Strong Resonant Optical Nonlinearity in Thin-Film Lithium Niobate

Thin-film lithium niobate is a promising integrated nonlinear photonics platform due to its strong second- and third-order nonlinear response. However, these off-resonant nonlinear mechanisms typically require high input optical powers beyond the microwatt level. In this work, we achieve optical nonlinearity at picowatt input power by incorporating the strong resonant nonlinear absorption of rare-earth ions into high-quality-factor thin-film lithium niobate ring resonators. By precisely controlling the output coupling strength of the resonator, we demonstrate multiple nonlinear response behaviors including optical limiting and negative differential transmission, in which the output power decreases with increasing input power. These nonlinear responses are well described by a model of an ensemble of inhomogeneously broadened three-level atoms coupled to a cavity. We also demonstrate optical bistability and hysteresis of the device with a bistable lifetime of approximately 3 ms. This versatile low-power nonlinear device can serve as a fundamental building block for nonlinear thin-film lithium niobate photonic circuits.