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Lithium niobate (LN), dubbed by many as the silicon of photonics, hasrecently risen to the forefront of chip-scale nonlinear optics research sinceits demonstration as an ultralow-loss integrated photonics platform. Due to itssignificant quadratic nonlinearity ($\chi^{(2)}$), LN inspires many importantapplications such as second-harmonic generation (SHG), spontaneous parametricdown-conversion, and optical parametric oscillation. Here, we demonstratehigh-efficiency SHG in dual-resonant, periodically poled z-cut LN microrings,where quasi-phase matching is realized by field-assisted domain engineering.Meanwhile, dual-band operation is accessed by optimizing the couplingconditions in fundamental and second-harmonic bands via a single pulleywaveguide. As a result, when pumping a periodically poled LN microring in thelow power regime at around 1617nm, an on-chip SHG efficiency of 250,000%/W isachieved, a state-of-the-art value reported among current integrated photonicsplatforms. An absolute conversion efficiency of 15% is recorded with a low pumppower of 115$\mu$W in the waveguide. Such periodically poled LN microrings alsopresent a versatile platform for other cavity-enhanced quasi-phase matched$\chi^{(2)}$ nonlinear optical processes.

Periodically poled thin film lithium niobate microring resonators with a second-harmonic generation efficiency of 250,000%/W