Efficient Second Harmonic Generation in a Reverse‐Polarization Dual‐Layer Crystalline Thin Film Nanophotonic Waveguide

Abstract Second harmonic generation (SHG), one of the most significant χ (2) nonlinear optical processes, plays a crucial role in a wide variety of optical and photonic applications. Designing various delicate schemes to achieve highly efficient SHG has become a long‐standing and challenging topic in nonlinear optics. Despite numerous success on SHG based on birefringent phase‐matching and quasi‐phase‐matching, so far, modal phase‐matching (MPM) for SHG in tightly light‐confined structures has still in its infancy. Here, a new scheme is proposed to realize efficient SHG via MPM by using a nanophotonic LiNbO 3 thin‐film waveguide consists of two bonded layers with internally reversed polarizations. In such a dual‐layer ridge waveguide based on lithium niobate on insulator (LNOI), upon optical excitation at 1574.6 nm, SHG is observed at 787.3 nm with a high conversion efficiency of 5540% W −1 cm −2 experimentally. This work advances the understanding on modal phase‐matched SHG and other quadratic optical nonlinear processes, offering additional strategies for developing high‐performance nonlinear photonic devices in on‐chip platforms.