High-efficiency second harmonic generation lithium niobate metasurfaces with electro-optic tunability

High-efficiency and actively tunable second-harmonic generation (SHG) in nonlinear metasurfaces is highly desirable for integrated nonlinear photonic applications. Here, we propose an electro-optically tunable lithium niobate metasurface consisting of an array of elliptical nanopillars with air-hole tetramers. By introducing structural asymmetry, quasi-bound states in the continuum (quasi-BIC) with a high-quality factor (high-Q) are formed, manifesting as a Fano resonance dominated by strong toroidal dipole excitation and enhanced optical field confinement. These resonant characteristics significantly strengthen the nonlinear light-matter interaction, yielding an optimized absolute SHG conversion efficiency of 3.2 × 10^-2 under a pump intensity of 5 kW/cm² in the O-band, corresponding to a normalized conversion efficiency of 6.4 × 10^-10 m²/W. In addition, the electro-optic effect of lithium niobate enables dynamic modulation of both the linear transmission spectrum and the SHG response under an externally applied voltage, and the designed lithium niobate metasurface achieves a wavelength tuning efficiency of 45 pm/V. By combining high-Q quasi-BIC resonances, efficient nonlinear frequency conversion, and active electro-optic tuning within a single metasurface platform, this work offers a promising route toward reconfigurable nonlinear photonic devices with potential applications in integrated frequency conversion, optical modulation, and photonic signal processing.