  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.
