  Optical parametric oscillators (OPOs) have been widely used for decades as
tunable, narrow linewidth, and coherent light sources for reaching long
wavelengths and are attractive for applications such as quantum random number
generation and Ising machines. To date, waveguide-based OPOs have suffered from
relatively high thresholds on the order of hundreds of milliwatts. With the
advance in integrated photonic techniques demonstrated by high-efficiency
second harmonic generation in aluminum nitride (AlN) photonic microring
resonators, highly compact and nanophotonic implementation of parametric
oscillation is feasible. Here, we employ phase-matched AlN microring resonators
to demonstrate low-threshold parametric oscillation in the telecom infrared
band with an on-chip efficiency up to 17% and milliwatt-level output power. A
broad phase-matching window is observed, enabling tunable generation of signal
and idler pairs over a 180 nm bandwidth across the C band. This result
establishes an important milestone in integrated nonlinear optics and paves the
way towards chip-based quantum light sources and tunable, coherent radiation
for spectroscopy and chemical sensing.
