Electro-optically tunable second-harmonic generation in lithium niobate metasurfaces boosted by Brillouin zone folding induced bound states in the continuum

Lithium niobate (LN) metasurfaces exhibit remarkable Pockels effect-driven electro-optic tunability, enabling dynamic control of optical responses through external electric fields. When combined with their high second-order nonlinear susceptibility (${\ensuremath{\chi}}^{(2)}$), this tunability is projected into the nonlinear landscape, realizing second-harmonic generation (SHG)-dominated functionalities in integrated photonics. However, achieving deep SHG modulation in LN metasurfaces remains challenging due to LN's limited refractive index tunability under practical driving voltages. To address this, we design an air-hole-structured LN metasurface by strategically adjusting air hole positions to induce Brillouin zone folding-enabled bound states in the continuum with ultrahigh $Q$ factors ($Q>{10}^{4}$). Numerical simulations demonstrate a $2.59%$ SHG conversion efficiency at $0.1\phantom{\rule{0.28em}{0ex}}\mathrm{MW}/{\mathrm{cm}}^{2}$ excitation and a modulation depth exceeding 0.99 under 15 V peak-to-peak voltage ($\mathrm{\ensuremath{\Delta}}{V}_{\text{pp}}$). This work establishes a compact framework for electrically tunable nonlinear optics, advancing applications in integrated quantum light sources and programmable photonic chips.