Second‐Harmonic Generation with a 440 000% W⁻¹ Conversion Efficiency in a Lithium Niobate Microcavity without Periodic Poling

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.