Excitation and sensing application of dual-bound states in the continuum in metallic terahertz metasurfaces

High-Q resonances driven by bound states in the continuum (BIC) in metasurfaces have recently attracted significant attention due to their promising applications in nanoscale lasers, sensors, and nonlinear optical devices. Here, we propose and experimentally verify the excitation of dual-BIC in metallic terahertz metasurfaces. The metasurfaces consist of a periodically arranged two joint split rings on a quartz substrate. Two modes driven by symmetry-protected BIC (mode III) and parameter-tuned BIC (mode II) besides dipole resonance (mode I) can be observed by ingeniously designing the two joint split rings. Such BIC modes can be transformed into quasi-BIC modes with ultrahigh-Q that are sensitive to the change of surrounding media and thus provide an excellent platform for sensing applications. Mode Ⅱ performs best and obtains an ultrahigh sensitivity of 200 GHz/RIU. A thinner substrate with a lower refractive index can further enhance the sensitivity in this case. Our work provides a new route to develop ultrahigh sensitivity optical sensors working at terahertz frequencies.