High-Q silicon nitride microresonators exhibiting low-power frequency comb initiation

Optical resonators with high quality factors (Qs) are promising for a variety of applications due to the enhanced nonlinearity and increased photonic density of states at resonances. In particular,frequency combs (FCs) can be generated through four-wave mixing in high-Q microresonators made from Kerr nonlinear materials such as silica,silicon nitride,magnesium fluoride,and calcium fluoride. These devices have potential for on-chip frequency metrology and high-resolution spectroscopy,high-bandwidth radiofrequency information processing,and high-data-rate telecommunications. Silicon nitride microresonators are attractive due to their compatibility with integrated circuit manufacturing; they can be cladded with silica for long-term stable yet tunable operation,and allow multiple resonators to be coupled together to achieve novel functionalities. Despite previous demonstrations of high-Q silicon nitride resonators,FC generation using silicon nitride microresonator chips still requires pump power significantly higher than those in whispering gallery mode resonators made from silica,magnesium,and calcium fluorides,which all have shown resonator Qs between 0.1 and 100 billion. Here,we report on a fabrication procedure that leads to the demonstration of "finger-shaped" Si₃N₄ microresonators with intrinsic Qs up to 17 million at a free spectrum range (FSR) of 24.7 GHz that are suitable for telecommunication and microwave photonics applications. The frequency comb onset power can be as low as 2.36 mW and broad,single FSR combs can be generated at a low pump power of 24 mW,both within reach of on-chip semiconductor lasers. Our demonstration is an important step toward a fully integrated on-chip FC source.