The inherent trade-off between efficiency and bandwidth of three-wave mixing processes in χ2 nonlinear waveguides is the major impediment for scaling down many well-established frequency conversion schemes onto the level of integrated photonic circuit. Here, we show that hybridization between modes of a silica microfiber and a LiNbO3 nanowaveguide, amalgamated with laminar χ2 patterning, offers an elegant approach for engineering broadband phase matching and high efficiency of three-wave mixing processes in an ultra-compact and natively fiber-integrated setup. We demonstrate exceptionally high normalized second harmonic generation (SHG) efficiency of up to ηnor ≈ 460% W−1 cm−2, combined with a large phase matching bandwidth of Δλ ≈ 100 nm (bandwidth-length product of Δλ · L ≈ 5 μm2) near the telecom bands, and extraordinary adjustment flexibility.
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Cai, L., Gorbach, A., Wang, Y., Hu, H., & Ding, W. (2018). Highly efficient broadband second harmonic generation mediated by mode hybridization and nonlinearity patterning in compact fiber-integrated lithium niobate nano-waveguides. Scientific Reports, 8(1), 1-9. . https://doi.org/10.1038/s41598-018-31017-0