Highly efficient second-harmonic generation in a double-layer thin-film lithium niobate waveguide

Thin-film lithium niobate (LN) has emerged as an ideal platform for efficient nonlinear wave-mixing processes due to its strong quadratic nonlinearity and high optical confinement. We demonstrate unprecedentedly efficient second-harmonic generation (SHG) in a double-layer thin-film LN waveguide. The modal overlap between fundamental and second-harmonic waves is significantly enhanced by the polarization-reversed double layers, leading to a normalized conversion efficiency higher than 10,000% W−1 cm−2 in theory. Under the low- and high-power pumping conditions, the measured normalized and absolute conversion efficiencies are 9600% W−1 cm−2 and 85%, respectively, substantially higher than state-of-the-art values among the reported SHGs in thin-film LN waveguides. Our results hold great promise for the development of efficient and scalable nonlinear photonic devices, with applications including metrology and quantum information processing.