Toward ultimate-efficiency frequency conversion in nonlinear optical microresonators.

Integrated nonlinear photonics has emerged as a transformative platform, enabling nanoscale nonlinear optical processes with substantial implications. Achieving efficient nonlinear frequency conversion in microresonators is paramount to fully unlocking this potential, yet the absolute conversion efficiency (ACE) remains fundamentally constrained by dissipative losses and intrinsic nonlinear effects. In this work, we establish a unified framework for second harmonic generation in microresonators, identifying a decisive factor that predicts the ACE limit under the nonlinear critical coupling (NCC) condition. Using this framework, we fabricate periodically poled lithium niobate microresonators and address the dispersive-dissipative suppression to approach the NCC condition. We achieve a record-high ACE of 61.3% with milliwatt-level pump powers toward the ultimate efficiency, with the potential for higher efficiency as the factor increases. These results provide a versatile paradigm for high-efficiency nonlinear optical devices, offering opportunities for advancements across classical and quantum photonic applications.