Light Chips Boosted by Wonder Material Promise Faster Data Transmission

Researchers are increasingly focused on integrating two-dimensional materials into silicon photonics to create more powerful and compact optical devices. Shahaz S. Hameed, Di Jin, and Aihao Zhao, from Swinburne University of Technology, along with Jiayang Wu, Junkai Hu, and Sebastien Cueff from INL, Institut des Nanotechnologies de Lyon, demonstrate a significant enhancement of self-phase modulation within silicon nitride waveguides by integrating monolayer molybdenum disulfide (MoS2) films. This work is particularly significant because it achieves a 27-fold increase in the nonlinear parameter and a Kerr coefficient for MoS2 nearly five orders of magnitude greater than silicon nitride, paving the way for substantially improved nonlinear optical performance in future integrated circuits.

This breakthrough demonstrates significantly enhanced self-phase modulation (SPM), a nonlinear optical process crucial for advanced photonic devices.

Researchers synthesized ultrahigh nonlinearity MoS2 monolayers via low-pressure chemical vapor deposition and precisely transferred them onto Si3N4 waveguides, controlling film length and placement using selectively opened windows in the chip’s silica cladding. Detailed measurements at telecom wavelengths revealed that hybrid devices incorporating a 1.4-mm-long MoS2 film exhibited substantially increased spectral broadening of sub-picosecond pulses compared to devices without MoS2.
Theoretical analysis of the experimental results indicates a ~27-fold increase in the nonlinear parameter (γ) for the hybrid MoS2/Si3N4 waveguides. This enhancement translates to an equivalent Kerr coefficient (n2) for MoS2 that is nearly five orders of magnitude greater than that of Si3N4. The work confirms the effectiveness of on-chip 2D material integration for boosting nonlinear optical performance.

This innovative approach addresses limitations in existing photonic integrated circuits, where materials like silicon, despite possessing higher Kerr coefficients, suffer from two-photon absorption at telecom wavelengths. Silicon nitride and doped silica glass offer negligible two-photon absorption but exhibit relatively low nonlinearities, hindering efficient SPM.

By incorporating MoS2, a material with exceptional Kerr nonlinearity, researchers have overcome this barrier, paving the way for compact, stable, and scalable optical devices. The findings have significant implications for broadband optical sources, optical spectroscopy, all-optical modulation, pulse compression, and optical coherence tomography.

Fabrication of silicon nitride waveguides and silica cladding definition were performed using electron beam lithography

Nitride waveguides integrated with molybdenum disulfide (MoS2) films were fabricated to demonstrate enhanced spectral broadening via self-phase modulation (SPM). The process commenced with the deposition of 330nm-thick silicon nitride (Si3N4) films using low-pressure chemical vapor deposition (LPCVD) in a two-step process for strain management and crack prevention.

Waveguide layouts were then patterned using 248-nm deep ultraviolet lithography (DUV) followed by fluorine-based dry etching within a 300-mm reactor at 32 mTorr pressure and 150°C. This etching employed a CF4-CH2F2-O2 chemistry, achieving an etch rate of 110nm/minute with a 1:1.5 selectivity of the lithography resist to Si3N4.

Following waveguide patterning, a 2.2-μm-thick silica upper cladding was deposited via high-density plasma enhanced chemical vapor deposition (HDP-PECVD). Subsequent lithography and reactive-ion etching (RIE) opened windows in the silica cladding, etching down to the boxing layer to expose the underlying Si3N4 waveguides.

The RIE process utilized CHF3 / CF4 / Ar gases, adjusted to provide an etch selectivity exceeding 5 for SiO2 over Si3N4. These opened windows facilitated the subsequent coating of the Si3N4 waveguides with MoS2 films. Monolayer MoS2 films were synthesized via LPCVD on sapphire substrates, controlling intrinsic atomic defects and ensuring large-area uniformity.

A ~5-μL droplet of ammonium molybdate tetrahydrate was drop-cast onto a sapphire substrate held at 750°C, dried at 110°C for 5 minutes, and then exposed to sulfur powder heated to 180°C with a 70-sccm argon gas flow at ~1 torr pressure. The resulting MoS2 film was transferred to the Si3N4 chip using a polymer-assisted transfer process involving spin-coating with polystyrene (PS), water-assisted delamination, stamping via van der Waals interactions, and PS removal with toluene. All fabricated Si3N4 waveguides were 2.0cm in length, while the MoS2 film coating lengths varied from 0.2mm to 1.4mm, allowing for precise control over the interaction length.

Enhanced nonlinear optical performance in silicon nitride waveguides via molybdenum disulfide integration is demonstrated

Spectral broadening of up to 2.4 was experimentally achieved in silicon nitride (Si3N4) waveguides integrated with monolayer molybdenum disulfide (MoS2) films. These hybrid devices demonstrate significantly enhanced nonlinear optical performance compared to devices without MoS2. Detailed self-phase modulation (SPM) measurements at telecom wavelengths revealed this increased spectral broadening for sub-picosecond optical pulses using a 1.4-mm-long MoS2 film.

Theoretical analysis of the experimental results indicates a 27-fold increase in the nonlinear parameter (γ) for the hybrid MoS2 / Si3N4 waveguides. Monolayer MoS2 films were synthesized via low-pressure chemical vapor deposition (LPCVD) and transferred onto Si3N4 waveguides with precise control over coating length and placement.

Window openings in the chip silica upper cladding facilitated selective film deposition on the waveguides. SPM measurements were performed with varying MoS2 film lengths and pulse peak powers reaching up to 91W. The equivalent Kerr coefficient (n2) of MoS2 was determined to be nearly five orders of magnitude higher than that of Si3N4.

This work confirms the effectiveness of on-chip integration of 2D MoS2 films for enhancing nonlinear optical performance in integrated devices. The increased nonlinear parameter (γ) and Kerr coefficient (n2) demonstrate the potential for improved broadband optical sources, optical spectroscopy, and all-optical modulation mechanisms. These findings suggest a pathway towards compact, stable, and scalable photonic integrated circuits with enhanced functionality at telecom wavelengths.

Enhanced nonlinearities via molybdenum disulfide integration into silicon nitride photonics enable compact and efficient optical devices

Researchers have demonstrated significantly enhanced spectral broadening of optical pulses within silicon nitride waveguides integrated with molybdenum disulfide films. This achievement leverages the substantial nonlinear optical properties of two-dimensional materials to improve the performance of integrated optical devices.

The process involves synthesizing high-quality monolayer molybdenum disulfide via low-pressure chemical vapour deposition and then precisely transferring it onto the silicon nitride waveguides, allowing for controlled coating length and placement. Detailed measurements of self-phase modulation reveal a broadening factor of up to 2.4 for devices incorporating a 1.4-millimetre-long molybdenum disulfide film.

Theoretical analysis indicates a 27-fold increase in the nonlinear parameter and a Kerr coefficient for molybdenum disulfide that is nearly five orders of magnitude greater than that of silicon nitride. These findings validate the effective integration of two-dimensional molybdenum disulfide films to boost the nonlinear optical characteristics of integrated devices at telecommunication wavelengths.

The authors acknowledge a trade-off between enhanced nonlinearity and increased optical loss introduced by the molybdenum disulfide film. The figure of merit, which balances nonlinearity and loss, is affected by waveguide length and linear attenuation. Future research could focus on optimising the integration process to minimise loss while maintaining high nonlinearity, potentially through improved material quality or waveguide design. This work establishes a promising pathway for developing advanced integrated optical components with enhanced functionality and performance.