Toward ultimate-efficiency frequency conversion in nonlinear optical microresonators

TL;DR

This paper develops a theoretical framework for optimizing second-harmonic generation in microresonators, achieving a record-high 61.3% conversion efficiency with potential for further improvements, advancing integrated nonlinear photonics.

Contribution

It introduces a unified theory predicting the upper limit of efficiency and demonstrates experimental realization approaching this limit in lithium niobate microresonators.

Findings

Achieved record-high 61.3% ACE in SHG microresonators.

Identified the key factor M predicting efficiency limits.

Demonstrated approach to near-critical coupling conditions.

Abstract

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.