Ultra-high-Q tunable microring resonators by slow light

Using spectral hole burning in an erbium-doped lithium niobate microring resonator, we achieve Q-factors exceeding 10^8, facilitated by strong dispersion that significantly reduces the group velocity. This reduction causes light to spend more time completing each round trip, thereby extending the ringdown time. The experimental data is modeled using modified Bloch equations, revealing an intensity-dependent dephasing rate that approaches the radiative limit (T2→2T1​) at higher field strengths. Finally, we demonstrate its potential as an on-chip MHz-level tunable optical filter by leveraging electro-optic tuning.