Nonlinearity Manipulation for Highly Efficient Modal Phase‐matched Second Harmonic Generation on Thin‐Film Lithium Niobate

Abstract While modal phase matching (MPM) shows great potential for adaptable engineering in quadratic nonlinear optical processes, poor mode overlap in traditional MPM persists as a key challenge. In this paper, a flexible domain engineering method are proposed and demonstrated for second‐harmonic generation (SHG) on thin‐film lithium niobate (TFLN). By meticulously tuning the electric field intensity applied to the poling electrodes with an Al 2 O 3 layer, the depth of domain inversion layer in TFLN can be controlled flexibly and locally, leading to the manipulation of nonlinearity, which effectively increases the mode overlap. SHG in the dual‐layer TFLN ridge waveguides has been experimentally realized with a normalized conversion efficiency of 8300% W −1 cm −2 . The flexible and fabrication‐friendly approach provides a versatile method for diverse MPM scenarios, and presents considerable potential for efficient quadratic nonlinear processes.