Second harmonic generation in an anisotropic lithium niobate metasurface governed by quasi-BICs

Resonant metasurfaces can greatly trap the light fields, so that they are widely used to enhance light-matter interactions at the nanoscale, such as promoting nonlinear effects of materials. Lithium niobate (LN) is an excellent nonlinear optical material and is often employed to generate harmonic signals. In this Letter, we numerically study the second harmonic generation (SHG) characteristics of the LN metasurface based on the quasi-bound states in the continuum (QBIC). The designed BIC and excited QBIC metasurfaces always hold C_4v symmetry, and the BIC is demonstrated to degenerate into two BICs due to the anisotropic characteristics of LN. Moreover, the excited two high Q-factor QBICs can effectively enhance the SHG in LN, although the device maintains C_4v symmetry, the SHG signal still shows polarization dependence. In addition, with the increase of Q-factor of quasi-BIC, the power and conversion efficiency (η) of SHG increase significantly. The calculated η can reach 6.04 × 10^-3 and can be further improved when the resonance mode is closer to BIC. These results have important implications for high-quality nonlinear light sources based on LN materials.