Arbitrary Electro-Optic Bandwidth and Frequency Control in Lithium Niobate Optical Resonators

Lithium niobate modulators are essential for optical communications infrastructure, and thin-film lithium niobate (TFLN) enables novel integrated photonic and quantum technologies. In situ tunability over the bandwidth and frequency of resonant systems would enable using photonic resonators as optical memories while also accommodating fabrication intolerance. We demonstrate bandwidth and frequency tuning over modes in a racetrack resonator by leveraging the electro-optic effect in lithium niobate. Our device, fabricated in TFLN atop a sapphire handle, consists of a racetrack resonator coupled to a feedback waveguide at two points. Applying DC bias voltage across the feedback loop introduces interference in the coupling. We observe a bandwidth tunability ratio of ~25 (max bandwidth/min bandwidth) at 1603.4 nm, with frequency tunability of ~680 MHz/V. We demonstrate that a Markovian input-output model does not fully capture this system's dynamics. Using scattering matrix theory, we derive a model to calibrate the mode's frequency and bandwidth tunability and to predict tuning with arbitrary bias voltage.