Tunable Switching of Quasi-bound States in the Continuum in Terahertz Metasurfaces

Bound states in the continuum (BIC) are a special type of optical resonance with strong field confinement and minimal energy loss, making them highly attractive for photonics applications. In this work, we propose a terahertz metasurface capable of supporting both symmetry-protected BIC and quasi-BIC (q-BIC) resonances. By intentionally breaking the structural symmetry, the optical response transitions from a symmetry-protected BIC to a leaky q-BIC resonance, enabling strong light-matter interaction. Especially, integrating the phase-change material vanadium dioxide (VO₂) enables reversible, thermally driven switching between BIC and q-BIC modes via its insulator-to-metal transition. Meanwhile, a maximum modulation depth of 96% can be achieved by tuning the conductivity of VO₂. Furthermore, the high quality-factor (Q-factor) of the q-BIC mode endows the hybrid metal-VO2 metasurface with remarkable sensing capabilities, achieving a sensitivity of 3708 GHz/RIU. This study advances the design of tunable terahertz devices, offering insights for dynamic modulators, switches, and high-performance sensors in the terahertz frequency range.