Etchless lithium niobate metasurfaces for enhanced second-harmonic generation via Fabry-Pérot bound states in the continuum

High-quality factor ($Q$ factor) optical resonators constitute the fundamental components of integrated nonlinear photonic devices. On the lithium niobate [${\mathrm{LiNbO}}_{3}$ (LN)] platform, conventional approaches for achieving high-$Q$ modes typically involve thin-film etching and the deliberate breaking of structural symmetry. However, these methods entail increased fabrication complexity and impose limitations on device scalability and tunability. In this work, we propose an etchless LN metasurface leveraging Fabry-P\'erot bound states in the continuum (FP-BICs) to enable efficient second-harmonic generation (SHG). This structure sandwiches an unetched LN thin film between two identical gratings. Using the hybrid coupling model and neglecting the influence of evanescent wave coupling, the approximate film thickness required to support an FP-BIC with an infinite $Q$ factor can be predicted. By detuning the film thickness by 1% from the exact FP-BIC condition, the FP-BIC can be converted into a high-$Q$ quasi-BIC, enabling a high-efficiency SHG of 2.72% under a pump intensity of $100\phantom{\rule{0.28em}{0ex}}\mathrm{kW}/{\mathrm{cm}}^{2}$, according to rigorous finite-element simulations. Furthermore, dynamic on-to-off modulation of SHG is demonstrated on this platform by applying an external voltage. This design eliminates the need for etching and symmetry-breaking operations, significantly simplifying the fabrication process and providing a new platform for tunable nonlinear photonics in on-chip light sources and quantum technologies.