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 d33 and strong optical field localization. Here, we first designed and fabricated a pulley-waveguide-coupled microring resonator with an intrinsic quality factor above 9.4 x10^5 on the reverse-polarized double-layer X-cut TFLN. In such a TFLN resonator without fine domain structures, second harmonic generation with an absolute (normalized) conversion efficiency of 30% (440,000% W-1), comparable to that in periodically poled lithium niobate (PPLN) microring resonators, was realized with a sub-microwatt continuous pump. This work reduces the dependence of high-efficiency nonlinear frequency conversion on PPLN microcavities that are difficult to prepare.