Highly Efficient Second-Harmonic Generation in a Double-Layer Thin-Film Lithium Niobate Waveguide

We demonstrate unprecedentedly efficient second-harmonic generation in a thin-film lithium niobate waveguide, with conversion efficiency as high as 9300% W
^-1
cm
^-2
achieved, which is enabled by greatly enhancing the modal overlap of the higher-order mode in polarization-reversed dual-layer lithium niobate.