Raman-Induced Platicon Formation

• Raman-induced platicon formation is a nonlinear optical phenomenon where stimulated Raman scattering seeds and stabilizes periodic dark solitons in normally dispersive cavities.
• The process leverages a modified Lugiato–Lefever equation that integrates Kerr and delayed Raman responses to lower detuning thresholds and broaden comb spectra.
• Experimental results in SiC, AlN, and fiber platforms demonstrate high coherence and efficiency, supporting applications in dual-comb spectroscopy and advanced photonic systems.

Raman-induced platicon formation is a recently identified nonlinear optical phenomenon where stimulated Raman scattering (SRS) acts not as a detrimental side effect, but as an enabling mechanism for the generation and stabilization of periodic dark soliton states—platicons—in resonators with normal group-velocity dispersion (GVD). Previously, SRS was considered antagonistic to Kerr-comb formation since it drains pump energy and destabilizes coherent soliton states. However, a series of experimental and theoretical investigations have demonstrated that the delayed Raman nonlinear response can be harnessed to seed, lock, and broaden platicon microcombs and related bright Kerr–Raman soliton states over a wide spectral range, with high efficiency and coherence.

1. Theoretical Foundations: Modified LLE with Raman Gain

The core theoretical description for Raman-induced platicon formation builds on the Lugiato–Lefever equation (LLE), which models the evolution of the intracavity field envelope. In systems where both Kerr and delayed Raman nonlinearities are significant, the generalized LLE assumes the form: $$\frac{\partial E}{\partial t} = \left[ -\frac{\kappa}{2} + i\,\delta\omega - i\sum_{k\geq2}\frac{\beta_k}{k!}(i\partial_\tau)^k \right] E + i\gamma L\,|E|^2E + i g_R L \left( h_R(\tau) * |E|^2 \right) E + \sqrt{ \kappa_{\rm ex} } E_{\rm in}$$ where $\kappa$ is the total cavity loss rate, $\delta\omega$ the pump-resonance detuning, $\beta_k$ the dispersion coefficients, $\gamma$ the Kerr coefficient, $g_R$ the Raman gain coefficient, and $h_R(\tau)$ is the normalized Raman response function. The Raman gain profile typically adopts a Lorentzian form centered at the Stokes shift $\Omega_R$: $$g_R(\Omega) = g_{R0} \frac{1}{1 + \left[ \frac{\Omega-\Omega_R}{\Gamma_R/2} \right]^2 }$$ with $\Omega_R$ characteristic of the material’s phonon mode and $\Gamma_R$ the Raman linewidth. In modal frameworks, the coupling between mode amplitudes $a_\mu$ is likewise extended to capture both Kerr and Raman-induced terms. Stimulated Raman scattering couples power from the pump mode at $\omega_p$ to Stokes modes at $\omega_s = \omega_p - \Omega_R$, producing a nonlocal delayed refractive index modulation that seeds new comb lines and enables strong interaction with the switching waves bounding the platicon state.

2. Dispersion Landscape and Platicon State Formation

Normal group-velocity dispersion ($\beta_2>0$) inhibits the formation of bright, sech-like solitons, favoring dark-pulse and flat-top platicon solutions. In microresonators such as 4H-SiC microrings (radius $\sim$43 \textmu m; width 2.1 \textmu m), the fundamental TE$_{00}$ mode exhibits strictly normal GVD within typical telecom bands and only transitions toward weak anomalous GVD at longer wavelengths. The emergence of platicons requires crossing a threshold where the nonlinear index shift and pump detuning permit the formation of flat-top dark pulses bounded by switching waves. The introduction of Raman gain effectively lowers the detuning threshold and injects a “dispersion kick,” enabling robust platicon formation even in the absence of structural mode crossings or engineered dispersion landscapes. In Fabry-Perot (FP) fiber cavities, this effect is observed as a rapid transformation from conventional platicon states to bright Kerr-Raman soliton structures with superimposed oscillatory tails, lockable via the Raman frequency.

3. Raman–Kerr Interplay: Cooperation vs. Competition

SRS has generally been regarded as competing with Kerr-driven four-wave mixing (FWM) in microcomb systems, stripping pump intensity and disrupting soliton steps. Experimental studies reveal that in materials and geometries where the free-spectral range (FSR) (e.g., ~400 GHz in SiC, ~230 GHz in AlN) significantly exceeds the Raman linewidth, discrete Stokes modes can build up cleanly. Resonant beating between pump and Stokes fields generates periodic refractive index gratings, facilitating parametric gain via FWM-Bragg scattering back into the pump band. This mutual coupling between spatially and spectrally separated comb components produces nonlocal index modulation, stabilizes the platicon state, and broadens the comb bandwidth. In synchronous operation, cross-phase modulation between pump and Stokes solitons enables multicolor microcomb generation.

4. Experimental Protocols and Observables

Diverse platforms have demonstrated Raman-induced platicon phenomena, including 4H-SiC microrings, AlN microresonators, and FP fiber cavities with ultra-low normal GVD. Key device properties include:

Platform	Geometry / FSR	GVD	Raman Shift	Efficiency / Bandwidth
SiC Ring	43 \textmu m, 400 GHz	$\beta_2 \sim 150$ ps/(nm·km), normal	23.3 THz (777 cm$^{-1}$)	$\sim$56%; 1500–1700 nm, extend >1800 nm
FP Fiber	5 cm silica, 1.8 GHz	$\beta_2 = 0.0149$ ps$^2$/km, normal	13 THz	>9000 lines; 1350–1700 nm; S+C+L bands
AlN Microring	200 \textmu m, 230 GHz	normal GVD	20 THz, $\Gamma_R\sim$30 GHz	>10 mW Stokes; spans 1425–1980 nm

Characteristic observables include transformation from noise-like regimes to clear soliton steps via red pump detuning, formation of flat-top platicon spectra (often extending over $>$20 THz), emergence of strong Stokes peaks in Raman band (offset by $\Omega_R$), and conversion efficiencies up to 56% (filtering out pump line). In FP fiber resonators, direct time-domain sampling reveals oscillations at the Raman frequency locked to switching wave edges, while in AlN microresonators “Stokes platicon” states manifest with simultaneous comb emission in both the pump and Stokes bands.

5. Coherence, Stability, and Spectral Control

Experiments report high coherence for Raman-induced platicon combs, with single-family comb line spacing (FSR) confirmed by beat-note measurements and phase noise figures comparable to conventional soliton microcombs (e.g., –100 dBc/Hz @1 kHz offset in FP resonators). Raman-enabled states persist over extended periods, supported by self-injection locking (SIL), thermal relaxations, and active stabilization using auxiliary lasers (thermal locking in SiC rings, multi-time-scale thermal self-adjustment in AlN platforms). In AlN systems, five distinct thermal relaxation processes mediate GHz-frequency sweeps of both pump and Stokes bands during turn-on, collectively stabilizing the comb state. Control of back-scattering phase and pump detuning enables switching between pure platicon and Stokes-platicon emission regimes, with spectral envelope tuning over ±100 nm achieved via fine adjustment of cavity parameters.

6. Comparison to Competing Paradigms and the Role of Dispersion

Unlike systems operating in anomalous GVD—where SRS is mainly associated with continuous self-frequency soliton shift, increased breather dynamics, and reduced coherence—Raman-enabled platicon formation in normal-dispersion cavities leverages SRS to lock switching waves, induce localized oscillatory features, and expand the stability windows for flat-top dark-pulse states. Numerical and analytic thresholds link the emergence of Kerr–Raman solitons to the overlap between platicon dispersive wave wings and the Raman gain peak, with well-defined excursion windows in detuning and pump power. Results confirm that the delayed nature of Raman response is essential for discrete switching wave locking and formation of bright, moving localized structures in contrast to the continuous regime in anomalous GVD.

7. Implications, Applications, and Future Directions

Raman-induced platicon formation marks a change in the microcomb design paradigm, demonstrating that SRS can be beneficially harnessed for broadband, efficient, low-noise comb generation in normally dispersive integrated cavities. Key implications and projected applications include:

• Removal of requirement for engineered avoided crossings or local dispersion anomalies in microresonator design.
• Record-high pump-to-comb conversion efficiencies for dark/dark-like soliton states.
• Expansion of comb coverage toward multi-octave bandwidth (1500–1850 nm demonstrated, potential for $>$2 octaves with tailored Raman shift).
• On-chip dual-comb spectroscopy (near- and mid-IR), frequency synthesis, petabit-class WDM communications, photonic LIDAR, and ultra-low-noise microwave and terahertz photonic oscillators.
• New opportunities to exploit narrow Raman lines in crystalline materials (AlN, LiNbO$_3$) for low-threshold, multi-band comb sources.
• Enrichment of microcomb nonlinear dynamics: coexistence of platicons, breathers, bright Kerr–Raman solitons, and cascaded Stokes/anti-Stokes emission accessible via deliberate Raman-Kerr interplay.

Recent experimental results in 4H-SiC (Li et al., 25 Jul 2025), FP fiber resonators (Li et al., 2023), and AlN microresonators (Ding et al., 16 Feb 2025) have firmly established Raman-induced platicon formation as a robust and versatile process, underlining its relevance for advanced photonic integration and nonlinear dynamics research. A plausible implication is that further refinement of Raman gain profiles and dispersion engineering may enable even broader comb spans and customizable mode locking in hybrid-integrated microresonator platforms.