Active control of quasi-bound states in the continuum in VO₂ metasurfaces for enhanced terahertz sensing

Bound states in the continuum (BICs) are resonant modes that reside within a light cone yet exhibiting perfect confinement, characterized by zero spectral width, no radiative leakage, and infinite lifetime. Incorporating BICs into actively tunable materials offers a promising route to enhance optoelectronic device performance. Here, we utilize vanadium dioxide (VO₂) metasurfaces to realize high-performance terahertz refractive index sensing by exploiting the theoretically infinite quality (Q) factor of BIC mode. Two types of BICs are identified in the VO₂ metasurface: symmetry-protected BIC (SP-BIC) and accidental BIC. By investigating quasi-BIC modes near these BICs, we demonstrate outstanding refractive index sensing sensitivity and figure of merit (FOM). Notably, the quasi-BIC mode associated with the accidental BIC maintains a high FOM across varying external environment refractive indices. Furthermore, we investigated the influence of VO₂ conductivity on sensing performance, showing that active tuning significantly enhances both sensing sensitivity and FOM. Q factor analysis and multipolar decomposition reveal that this improvement arises from the tunable nature of the accidental BIC. Additionally, incorporating an optical gain substrate mitigates performance degradation due to intrinsic losses. This work integrates BIC physics with active materials to achieve superior terahertz sensing, offering new insights for the design of advanced sensor devices.