Resonance and Topological Characteristics of BICs Based on Geared Structure THz Metasurface for Sensing Applications

In optical applications, optical resonators with high quality factors (Q) are crucial for enhancing light-matter interactions. Nevertheless, achieving high-Q factors in optical devices is often challenging. Bound states in the continuum (BIC) in the continuous spectrum provide an important physical mechanism to overcome this challenge as they can significantly increase the Q factor of optical resonators. Here, we have revealed two symmetry-protected BICs in all-dielectric geared-structure metasurface, which is topologically protected by polarization vortices. When symmetry of the metasurface is broken, BIC transforms into quasi-BIC (QBIC) states with ultra-high Q factors. As the asymmetry parameter continues to increase, the Q factors exhibits an anomalous increase beyond the typical inverse square law that can be attributed to the strong mode coupling between the two isolated bands. Interestingly, these narrow linewidth, low-loss QBIC resonances are highly sensitive to changes in the refractive index of the analyte to be measured, with an impressive figure of merit (FoM) of 159033 and detection sensitivity of 338 GHz/RIU. These works provide a method for the development of THz photonics devices such as THz sensors, and THz resonators based on BIC.