Ultra-low-loss on-chip resonators with sub-milliwatt parametric oscillation threshold

On-chip optical resonators have the promise of revolutionizing numerous fields, including metrology and sensing; however, their optical losses have always lagged behind those of their larger discrete resonator counterparts based on crystalline materials and silica microtoroids. Silicon nitride (Si3N4) ring resonators open up capabilities for frequency comb generation, optical clocks, and high-precision sensing on an integrated platform. However, simultaneously achieving a high quality factor (Q) and high confinement in Si3N4 (critical for nonlinear processes, for example) remains a challenge. Here we show that addressing surface roughness enables overcoming the loss limitations and achieving high-confinement on-chip ring resonators with Q of 37 million for a ring of 2.5 mu m width and 67 million for a ring of 10 mu m width. We show a clear systematic path for achieving these high Qs, and these techniques can also be used to reduce losses in other material platforms independent of geometry. Furthermore, we demonstrate optical parametric oscillation using the 2.5 mu m wide ring with sub-milliwatt pump powers and extract the loss limited by the material absorption in our films to be 0.13 +/- 0.05 dB/m, which corresponds to an absorption-limited Q of at least 170 million by comparing two resonators with different degrees of confinement. Our work provides a chip-scale platform for applications such as ultralow-power frequency comb generation, laser stabilization, and sideband-resolved optomechanics. (C) 2017 Optical Society of America