High-Q integrated photonic microresonators on 3C-SiC-on-insulator (SiCOI) platform

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

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.

Entangled photon pair generation in an integrated SiC platform

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) × 103 pairs/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 heralded g(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.

High-Q adiabatic micro-resonators on a wafer-scale ion-sliced 4H-silicon carbide-on-insulator platform

A 4H-silicon carbide-on-insulator (4H-SiCOI) has emerged as a prominent material contender for integrated photonics owing to its outstanding material properties such as CMOS compatibility, high refractive index, and high second- and third-order nonlinearities. Although various micro-resonators have been realized on the 4H-SiCOI platform, enabling numerous applications including frequency conversion and electro-optical modulators, they may suffer from a challenge associated with spatial mode interactions, primarily due to the widespread use of multimode waveguides. We study the suppression of spatial mode interaction with Euler bends, and demonstrate micro-resonators with improved Q values above 1 × 105 on ion-sliced 4H-SiCOI platform with a SiC thickness nonuniformity less than 1%. The spatial-mode-interaction-free micro-resonators reported on the CMOS-compatible wafer-scale 4H-SiCOI platform would constitute an important ingredient for the envisaged large-scale integrated nonlinear photonic circuits.

High-Quality Amorphous Silicon Carbide for Hybrid Photonic Integration Deposited at a Low Temperature

Integrated photonic platforms have proliferated in recent years, each demonstrating its unique strengths and shortcomings. Given the processing incompatibilities of different platforms, a formidable challenge in the field of integrated photonics still remains for combining the strengths of different optical materials in one hybrid integrated platform. Silicon carbide is a material of great interest because of its high refractive index, strong second- and third-order nonlinearities, and broad transparency window in the visible and near-infrared range. However, integrating silicon carbide (SiC) has been difficult, and current approaches rely on transfer bonding techniques that are time-consuming, expensive, and lacking precision in layer thickness. Here, we demonstrate high-index amorphous silicon carbide (a-SiC) films deposited at 150 °C and verify the high performance of the platform by fabricating standard photonic waveguides and ring resonators. The intrinsic quality factors of single-mode ring resonators were in the range of Qint = (4.7-5.7) × 105 corresponding to optical losses between 0.78 and 1.06 dB/cm. We then demonstrate the potential of this platform for future heterogeneous integration with ultralow-loss thin SiN and LiNbO3 platforms.

Inverse-designed silicon carbide quantum and nonlinear photonics

Inverse design has revolutionized the field of photonics, enabling automated development of complex structures and geometries with unique functionalities unmatched by classical design. However, the use of inverse design in nonlinear photonics has been limited. In this work, we demonstrate quantum and classical nonlinear light generation in silicon carbide nanophotonic inverse-designed Fabry-Pérot cavities. We achieve ultra-low reflector losses while targeting a pre-specified anomalous dispersion to reach optical parametric oscillation. By controlling dispersion through inverse design, we target a second-order phase-matching condition to realize second- and third-order nonlinear light generation in our devices, thereby extending stimulated parametric processes into the visible spectrum. This first realization of computational optimization for nonlinear light generation highlights the power of inverse design for nonlinear optics, in particular when combined with highly nonlinear materials such as silicon carbide.

χ(2) nonlinear photonics in integrated microresonators

Second-order (χ⁽²⁾) optical nonlinearity is one of the most common mechanisms for modulating and generating coherent light in photonic devices. Due to strong photon confinement and long photon lifetime, integrated microresonators have emerged as an ideal platform for investigation of nonlinear optical effects. However, existing silicon-based materials lack a χ⁽²⁾ response due to their centrosymmetric structures. A variety of novel material platforms possessing χ⁽²⁾ nonlinearity have been developed over the past two decades. This review comprehensively summarizes the progress of second-order nonlinear optical effects in integrated microresonators. First, the basic principles of χ⁽²⁾ nonlinear effects are introduced. Afterward, we highlight the commonly used χ⁽²⁾ nonlinear optical materials, including their material properties and respective functional devices. We also discuss the prospects and challenges of utilizing χ⁽²⁾ nonlinearity in the field of integrated microcavity photonics.

A 3C-SiC-on-Insulator-Based Integrated Photonic Platform Using an Anodic Bonding Process with Glass Substrates

Various crystalline silicon carbide (SiC) polytypes are emerging as promising photonic materials due to their wide bandgap energies and nonlinear optical properties. However, their wafer forms cannot readily provide a refractive index contrast for optical confinement in the SiC layer, which makes it difficult to realize a SiC-based integrated photonic platform. In this paper, we demonstrate a 3C-SiC-on-insulator (3C-SiCoI)-based integrated photonic platform by transferring the epitaxial 3C-SiC layer from a silicon die to a borosilicate glass substrate using anodic bonding. By fine-tuning the fabrication process, we demonstrated nearly 100% area transferring die-to-wafer bonding. We fabricated waveguide-coupled microring resonators using sulfur hexafluoride (SF6)-based dry etching and demonstrated a moderate loaded quality (Q) factor of 1.4 × 105. We experimentally excluded the existence of the photorefractive effect in this platform at sub-milliwatt on-chip input optical power levels. This 3C-SiCoI platform is promising for applications, including large-scale integration of linear, nonlinear and quantum photonics.

Novel Photonic Applications of Silicon Carbide

Silicon carbide (SiC) is emerging rapidly in novel photonic applications thanks to its unique photonic properties facilitated by the advances of nanotechnologies such as nanofabrication and nanofilm transfer. This review paper will start with the introduction of exceptional optical properties of silicon carbide. Then, a key structure, i.e., silicon carbide on insulator stack (SiCOI), is discussed which lays solid fundament for tight light confinement and strong light-SiC interaction in high quality factor and low volume optical cavities. As examples, microring resonator, microdisk and photonic crystal cavities are summarized in terms of quality (Q) factor, volume and polytypes. A main challenge for SiC photonic application is complementary metal-oxide-semiconductor (CMOS) compatibility and low-loss material growth. The state-of-the-art SiC with different polytypes and growth methods are reviewed and a roadmap for the loss reduction is predicted for photonic applications. Combining the fact that SiC possesses many different color centers with the SiCOI platform, SiC is also deemed to be a very competitive platform for future quantum photonic integrated circuit applications. Its perspectives and potential impacts are included at the end of this review paper.

Optical bi-stability in cubic silicon carbide microring resonators

We measure the photothermal nonlinear response in suspended cubic silicon carbide (3C-SiC) and 3C-SiC-on-insulator (SiCOI) microring resonators. Bi-stability and thermo-optic hysteresis is observed in both types of resonators, with the suspended resonators showing a stronger response. A photothermal nonlinear index of 4.02×10^-15 m²/W is determined for the suspended resonators, while the SiCOI resonators demonstrate one order of magnitude lower photothermal nonlinear index of 4.32×10^-16 m²/W. Cavity absorption and temperature analysis suggest that the differences in thermal bi-stability are due to variations in waveguide absorption, likely from crystal defect density differences throughout the epitaxially grown layers. Furthermore, coupled mode theory model shows that the strength of the optical bi-stability, in suspended and SiCOI resonators can be engineered for high power or nonlinear applications.

Integrated silicon carbide electro-optic modulator

Owing to its attractive optical and electronic properties, silicon carbide is an emerging platform for integrated photonics. However an integral component of the platform is missing—an electro-optic modulator, a device which encodes electrical signals onto light. As a non-centrosymmetric crystal, silicon carbide exhibits the Pockels effect, yet a modulator has not been realized since the discovery of this effect more than three decades ago. Here we design, fabricate, and demonstrate a Pockels modulator in silicon carbide. Specifically, we realize a waveguide-integrated, small form-factor, gigahertz-bandwidth modulator that operates using complementary metal-oxide-semiconductor (CMOS)-level voltages on a thin film of silicon carbide on insulator. Our device is fabricated using a CMOS foundry compatible fabrication process and features no signal degradation, no presence of photorefractive effects, and stable operation at high optical intensities (913 kW/mm²), allowing for high optical signal-to-noise ratios for modern communications. Our work unites Pockels electro-optics with a CMOS foundry compatible platform in silicon carbide.

Electro-optic modulator is used to encode electrical signals onto light. Here the authors demonstrate an electro-optic modulator, based on Silicon Carbide, which can be useful for quantum and optical communications.

High-Q ultrasensitive integrated photonic sensors based on slot-ring resonator on a 3C-SiC-on-insulator platform

We demonstrate, to the best of our knowledge, the first high-Q silicon carbide (SiC) integrated photonic sensor based on slot-ring resonators on a 3C-SiC-on-insulator (SiCOI) platform. We experimentally demonstrate an intrinsic Q of 17,400 at around 1310 nm wavelength for a slot-ring resonator with 40 µm radius with water cladding. By applying different concentrations of a sodium chloride (NaCl) solution that covers the devices, measured bulk sensitivities of 264-300 nm/RIU (refractive index unit) are achieved in the slot-ring resonator with a 400-450 nm rail width and a 100-200 nm slot width. The device performance for biomolecular layer sensing (BMLS) is proved by the detection of the cardiac biomarker troponin with 248-322 pm/nm surface sensitivity. The reported slot-ring resonators can be of great interest for diverse sensing applications from visible to infrared wavelengths.

Optical super-resonance in a customized P T-symmetric system of hybrid interaction

We investigate the optical resonances in coupled meta-atoms with hybrid interaction pathways. One interaction pathway is the directly near-field coupling between the two meta-atoms. The other interaction pathway is via the continuum in a waveguide functioned as a common bus connecting them. We show that by properly introducing gain or loss into the meta-atoms, the hybrid optical system becomes parity-time (P T) symmetric, in which the effective coupling rate can be customized by manipulating the length of the waveguide. At the exact phase of the customized P T symmetry, the coupled meta-atoms support discrete super-resonant modes that can be observed from the transmission spectra as extremely sharp peaks. At an exception point where the eigenmodes coalesce, albeit the transmission curve is flat, a high-Q factor of the localized field in the meta-atoms can be obtained. Similarities of the super-resonance with the bound states in the continuum (BICs) are discussed. This investigation promotes our understanding about the ways in realizing high-Q optical resonance especially by manipulating the distributions of loss and gain via the concepts of P T and BICs. Many attractive applications are expected.

Racetrack microresonator based electro-optic phase shifters on a 3C silicon-carbide-on-insulator platform

We report, to the best of our knowledge, the first demonstration of integrated electro-optic (EO) phase shifters based on racetrack microresonators on a 3C silicon-carbide-on-insulator (SiCOI) platform working at near-infrared wavelengths. By applying DC voltage in the crystalline axis perpendicular to the waveguide plane, we have observed optical phase shifts from the racetrack microresonators whose loaded quality ($ Q $) factors are $\sim\! {30,\!000}$. We show voltage-length product (${{V}_{\pi}} \cdot {{L}_{ \pi}}$) of ${118}\;{{\rm V}\cdot{\rm cm}}$, which corresponds to an EO coefficient ${{r}_{41}}$ of 2.6 pm/V. The SiCOI platform can be used to realize tunable silicon carbide integrated photonic devices that are desirable for applications in nonlinear and quantum photonics over a wide bandwidth that covers visible and infrared wavelengths.

Single-crystal 3C-SiC-on-insulator platform for integrated quantum photonics

Photonic quantum information processing and communication demand highly integrated device platforms, which can offer high-fidelity control of quantum states and seamless interface with fiber-optic networks simultaneously. Exploiting the unique quantum emitter characteristics compatible with photonic transduction, combined with the outstanding nonlinear optical properties of silicon carbide (SiC), we propose and numerically investigate a single-crystal cubic SiC-on-insulator (3C-SiCOI) platform toward multi-functional integrated quantum photonic circuit. Benchmarking with the state-of-the-art demonstrations on individual components, we have systematically engineered and optimized device specifications and functions, including state control via cavity quantum electrodynamics and frequency conversion between quantum emission and telecommunication wavelengths, while also considering the manufacturing aspects. This work will provide concrete guidelines and quantitative design considerations for realizing future SiCOI integrated photonic circuitry toward quantum information applications.

Silicon carbide zipper photonic crystal optomechanical cavities

We demonstrate a silicon carbide (SiC) zipper photonic crystal optomechanical cavity. The device is on a 3C-SiC-on-silicon platform and has a compact footprint of ∼30 × 1 μm. The device shows an optical quality of 2800 at telecom and a mechanical quality of 9700 at 12 MHz with an effective mass of ∼3.76 pg. The optical mode and mechanical mode exhibit strong nonlinear interaction, namely, the quadratic spring effect, with a nonlinear spring constant of 3.3 × 10⁴ MHz²/nm. The SiC zipper cavity is potentially useful in sensing and metrology in harsh environments.

High-Q suspended optical resonators in 3C silicon carbide obtained by thermal annealing

We fabricate suspended single-mode optical waveguides and ring resonators in 3C silicon carbide (SiC) that operate at telecommunication wavelength, and leverage post-fabrication thermal annealing to minimize optical propagation losses. Annealed optical resonators yield quality factors of over 41,000, which corresponds to a propagation loss of 7 dB/cm, and is a significant improvement over the 24 dB/cm in the case of the non-annealed chip. This improvement is attributed to the enhancement of SiC crystallinity and a significant reduction of waveguide surface roughness, from 2.4 nm to below 1.7 nm. The latter is attributed to surface layer oxide growth during the annealing step. We confirm that the thermo-optic coefficient, an important parameter governing high-power and temperature-dependent performance of SiC, does not vary with annealing and is comparable to that of bulk SiC. Our annealing-based approach, which is especially suitable for suspended structures, offers a straightforward way to realize high-performance 3C-SiC integrated circuits.

4H-SiC microring resonators for nonlinear integrated photonics

We demonstrate enhanced four-wave mixing (FWM) in high-quality factor, high-confinement 4H-SiC microring resonators via continuous-wave FWM. With the large power buildup effect of the microring resonator, -21.7  dBFWM conversion efficiency is achieved with 79 mW pump power. Thanks to the strong light confinement in SiC-on-insulator (SiCOI) waveguides with submicrometer cross-sectional dimensions, a high nonlinear parameter wγ of 7.4±0.9  W^-1 m^-1 is obtained, from which the nonlinear refractive index (n₂) of 4H-SiC is estimated to be (6.0±0.6)×10^-19  m²/W at the telecom wavelengths. Besides, we are able to engineer the dispersion of a SiCOI waveguide to achieve 3 dB FWM conversion bandwidth of more than 130 nm. This work represents a step toward enabling all-optical signal processing functionalities using highly nonlinear SiCOI waveguides.

High-Q microresonators integrated with microheaters on a 3C-SiC-on-insulator platform

We demonstrate, to the best of our knowledge, the first thermally reconfigurable high-Q silicon carbide (SiC) microring resonators with integrated microheaters on a 3C-SiC-on-insulator platform. We extract a thermo-optic coefficient of around 2.67×10^-5/K for 3C-SiC from wavelength shift of a resonator heated by a hot plate. Finally, we fabricate a 40-μm-radius microring resonator with intrinsic Q of 139,000 at infrared wavelengths (∼1550  nm) after integration with a NiCr microheater. By applying current through the microheater, a resonance shift of 30 pm/mW is achieved in the microring, corresponding to ∼50  mW per π phase shift. This platform offers an easy and reliable way for integration with electronic devices as well as great potential for diverse integrated optics applications.

Single photon detection with superconducting nanowires on crystalline silicon carbide

Silicon carbide (SiC) is among the most promising optical materials for the realization of classical and quantum photonics, due to the simultaneous presence of quantum emitters and a non-centrosymmetric crystal structure. In recent years, progress have been made in the development of SiC integrated optical components making this a mature platform for the implementation of quantum experiments on chip. Toward this scope, the fabrication of a single photon detector that can be implemented on top of a photonic circuit is essential to achieve a monolithic integration of all the fundamental building blocks required for photonic quantum technologies. Here we demonstrate for the first time single photon detection with superconducting nanowires on top of a bare 3C SiC layer using a novel approach for the fiber-to-detector coupling that allows the optical characterization of multiple detectors without the use of neither cryogenic positioners nor the micromachining of the substrate.

Silicon carbide double-microdisk resonator

We demonstrate the first silicon carbide (SiC) double-microdisk resonator (DMR). The device has a compact footprint with a radius of 24 μm and operates in the ITU high frequency range (3-30 MHz). We develop a multi-layer nanofabrication recipe that yields high optical quality (Q∼10⁵) for the SiC DMR. Because of its strong optomechanical interaction, we observe the thermal-Brownian motions of mechanical modes in a SiC DMR directly at room temperature for the first time, to the best of our knowledge. The observed mechanical modes include fundamental/second-order common modes and fundamental differential (D1) modes. The D1 modes have high mechanical qualities >3800 at around 18.4 MHz tested in vacuum. We further show that optomechanical interactions, including linear and nonlinear optomechanical spring effects, can be observed in a SiC DMR at sub-milliwatt optical power. The SiC DMR has great potential for low-power optomechanical sensing applications in harsh environments.

High-quality factor, high-confinement microring resonators in 4H-silicon carbide-on-insulator

Silicon carbide (SiC) exhibits promising material properties for nonlinear integrated optics. We report on a SiC-on-insulator platform based on crystalline 4H-SiC and demonstrate high-confinement SiC microring resonators with sub-micron waveguide cross-sectional dimensions. The Q factor of SiC microring resonators in such a sub-micron waveguide dimension is improved by a factor of six after surface roughness reduction by applying a wet oxidation process. We achieve a high Q factor (73,000) for such devices and show engineerable dispersion from normal to anomalous dispersion by controlling the waveguide cross-sectional dimension, which paves the way toward nonlinear applications in SiC microring resonators.