Flatband Resonance Enhanced Second-Harmonic Generation in Thin-Film Lithium Niobate Moiré Microcavity

Optical nonlinear processes are fundamental to numerous applications, from frequency conversion to quantum technologies. Although subwavelength integrated photonic structures can enhance nonlinear interactions, they rarely exploit the broad oblique wavevector components of tightly focused pump beams, restricting the nonlinear conversion efficiency. Here, we demonstrate efficient second-harmonic generation (SHG) pumped by tightly focused beams containing broad oblique wavevector components in thin-film lithium niobate (TFLN) moiré superlattice microcavity. Specifically, we experimentally achieve flatband enhanced SHG in a 4.41°-twisted moiré cavity with a high quality (Q) factor of ∼1100 and a normalized SHG conversion efficiency of (2.1 ± 0.4) × 10^-4 cm²/GW at an extremely low pump power density of 1.1 ± 0.2 MW/cm². The synergy of high Q, flatband dispersion, and broad angular acceptance maximizes pump utilization, enabling high-efficiency nonlinear conversion. This breakthrough establishes TFLN moiré superlattices as a versatile platform for compact, integrated nonlinear photonics and next-generation quantum light sources.