Dual bound states in the continuum in near-field coupled terahertz metasurfaces

Abstract The concept of bound states in the continuum (BICs) originates from quantum mechanics, in which a resonant (bound) state can persist within a range of unbound (or non-resonating) states. BICs are designated as completely non-radiative modes represented by infinite (ideally) Q -factors. Recently, the concept of BICs or quasi-BICs (q-BICs) has garnered considerable attention in the context of terahertz metasurfaces. We demonstrate the excitation of q-BICs in near-field coupled dual resonator-based terahertz metasurfaces, which evolve into BICs through structural symmetry protection. Our distinctive yet relatively straightforward design demonstrates dual pathways for the excitation of BICs for orthogonal electric field polarizations of the probing terahertz beam. Our investigations reveal that for one polarization q-BICs are excited through inductive–capacitive resonance coupling, while dipole–dipole resonance coupling is responsible for BIC effects in the orthogonal probe polarization. Simultaneous realization of BIC effects for orthogonal probe polarizations holds great potential towards the actualization of multi-frequency resonant cavities.