption peak position and intensity of such metamaterial sensor could be adjustable. The proposed terahertz metamaterial sensor with such high sensitivity and easily tunable properties has broad application prospects in research fields including biomedical sensing, disease diagnosis, and also trace detection of hazardous substances.