Broadband birefringence phase-matched second-harmonic generation in a slightly curved lithium niobate-on-insulator waveguide.

Lithium niobate-on-insulator (LNOI) waveguides, leveraging their excellent optical field confinement and high nonlinear coefficients, have become an ideal platform for achieving optical frequency conversion. Phase-matching interaction strength and bandwidth are critical factors in this process. Here, we propose a scheme using micrometer-scale, slightly curved waveguides on x-cut LNOI to achieve broadband birefringence phase matching (BPM). Experimentally, we observed a broadband second-harmonic generation (SHG) with a bandwidth exceeding 100 nm in the near-infrared regime, achieving the highest, to the best of our knowledge, normalized conversion efficiency of 1.38%/(W⋅cm2) at 1560 nm. For 500 fs pulsed light pumping, an absolute conversion efficiency of over 10% is achieved at a low input power. The approach is suitable for frequency conversion of ultrashort lasers on a chip. This approach presents a promising alternative to periodically poled lithium niobate (PPLN) for broadband frequency conversion applications.