Leveraging Raman response in X-cut thin-film lithium tantalate for ultrabroadband combs and polychromatic visible light

Technical Summary: Leveraging Raman Response in X‑cut Thin‑Film Lithium Tantalate for Ultrabroadband Combs and Polychromatic Visible Light

Problem Statement
Integrated photonic platforms based on thin-film lithium niobate (TFLN) and thin-film lithium tantalate (TFLT) offer a unique combination of second-order ($\chi^{(2)}$) and third-order ($\chi^{(3)}$) nonlinearities, making them ideal for monolithic integration of electro-optic functions and Kerr frequency combs. However, a significant barrier exists for X-cut platforms, which are preferred for electro-optic modulators due to their large $r_{33}$ coefficient. X-cut TFLN and TFLT exhibit a strong Raman response. Historically, this strong Raman scattering has been viewed as a parasitic competitor to Kerr four-wave mixing (FWM), often hindering the generation of broadband Kerr combs. Previous strategies to mitigate this involved suppressing Raman gain through dissipation engineering (adding loss) or restricting device geometry to specific crystal orientations, which limited dispersion engineering flexibility. Consequently, high-performance Kerr combs have largely been restricted to Z-cut platforms, preventing the monolithic integration of electro-optic modulation and broadband comb generation on a single X-cut chip.

Methodology
The authors propose a paradigm shift: rather than suppressing the Raman response, they leverage it through coupling-defined dissipation engineering.

• Design Strategy: Instead of modifying the microring waveguide geometry (which would alter intrinsic dispersion), the researchers kept the microring geometry fixed and engineered the coupling region. By varying the bus–ring gap ($g$), coupling angle ($\theta_{couple}$), and waveguide widths ($w_{bus}, w_{ring}$), they introduced wavelength-dependent external loss.
• Mechanism: This approach allows for the independent tuning of the thresholds for Raman scattering and Kerr FWM. The external coupling rate $\kappa_{ex}$ becomes a function of wavelength, creating different loss conditions for the pump, Raman Stokes modes, and Kerr sidebands.
• Simulation and Modeling: The team utilized a generalized Lugiato–Lefever equation (gLLE) incorporating a delayed Raman response and the calculated wavelength-dependent coupling rates to simulate frequency comb dynamics. This allowed them to predict transitions between pure Kerr, pure Raman, and synergistic Raman–Kerr states.
• Fabrication: Devices were fabricated on 600-nm-thick X-cut TFLT-on-insulator (LTOI) wafers using electron-beam lithography and inductively coupled plasma (ICP) etching. The microrings feature a 100 $\mu$m radius and support anomalous dispersion.

Key Results
By systematically varying the coupling conditions, the authors demonstrated four distinct nonlinear regimes on the same X-cut TFLT platform:

1. Pure Kerr Comb: In weakly coupled devices, the system generates a stable Kerr comb spanning 450 nm. This represents the broadest Kerr comb reported to date on an X-cut TFLT platform, achieved with an on-chip pump power of ~50 mW.
2. Raman-Dominated States: By increasing the coupling strength (reducing the gap), the system transitions to Raman lasing. In specific configurations, a "dual Raman comb" was observed, featuring cascaded Stokes and anti-Stokes components, a phenomenon first observed in an LT microring.
3. Raman–Kerr Synergistic Comb: In strongly coupled devices, the Raman and Kerr thresholds are simultaneously satisfied. The Raman process seeds localized comb lines, which are then filled in by Kerr FWM. This results in an ultrabroadband comb spanning 650 nm (~1190–1840 nm), the broadest reported on X-cut TFLN/TFLT platforms.
4. Polychromatic Visible Emission: In the strongly coupled regime, the dense near-infrared Raman–Kerr comb lines serve as a reservoir for cascaded $\chi^{(2)}$ sum-frequency processes. A single near-infrared pump (1556.55 nm) generates visible emission across eight distinct bands from violet (389.1 nm) to deep red (725 nm). Notably, this includes the first on-chip observation of yellow (580 nm) and orange (603 nm) emission generated via cascaded $\chi^{(2)}$ processes involving second harmonics and Raman sidebands.

Significance and Claims
The paper claims to fundamentally transform the role of the Raman response in X-cut nonlinear photonics.

• From Parasitic to Enabling: The work demonstrates that a strong Raman response, traditionally a hindrance to Kerr combs, can be engineered to act as an enabling mechanism. By controlling dissipation via coupling, the Raman effect is transformed from a competitor into a cooperative partner that broadens the spectral coverage and facilitates complex frequency conversion.
• Monolithic Integration: The results establish X-cut TFLT as a viable monolithic platform capable of simultaneously supporting high-performance electro-optic functions (via the X-cut orientation) and complex nonlinear light sources (Kerr combs and visible generation).
• Spectral Breadth: The achievement of a 650 nm Raman–Kerr comb and multi-band visible emission on a single chip sets a new benchmark for spectral coverage on X-cut platforms, surpassing previous TFLN and TFLT records which were limited to narrower spans (typically <300 nm) and lacked visible emission.
• Future Potential: The authors suggest that this platform, capable of generating multiple precisely spaced optical frequencies in the visible and near-infrared, holds promise for applications such as chip-scale atomic clocks and dual-comb spectroscopy, though they note that further optimization of spectral flatness and conversion efficiency is required.