Flexible dimer metasurfaces for terahertz quasi-BIC resonances and sensing

Metasurface-based quasi-bound states in the continuum (quasi-BIC) have emerged as a powerful platform for high-performance optical sensing, owing to their ultra-high quality (Q) factors that greatly strengthen light-analyte interactions. Conventional quasi-BIC excitation schemes primarily rely on symmetry breaking of individual resonant unit cells, which inherently constrain structural design and performance modulation. To address this constraint, this work proposes what we believe to be a novel terahertz (THz) metasurface architecture constructed with H-shaped dimer resonators on a flexible polyimide (PI) substrate. By introducing the asymmetric parameter between two closely spaced subwavelength resonators, the transitions from BIC to quasi-BIC are experimentally realized. Combined with the multipole decomposition and near-field analysis, it is revealed that the generation of quasi-BIC mode in H-shaped dimer metasurface is dominated by electric quadrupole (EQ) modes. As a proof of concept, we perform biosensing experiments using bovine serum albumin (BSA) as a model analyte. The results reveal that the quasi-BIC resonance exhibits observable emergence and disappearance with increasing BSA concentration. This work provides theoretical and experimental support for the development of flexible THz quasi-BIC devices and presents promising application prospects in the field of THz biodetection.