Lithium niobate quadratic integrated nonlinear photonics: enabling ultra-wide bandwidth and ultrafast photonic engines

TL;DR

This Perspective argues that χ^(2) nonlinear photonics in thin-film lithium niobate (TFLN) provides a powerful path to ultra-wide bandwidth and ultrafast on-chip photonic engines. By leveraging the strong χ^(2) nonlinearity, quasi-phase matching, and exceptionally efficient electro-optic effects, LN enables rapid, wide-range wavelength tuning and high-peak-power ultrafast pulse generation through architectures including dispersion-engineered OPOs, FM-OPOs, E-O combs, χ^(2) SCG, and synchronously pumped OPOs. Key quantitative highlights include normalized nonlinear conversion efficiencies of η0 > 1000 %/W·cm^2 and near-degenerate parametric gain spanning from ~900 nm to ~1350 nm, with broader tunability achievable via cascaded or DFG-assisted approaches. The authors discuss practical challenges (e.g., photorefractive effects in UV-visible, mode management in broadband OPOs) and outline a roadmap toward monolithic, multi-functional photonic systems for sensing, communications, and quantum information processing.

Abstract

Integrated photonic coherent light sources capable of generating emission with broad spectral coverage and ultrashort pulse durations are critical for both fundamental science and emerging technologies. In this Perspective, we start by discussing emerging quantum and classical photonic applications from the standpoint of operating wavelength and timescale, highlighting the technological gaps that persist in current integrated photonic light sources. Next, we introduce the unique properties of lithium niobate-based integrated quadratic nonlinear photonics, and discuss several promising strategies that exploit this platform to realize wavelength-tunable continuous wave light sources and broadband, ultra-short light pulse generation. We also assessed their advantages and limitations while discussing potential solutions. Finally, we outline future prospects and challenges that need to be addressed, aiming at inspiring continued research and innovation in this rapidly evolving field.

Figures (13)

• Figure 1: Non-exhaustive summary of emerging classical and quantum photonic applications across different wavelengths and timescales. Sub-figure (a) is adapted from Ref. schotz2019perspective; (b) is adapted from Ref. epping2015chip; (d) is adapted from Ref. lu2024emerging; (e) is adapted from Ref. mcmahon2016fully; (i) is adapted from Ref. lanin2014time. Subfigures (c), (f), (h), (j) and (k) were generated with the assistance of ChatGPT for illustrative purposes. It ought to be noted that many applications can span a large portion of the wavelength and timecale, either inherently or depending on the specific application instance.
• Figure 2: Lithium niobate $\chi^{(2)}$ integrated nonlinear photonics and its key features. (a) Schematic of PPLN nanophotonic waveguide. Arrows denote the orientations of ferroelectric domains. (b) Scanning electron microscope (SEM) image of the PPLN nanophotonic waveguide. (c) Second harmonic microscope image of the periodic poling region in (b). (d) Simulated $\chi^{(2)}$ SHG process with (red) and without (blue) QPM (adapted from Ref. guo2022femtojoule). (e) Calculated maximum normalized conversion efficiency as a function of FH frequency for different material platforms (adapted from Ref. jankowski2021dispersion). (f) GVM (red) and GVD (blue) for the quasi-TE modes of a dispersion-engineered LN nanophotonic waveguide, with the corresponding cross-sectional geometry and optical mode profiles shown in the insets (adapted from Ref. guo2022femtojoule).
• Figure 3: Fast E-O wavelength-tunable CW lasers on TFLN. (a) Schematic of fast wavelength-tunable self-injection laser. Laser wavelength tuning is achieved by applying a voltage signal on the tungsten electrodes (yellow) on TFLN. (b) False-colour SEM image of a heterogeneous Si$_3$N$_4$–LiNbO$_3$ waveguide cross-section. (c) Laser frequency change with time upon applying a linearly-modulated electrode voltage. The blue shaded region indicates the self-injection locking of the laser frequency to the microring resonance. (d) Time-frequency spectrograms of the heterodyne beatnotes at different modulation frequencies. The bottom shows the corresponding frequency deviation. (e) Schematic of a Pockels laser integrating a RSOA with external cavity DBR mirror. (f) Photo of the Pockels laser device. (g) The cross section of the e-DBR section. (h) Time-frequency spectrograms of the heterodyne beatnote and the corresponding frequency deviation. (i) Laser frequency tuning efficiency versus modulation speed. (a)-(d) are adapted from Ref. snigirev2023ultrafast, (e)-(i) are adapted from Ref. xue2025pockels.
• Figure 4: (a) Concept of the ultra-wide band light sources based on dispersion-engineered OPA or OPO. (b) Calculated optical parametric gain as a function of wavelength and externally induced perturbation wavevector $\delta k$ for a 7-mm-long PPLN nanophotonic waveguide with minimized $\beta_2$ and $\beta_4$, under a CW pump power of 1 W at 532 nm. (c–e) Examples of singly- (c), doubly-(d), and triply-resonant (e) TFLN nanophotonic OPOs. OPOs can also be implemented in linear cavity geometries using distributed Bragg reflectors kellner2025low. (f) Upper panel: representative output spectra for a few doubly-resonant OPOs on the same chip. Lower panel: measured OPO wavelength (colored dots) versus the corresponding pump wavelength along with the theoretically calculated tuning curves (solid lines). (g) Upper panel: Singly-resonant OPO output wavelength tuning curves at different temperatures. Solid lines are theoretical tuning curves. Points marked with a blue outline have their output optical spectra plotted in the lower panel. (f) is adapted from Ref. ledezma2023octave, (g) is adapted from Ref. hwang2023mid.
• Figure 5: (a) Schematic illustration of the operating principle of the FM-OPO. (b) Output spectrum (blue) of the FM-OPO showing operation at the signal and idler frequencies. (a) and (b) are adapted from Ref. stokowski2024integrated.