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ACS Photonics

Strong Resonant Optical Nonlinearity in Thin-Film Lithium Niobate

Yuqi Zhao, Dylan L. Renaud, Mohammad Habibur Rahaman, Neil E. Sinclair, Marko Lončar, Edo Waks A

Published Jan 21, 2026• 0 citations

Abstract

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.

DOI: 10.1021/acsphotonics.5c01586

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As an active material with favorable linear and nonlinear optical properties, thin-film lithium niobate has demonstrated its potential in integrated photonics. Integration with rare-earth ions, which are promising candidates for quantum memories and transducers, will enrich the system with new applications in quantum information processing. Here, we investigate the optical properties at 1.5 μm wavelengths of rare-earth ions (Er3+) implanted in thin-film lithium niobate waveguides and micro-ring resonators. Optical quality factors near a million after post-annealing show that ion implantation damage can be repaired. The transition linewidth and fluorescence lifetime of erbium ions are characterized. The ion-cavity coupling is observed through a Purcell enhanced fluorescence from which a Purcell factor of 3.8 ± 0.5, compared with waveguide lifetime, is extracted. This platform is compatible with top-down lithography processes and leads to a scalable path for controlling spin-photon interfaces in photonic circuits.

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Abstract On-chip optical filters are fundamental components in optical signal processing. While rare-earth ion-doped crystals offer ultra-narrow optical filtering via spectral hole burning, their applications have primarily been limited to those using bulk crystals, restricting their utility. In this work, we demonstrate cavity-enhanced spectral filtering based on rare-earth ions in an integrated nonlinear optical platform. We incorporate rare-earth ions into high quality-factor ring resonators patterned in thin-film lithium niobate. By spectral hole burning at 4 K in a critically coupled resonance mode, we achieve bandpass filters ranging from 7 MHz linewidth, with 13.0 dB of extinction, to 24 MHz linewidth, with 20.4 dB of extinction. By reducing the temperature to 100 mK to eliminate phonon broadening, we achieve an even narrower linewidth of 681 kHz, which is comparable to the narrowest filter linewidth demonstrated in an integrated photonic device, while only requiring a small device footprint. Moreover, the cavity enables reconfigurable filtering by varying the cavity coupling rate. For instance, as opposed to the bandpass filter, we demonstrate a bandstop filter utilizing an under-coupled ring resonator. Such versatile integrated spectral filters with high extinction ratio and narrow linewidth could serve as fundamental components for optical signal processing and optical memories on-a-chip.