Toward ultimate-efficiency frequency conversion in nonlinear optical microresonators

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