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.

Demand for high levels of quality and safety in agricultural products and food requires appropriate analytical techniques for analysis both during and after production. Desirable techniques should be quick, easy, and safe to use; require minimal or no sample preparation; avoid sample destruction; and be accurate. In this study, the recent technical applications of terahertz spectroscopy to identify and classify, qualitatively and quantitatively analyze, evaluate, and safely control agricultural products and food are reviewed. The challenges and future outlook of terahertz spectroscopy are discussed.