Ultra high-Q tunable microring resonators enabled by slow light

High-Q nanophotonic resonators are crucial for many applications in classical and quantum optical processing, communication, and sensing. We achieve ultra-high quality factors via a method previously limited to bulk systems, preparing a highly transparent and strongly dispersive medium within the resonator that causes a reduction in the group velocity and a corresponding increase in the quality factor. We implement this via spectral hole burning in erbium-doped thin-film lithium niobate microring resonators, and show Q-factors enhanced by nearly three orders of magnitude to exceed 108. Additionally, we show dynamic control of the resonances via electro-optic tuning. Finally, we present a theoretical model for our experimentally observed resonator linewidths, which are not well-described by the standard Bloch equations. Our results show a dramatic reduction in the erbium dephasing rate under a strong optical drive, leading to much narrower linewidths than would otherwise be expected given the large circulating intensity in the resonator. Light can be slowed to extreme degrees in engineered media, giving rise to unique behavior. Here, the authors show that slow light enables the formation of ultra-narrow tunable mirroring resonators, introducing a new paradigm for controlling optical responses on a photonic chip.