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

Other text in scientific journal, 2026

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) moire superlattice microcavity. Specifically, we experimentally achieve flatband enhanced SHG in a 4.41 degrees-twisted moire cavity with a high quality (Q) factor of similar to 1100 and a normalized SHG conversion efficiency of (2.1 +/- 0.4) & times; 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 moire superlattices as a versatile platform for compact, integrated nonlinear photonics and next-generation quantum light sources.