Second‐Harmonic Generation with a 440 000% W<sup>−1</sup> Conversion Efficiency in a Lithium Niobate Microcavity without Periodic Poling
Xiao Wu, Zhenzhong Hao, Li Zhang, Di Jia, Rui Ma, Can Tao, Feng Gao, Fang Bo, Guoquan Zhang, Jingjun Xu
Abstract
Abstract Thin‐film lithium niobate (TFLN) enables extremely high‐efficiency second‐order nonlinear optical effects due to large nonlinear coefficient d 33 and strong optical field confinement. Here, a pulley‐waveguide‐coupled microring resonator with an intrinsic quality factor above 9.4 × 10 5 on the reverse‐polarized double‐layer X‐cut TFLN is designed and fabricated. In such a TFLN resonator without fine domain structures, second harmonic generation with a normalized conversion efficiency of 440,000% W −1 with weak pump power, approaching that in periodically poled lithium niobate (PPLN) microring resonators, and an absolute conversion efficiency of 30% is realized with a sub‐milliwatt continuous pump. This work reduces the dependence of high‐efficiency nonlinear frequency conversion on PPLN microcavities that are difficult to prepare.