Developing natural “free space” frequency upconversion is essential for photonic integrated circuits. In a single-crystal lithium niobate thin film planar waveguide of less than 1 μm thickness, we achieve type I and type II mode phase-matching conditions simultaneously for this thin film planar waveguide. Finally, by employing the mode phase matching of e+e→e with d33 at 1018 nm, we successfully achieve a green second-harmonic wave output with the conversion efficiency of 0.12%/(W·cm2), which ve
