[Paper Review] Octave-spanning microcomb generation in 4H-silicon-carbide-on-insulator photonics platform

Lutong Cai, Jingwei Li+2 more|arXiv (Cornell University)|Nov 21, 2021

Advanced Fiber Laser Technologies30 references4 citations

TL;DR

This paper demonstrates octave-spanning microcomb generation in a 4H-silicon-carbide-on-insulator (4H-SiCOI) photonic platform for the first time. By optimizing nanofabrication to achieve >1 million intrinsic Q-factors in 36-µm-radius microring resonators and engineering dispersion via waveguide width tuning in the fundamental transverse-electric (TE₀₀) mode, the authors achieve a broadband comb spanning 1100–2400 nm with on-chip power near 120 mW, marking a critical step toward chip-scale f-2f self-referencing.

ABSTRACT

Silicon carbide has recently emerged as a promising photonics material due to its unique properties, including possessing strong second- and third-order nonlinear coefficients and hosting various color centers that can be utilized for a wealth of quantum applications. Here, we report the design and demonstration of octave-spanning microcombs in a 4H-silicon-carbide-on-insulator platform for the first time. Such broadband operation is enabled by optimized nanofabrication achieving >1 million intrinsic quality factors in a 36-$μ$m-radius microring resonator, and careful dispersion engineering by investigating the dispersion properties of different mode families. For example, for the fundamental transverse-electric mode whose dispersion can be tailored by simply varying the microring waveguide width, we realized a microcomb spectrum covering the wavelength range from 1100 nm to 2400 nm with an on-chip power near 120 mW. While the observed comb state is verified to be chaotic and not soliton, attaining such a large bandwidth is a crucial step towards realizing $f$-2$f$ self-referencing. In addition, we have also observed coherent soliton-crystal state for the fundamental transverse-magnetic mode, which exhibits stronger dispersion than the fundamental transverse-electric mode and hence a narrower bandwidth.

Motivation & Objective

• To achieve octave-spanning microcomb generation in a 4H-silicon-carbide-on-insulator (4H-SiCOI) photonic platform for integrated frequency metrology.
• To overcome the challenge of limited bandwidth in prior SiC-based microcombs by enabling dispersion engineering through waveguide width tuning.
• To demonstrate high intrinsic Q-factors (>1 million) in compact SiC microring resonators via optimized nanofabrication.
• To enable chip-scale f-2f self-referencing by generating broadband combs spanning more than one octave.
• To evaluate the Kerr nonlinearity of the SiC material by comparing experimental comb spectra with LLE simulations.

Proposed method

• Utilized a 4H-SiCOI platform with 500 nm SiC thickness and 100 nm buried oxide layer, fabricated using direct wafer bonding and polishing techniques.
• Engineered dispersion in the fundamental TE₀₀ mode by varying the microring waveguide width from 1.6 to 1.9 µm to achieve anomalous group-velocity dispersion (GVD) over a broad wavelength range.
• Achieved high intrinsic Q-factors (>1 million) through optimized nanofabrication, including sidewall angles of 80–85°, minimizing scattering losses.
• Employed a grating coupler for input coupling and butt coupling to a polished chip facet for broadband output collection to resolve the full 1100–2400 nm spectrum.
• Used two optical spectrum analyzers (OSAs) covering 600–1700 nm and 1200–2400 nm to measure the full comb bandwidth.
• Performed numerical simulations using the Lugiato-Lefever equation (LLE) with γ ≈ 2.1 W⁻¹m⁻¹ and Q_intrinsic = 1.25 million to model comb dynamics and infer material nonlinearity.

Experimental results

Research questions

• RQ1 Can octave-spanning microcombs be generated in a 4H-SiCOI platform through dispersion engineering and high-Q nanofabrication?
• RQ2 What is the maximum achievable comb bandwidth in the TE₀₀ mode of a 4H-SiCOI microring by tuning waveguide width?
• RQ3 How does the intrinsic Q-factor of 4H-SiCOI microrings compare to state-of-the-art platforms, and what enables >1 million Q in this work?
• RQ4 Can the observed comb states support f-2f self-referencing, and what is the nature of the comb (chaotic vs. soliton) in this platform?
• RQ5 What is the effective Kerr nonlinearity of the 4H-SiC material used, as inferred from experimental-comb spectra and LLE simulations?

Key findings

• An octave-spanning microcomb was demonstrated for the first time in any SiC-based nanophotonic platform, covering 1100–2400 nm with a 36-µm-radius microring resonator.
• The comb spectrum was achieved using the fundamental TE₀₀ mode with waveguide width of 1.8–1.9 µm, resulting in a spectral bandwidth exceeding 150 THz.
• An on-chip pump power of approximately 120 mW was required to generate the broadband comb, with the output collected via butt coupling to a lensed fiber.
• Beat-note measurements confirmed the comb state as a modulation-instability (MI) comb, not a coherent soliton, due to chaotic dynamics.
• The effective Kerr nonlinearity was estimated as n₂ = (3.0 ± 1.0) × 10⁻¹⁹ m²/W, corresponding to γ ≈ 2.1 W⁻¹m⁻¹, which is lower than previously reported values in other SiC samples.
• A coherent soliton-crystal state was observed in the TM₀₀ mode, though with a narrower bandwidth due to stronger dispersion compared to the TE₀₀ mode.