Microfiber-Lithium Niobate on Insulator Hybrid Waveguides for Efficient and Reconfigurable Second-Order Optical Nonlinearity on a Chip

Highly efficient broadband second harmonic generation mediated by mode hybridization and nonlinearity patterning in compact fiber-integrated lithium niobate nano-waveguides

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 η_nor ≈ 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.