Mode phase matching (MPM) is a widely used phase-matching technique in χ(2) nonlinear thin film lithium niobate waveguides for efficient wavelength conversion. Unfortunately, the conversion efficiency is limited by the reduction in the spatial overlap of the three interacting waves due to the intrinsic mode mismatch of spatial symmetry. However, changing the χ(2) distribution of the waveguide can greatly increase the mode overlap factor, resulting in high second-harmonic conversion efficiency. In this work, we have optimized the structure of the semi-nonlinear photonic waveguide to greatly attenuate the shift of the central wavelength due to the fine structure of the waveguide while ensuring a similar conversion efficiency. This designed waveguide is more suitable for devices with high wavelength range requirements. Then, we propose a MPM scheme, which can achieve a large mode overlap factor of 0.8019 and a theoretical normalized SHG conversion efficiency of up to 10,872 % W − 1 cm − 2 by modifying the χ(2) distribution of the waveguide. The proposed working principle is also applicable to other quadratic nonlinear platforms and has great potential for applications in nonlinear optics.

中文翻译：

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基于薄膜铌酸锂的二次谐波非对称波导设计

模式相位匹配 (MPM) 是 χ(2) 非线性薄膜铌酸锂波导中广泛使用的相位匹配技术，可实现高效波长转换。不幸的是，由于空间对称性的固有模式失配，三个相互作用波的空间重叠减少限制了转换效率。然而，改变波导的χ(2)分布可以大大增加模式重叠因子，从而获得较高的二次谐波转换效率。在这项工作中，我们优化了半非线性光子波导的结构，以极大地衰减由于波导的精细结构而导致的中心波长的偏移，同时确保类似的转换效率。这种设计的波导更适合波长范围要求较高的器件。然后，我们提出一个MPM方案，通过修改波导的χ(2)分布，可以实现0.8019的大模式重叠因子和高达10,872%W−1cm−2的理论归一化SHG转换效率。所提出的工作原理也适用于其他二次非线性平台，在非线性光学中具有巨大的应用潜力。