Enhanced selfphase modulation in silicon nitride waveguides with integrated 2D MoS2 films

On-chip integration of 2D materials provides a promising route towards next-generation integrated optical devices with performance beyond existing limits. Here, significantly enhanced spectral broadening induced by self-phase modulation (SPM) is experimentally demonstrated in silicon nitride (Si3N4) waveguides integrated with 2D monolayer molybdenum disulfide (MoS2) films. Monolayer MoS2 films with ultrahigh optical nonlinearity are synthesized via low-pressure chemical vapor deposition (LPCVD) and subsequently transferred onto Si3N4 waveguides, with precise control of the film coating length and placement achieved by selectively opening windows on the chip silica upper cladding. Detailed SPM measurements at telecom wavelengths are performed using fabricated waveguides with various MoS2 film coating lengths. Compared to devices without MoS2, increased spectral broadening of sub-picosecond optical pulses is observed for the hybrid devices, achieving a broadening factor of up to ~ 2.4 for a device with a 1.4-mm-long MoS2 film. Theoretical fitting of the experimental results further reveals an increase of up to ~27 fold in the nonlinear parameter (γ) for the hybrid MoS2 / Si3N4 waveguides and an equivalent Kerr coefficient (n2) of MoS2 nearly 5 orders of magnitude higher than Si3N4. These results confirm the effectiveness of on-chip integration of 2D MoS2 films to enhance the nonlinear optical performance of integrated photonic devices.

💡 Research Summary

In this work the authors demonstrate a substantial enhancement of self‑phase modulation (SPM) in silicon nitride (Si₃N₄) waveguides by integrating monolayer molybdenum disulfide (MoS₂) films. High‑quality, large‑area MoS₂ monolayers are synthesized by low‑pressure chemical vapor deposition (LPCVD) on sapphire substrates, where the growth parameters are tuned to control sulfur‑vacancy defects. The films are transferred onto pre‑fabricated Si₃N₄ waveguides using a polymer‑assisted (polystyrene) pick‑up and release technique. Prior to transfer, windows are opened in the SiO₂ upper cladding by reactive‑ion etching, exposing the Si₃N₄ core over lengths (Lc) ranging from 0.2 mm to 1.4 mm. This approach enables precise spatial control of the MoS₂ interaction region while keeping the rest of the waveguide cladding intact.

The Si₃N₄ waveguides themselves are fabricated via a CMOS‑compatible, crack‑free LPCVD process (two 330 nm Si₃N₄ layers), followed by deep‑UV lithography and fluorine‑based dry etching. Inverse‑taper edge couplers (≈120 nm tip width, 300 µm length) provide fiber‑to‑chip coupling with ~7.5 dB per facet loss (optimizable to 4–5 dB). TE‑polarized light is used throughout because the evanescent field of the TE mode interacts strongly with the in‑plane optical response of the 2‑D material.

Linear loss measurements with a continuous‑wave (CW) laser at 1550 nm show that the additional propagation loss introduced by the MoS₂ coating is negligible (<0.2 dB) across all window lengths, confirming that the monolayer does not significantly increase absorption or scattering. For the nonlinear experiments, sub‑picosecond pulses (peak powers up to ~91 W) are launched into the devices. The output spectra are recorded with an optical spectrum analyzer, and the spectral broadening factor (BF) is extracted as the ratio of the 3‑dB bandwidth after propagation to that of the input pulse.

Hybrid devices exhibit markedly larger spectral broadening than bare Si₃N₄ waveguides. The device with a 1.4 mm MoS₂ coating reaches a BF of ~2.4, while the uncoated reference remains near unity under identical conditions. By fitting the measured BF versus input peak power to the analytical solution of the nonlinear Schrödinger equation (including linear loss), the effective nonlinear parameter γ of the hybrid waveguide is found to be up to 27 times higher than that of the bare Si₃N₄ waveguide. Translating this enhancement into an equivalent Kerr coefficient for the MoS₂ layer yields n₂ ≈ 5 × 10⁴ times the Si₃N₄ value, i.e., an increase of roughly five orders of magnitude.

Finite‑element mode simulations (COMSOL) reveal that the fundamental TE mode overlaps the monolayer primarily on the top surface (≈0.027 % overlap) and only minimally near the sidewalls (≈0.0025 %). The monolayer’s refractive index (n≈3.8) and extinction coefficient (k≈0.107) at 1550 nm, measured previously by ellipsometry, are sufficient to produce a strong intensity‑dependent phase shift despite the sub‑nanometer thickness. The polymer‑assisted transfer leaves a small air gap between the MoS₂ and the waveguide sidewalls, but this does not appreciably affect the mode profile.

The study demonstrates several key advances: (1) a scalable, CMOS‑compatible process for integrating 2‑D materials onto low‑loss Si₃N₄ photonic platforms; (2) the ability to engineer the length and position of the nonlinear region by patterning the cladding, enabling custom nonlinear circuit designs; (3) verification that monolayer MoS₂ can provide ultrahigh Kerr nonlinearity without incurring significant linear loss, thereby overcoming the two‑photon‑absorption limitations of silicon at telecom wavelengths.

The authors suggest that the approach can be extended to other transition‑metal dichalcogenides (e.g., WS₂, WSe₂) or heterostructures (graphene/MoS₂) to further tailor the nonlinear response. Potential applications include on‑chip supercontinuum generation, all‑optical signal processing, pulse compression, and quantum photonic circuits where strong, low‑loss Kerr nonlinearity is essential. This work thus establishes a practical pathway toward high‑performance, nonlinear integrated photonics by leveraging the extraordinary optical properties of atomically thin 2‑D materials.