Design of asymmetric waveguide for second harmonic generation based on thin film lithium niobate

Mode phase matching (MPM) is a widely used phase-matching technique in ?(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 ?((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 ?((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.