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 their larger discrete resonator counterparts based on crystalline materials and flowable glass. Silicon nitride (Si3N4) ring resonators open up capabilities for optical routing, frequency comb generation, optical clocks and high precision sensing on an integrated platform. However, simultaneously achieving high quality factor and high confinement in Si3N4 (critical for nonlinear processes for example) remains a challenge. Here, we show that addressing surface roughness enables us to overcome the loss limitations and achieve high-confinement, on-chip ring resonators with a quality factor (Q) of 37 million for a ring with 2.5 μm width and 67 million for a ring with 10 μm width. We show a clear systematic path for achieving these high quality factors. Furthermore, we extract the loss limited by the material absorption in our films to be 0.13 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 ultra-low power frequency comb generation, high precision sensing, laser stabilization and sideband resolved optomechanics.