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 x 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.