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1. Demonstration of 4H-silicon carbide on an aluminum nitride integrated photonic platform

   https://doi.org/10.1364/OL.521157  

   Wang, Ruixuan; Li, Jingwei; Cai, Lutong; Li, Qing (January 2024, Optics Letters)

   The existing silicon-carbide-on-insulator photonic platform utilizes a thin layer of silicon dioxide under silicon carbide (SiC) to provide optical confinement and mode isolation. Here, we replace the underneath silicon dioxide layer with 1-µm-thick aluminum nitride and demonstrate a 4H-silicon-carbide-on-aluminum-nitride integrated photonic platform for the first time to our knowledge. Efficient grating couplers, low-loss waveguides, and compact microring resonators with intrinsic quality factors up to 210,000 are fabricated. In addition, by undercutting the aluminum nitride layer, the intrinsic quality factor of the silicon carbide microring is improved by nearly one order of magnitude (1.8 million). Finally, an optical pump–probe method is developed to measure the thermal conductivity of the aluminum nitride layer, which is estimated to be over 30 times of that of silicon dioxide. 

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2. Octave-spanning microcomb generation in 4H-silicon-carbide-on-insulator photonics platform

   https://doi.org/10.1364/PRJ.449267  

   Cai, Lutong; Li, Jingwei; Wang, Ruixuan; Li, Qing (March 2022, Photonics Research)

   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, to our knowledge. Such broadband operation is enabled by optimized nanofabrication achieving $>1$million intrinsic quality factors in a 36-μm-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 $f-2f$self-referencing. In addition, we also observed a 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. 

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3. Triangular quantum photonic devices with integrated detectors in silicon carbide

   https://doi.org/10.1088/2633-4356/acc302  

   Majety, Sridhar; Strohauer, Stefan; Saha, Pranta; Wietschorke, Fabian; Finley, Jonathan J.; Müller, Kai; Radulaski, Marina (March 2023, Materials for Quantum Technology)

   Abstract Triangular cross-section silicon carbide (SiC) photonic devices have been studied as an efficient and scalable route for integration of color centers into quantum hardware. In this work, we explore efficient collection and detection of color center emission in a triangular cross-section SiC waveguide by introducing a photonic crystal mirror on its one side and a superconducting nanowire single photon detector (SNSPD) on the other. Our modeled triangular cross-section devices with a randomly positioned emitter have a maximum coupling efficiency of 89% into the desired optical mode and a high coupling efficiency ( $>$75%) in more than half of the configurations. For the first time, NbTiN thin films were sputtered on 4H-SiC and the electrical and optical properties of the thin films were measured. We found that the transport properties are similar to the case of NbTiN on SiO₂substrates, while the extinction coefficient is up to 50% higher for 1680 nm wavelength. Finally, we performed finite-difference time-domain simulations of triangular cross-section waveguide integrated with an SNSPD to identify optimal nanowire geometries for efficient detection of light from transverse electric and transverse magnetic polarized modes. 

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4. Entangled photon pair generation in an integrated SiC platform

   https://doi.org/10.1038/s41377-024-01443-z  

   Rahmouni, Anouar; Wang, Ruixuan; Li, Jingwei; Tang, Xiao; Gerrits, Thomas; Slattery, Oliver; Li, Qing; Ma, Lijun (December 2024, Light: Science & Applications)

   Abstract Entanglement plays a vital role in quantum information processing. Owing to its unique material properties, silicon carbide recently emerged as a promising candidate for the scalable implementation of advanced quantum information processing capabilities. To date, however, only entanglement of nuclear spins has been reported in silicon carbide, while an entangled photon source, whether it is based on bulk or chip-scale technologies, has remained elusive. Here, we report the demonstration of an entangled photon source in an integrated silicon carbide platform for the first time. Specifically, strongly correlated photon pairs are efficiently generated at the telecom C-band wavelength through implementing spontaneous four-wave mixing in a compact microring resonator in the 4H-silicon-carbide-on-insulator platform. The maximum coincidence-to-accidental ratio exceeds 600 at a pump power of 0.17 mW, corresponding to a pair generation rate of (9 ± 1) × 10³pairs/s. Energy-time entanglement is created and verified for such signal-idler photon pairs, with the two-photon interference fringes exhibiting a visibility larger than 99%. The heralded single-photon properties are also measured, with the heraldedg⁽²⁾(0) on the order of 10⁻³, demonstrating the SiC platform as a prospective fully integrated, complementary metal-oxide-semiconductor compatible single-photon source for quantum applications. 

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5. Investigation of the electro-optic effect in high-Q 4H-SiC microresonators

   https://doi.org/10.1364/OL.482844  

   Wang, Ruixuan; Li, Jingwei; Cai, Lutong; Li, Qing (March 2023, Optics Letters)

   Silicon carbide (SiC) recently emerged as a promising photonic and quantum material owing to its unique material properties. In this work, we carried out an exploratory investigation of the Pockels effect in high-quality-factor (high-Q) 4H-SiC microresonators and demonstrated gigahertz-level electro-optic modulation for the first time. The extracted Pockels coefficients show certain variations among 4H-SiC wafers from different manufacturers, with the magnitudes ofr₁₃andr₃₃estimated to be in the range of (0.3–0.7) pm/V and (0–0.03) pm/V, respectively. 

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