Toroidal dipole Fano resonances driven by bound states in the continuum of lithium niobate metasurface for efficient electro-optic modulation

Free-space optical applications benefit greatly from actively reconfigurable metasurfaces. While static metasurfaces have demonstrated remarkable capabilities in manipulating optical wave characteristics, their fixed electromagnetic response faces a challenge for active device demands. This work presents an all-dielectric metasurface of lithium niobate with high Q-factor Fano resonances for enhanced electro-optic modulation. The resonance mechanism is investigated through multipole decomposition and near-field analysis, revealing that toroidal dipole contributions are dominant in all found three resonances. The strongest resonance arises from symmetry-protected bound states in the continuum (SP-BIC), and it is excited in an asymmetric metasurface by introducing a perturbation in the symmetric nanohole clusters, to enable polarization-independent operation. We numerically achieve a tuning efficiency of 0.65 nm/V and π-phase retardation driven by a bias of 0.53 V, due to the strong field confinement of quasi-BIC mode. Our work provides a scheme for compact, high-performance, and electrically tunable metasurfaces that can help in the development of spatial optical switches as well as tunable micro-lenses and displays.