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Thin-film lithium niobate (TFLN) enables extremely high-efficiencysecond-order nonlinear optical effects due to large nonlinear coefficient d33and strong optical field localization. Here, we first designed and fabricated apulley-waveguide-coupled microring resonator with an intrinsic quality factorabove 9.4 x10^5 on the reverse-polarized double-layer X-cut TFLN. In such aTFLN resonator without fine domain structures, second harmonic generation withan absolute (normalized) conversion efficiency of 30% (440,000% W-1),comparable to that in periodically poled lithium niobate (PPLN) microringresonators, was realized with a sub-microwatt continuous pump. This workreduces the dependence of high-efficiency nonlinear frequency conversion onPPLN microcavities that are difficult to prepare.

Second-harmonic generation with a 440,000% W-1 conversion efficiency in a lithium niobate microcavity without periodic poling