Gate-tunable frequency combs in graphene–nitride microresonators

Peregrine solitons and resonant radiation in cubic and quadratic media

We present the fascinating phenomena of resonant radiation emitted by transient rogue waves in cubic and quadratic nonlinear media, particularly those shed from Peregrine solitons, one of the main wavepackets used today to model real-world rogue waves. In cubic media, it turns out that the emission of radiation from a Peregrine soliton can be attributed to the presence of higher-order dispersion, but is affected by the intrinsic local longitudinal variation of the soliton wavenumber. In quadratic media, we reveal that a two-color Peregrine rogue wave can resonantly radiate dispersive waves even in the absence of higher-order dispersion, subjected to a phase-matching mechanism that involves the second-harmonic wave, and to a concomitant difference-frequency generation process. In both cubic and quadratic media, we provide simple analytic criteria for calculating the radiated frequencies in terms of material parameters, showing excellent agreement with numerical simulations.

Dispersive Fourier transform based dual-comb ranging

Laser-based light detection and ranging (LIDAR) offers a powerful tool to real-timely map spatial information with exceptional accuracy and owns various applications ranging from industrial manufacturing, and remote sensing, to airborne and in-vehicle missions. Over the past two decades, the rapid advancements of optical frequency combs have ushered in a new era for LIDAR, promoting measurement precision to quantum noise limited level. For comb LIDAR systems, to further improve the comprehensive performances and reconcile inherent conflicts between speed, accuracy, and ambiguity range, innovative demodulation strategies become crucial. Here we report a dispersive Fourier transform (DFT) based LIDAR method utilizing phase-locked Vernier dual soliton laser combs. We demonstrate that after in-line pulse stretching, the delay of the flying pulses can be identified via the DFT-based spectral interferometry instead of temporal interferometry or pulse reconstruction. This enables absolute distance measurements with precision starting from 262 nm in single shot, to 2.8 nm after averaging 1.5 ms, in a non-ambiguity range over 1.7 km. Furthermore, our DFT-based LIDAR method distinctly demonstrates an ability to completely eliminate dead zones. Such an integration of frequency-resolved ultrafast analysis and dual-comb ranging technology may pave a way for the design of future LIDAR systems.

Long-distance and high-precision ranging with dual-comb nonlinear asynchronous optical sampling

Precise distance metrology and measurements play an important role in many fields of scientific research and industrial manufacture. Dual-comb laser ranging combines sub-wavelength ranging precision, large non-ambiguity range, and high update rate, making it the most promising candidate in precise distance metrology and measurements. However, previous demonstrations of dual-comb ranging suffer from short working distances, limited by the decoherence of lasers in interferometric schemes or by the low sensitivity of the photodetectors in response to the sparse echo photons. Here, we propose and demonstrate time-of-flight laser ranging with dual-comb nonlinear asynchronous optical sampling and photon counting by a fractal superconducting nanowire single-photon detector, achieving ranging precision of 6.2 micrometers with an acquisition time of 100 ms and 0.9 micrometers with an acquisition time of 1 s in measuring the distance of an outdoor target approximately 298 m away.

Scalable stable comb-to-tone integrated RF photonic drive for superconducting qubits

The recent advent of quantum computing has the potential to overhaul security, communications, and scientific modeling. Superconducting qubits are a leading platform that is advancing noise-tolerant intermediate-scale quantum processors. The implementation requires scaling to large numbers of superconducting qubits, circuit depths, and gate speeds, wherein high-purity RF signal generation and effective cabling transport are desirable. Fiber photonic-enhanced RF signal generation has demonstrated the principle of addressing both signal generation and transport requirements, supporting intermediate qubit numbers and robust packaging efforts; however, fiber-based approaches to RF signal distribution are often bounded by their phase instability. Here, we present a silicon photonic integrated circuit-based version of a photonic-enhanced RF signal generator that demonstrates the requisite stability, as well as a path towards the necessary signal fidelity.

Harnessing sub-comb dynamics in a graphene-sensitized microresonator for gas detection

Since their inception, frequency combs generated in microresonators, known as microcombs, have sparked significant scientific interests. Among the various applications leveraging microcombs, soliton microcombs are often preferred due to their inherent mode-locking capability. However, this choice introduces additional system complexity because an initialization process is required. Meanwhile, despite the theoretical understanding of the dynamics of other comb states, their practical potential, particularly in applications like sensing where simplicity is valued, remains largely untapped. Here, we demonstrate controllable generation of sub-combs that bypasses the need for accessing bistable regime. And in a graphene-sensitized microresonator, the sub-comb heterodynes produce stable, accurate microwave signals for high-precision gas detection. By exploring the formation dynamics of sub-combs, we achieved 2 MHz harmonic comb-to-comb beat notes with a signal-to-noise ratio (SNR) greater than 50 dB and phase noise as low as - 82 dBc/Hz at 1 MHz offset. The graphene sensitization on the intracavity probes results in exceptional frequency responsiveness to the adsorption of gas molecules on the graphene of microcavity surface, enabling detect limits down to the parts per billion (ppb) level. This synergy between graphene and sub-comb formation dynamics in a microcavity structure showcases the feasibility of utilizing microcombs in an incoherent state prior to soliton locking. It may mark a significant step toward the development of easy-to-operate, systemically simple, compact, and high-performance photonic sensors.

Transient cavity-cavity strong coupling at terahertz frequency on LiNbO3 chips

Terahertz (THz) microcavities have garnered considerable attention for their ability to localize and confine THz waves, allowing for strong coupling to remarkably enhance the light-matter interaction. These properties hold great promise for advancing THz science and technology, particularly for high-speed integrated THz chips where transient interaction between THz waves and matter is critical. However, experimental study of these transient time-domain processes requires high temporal and spatial resolution since these processes, such as THz strong coupling, occur in several picoseconds and microns. Thus, most literature studies rarely cover temporal and spatial processes at the same time. In this work, we thoroughly investigate the transient cavity-cavity strong-coupling phenomena at THz frequency and find a Rabi-like oscillation in the microcavities, manifested by direct observation of a periodic energy exchange process via a phase-contrast time-resolved imaging system. Our explanation, based on the Jaynes-Cummings model, provides theoretical insight into this transient strong-coupling process. This work provides an opportunity to deeply understand the transient strong-coupling process between THz microcavities, which sheds light on the potential of THz microcavities for high-speed THz sensor and THz chip design.

Repetition rate tuning and locking of solitons in a microrod resonator

Recently, there has been significant interest in the generation of coherent temporal solitons in optical microresonators. In this Letter, we present a demonstration of dissipative Kerr soliton generation in a microrod resonator using an auxiliary-laser-assisted thermal response control method. In addition, we are able to control the repetition rate of the soliton over a range of 200 kHz while maintaining the pump laser frequency, by applying external stress tuning. Through the precise control of the PZT voltage, we achieve a stability level of 3.9 × 10-10 for residual fluctuation of the repetition rate when averaged 1 s. Our platform offers precise tuning and locking capabilities for the repetition frequency of coherent mode-locked combs in microresonators. This advancement holds great potential for applications in spectroscopy and precision measurements.

Coherently parallel fiber-optic distributed acoustic sensing using dual Kerr soliton microcombs

Fiber-optic distributed acoustic sensing (DAS) has proven to be a revolutionary technology for the detection of seismic and acoustic waves with ultralarge scale and ultrahigh sensitivity, and is widely used in oil/gas industry and intrusion monitoring. Nowadays, the single-frequency laser source in DAS becomes one of the bottlenecks limiting its advance. Here, we report a dual-comb-based coherently parallel DAS concept, enabling linear superposition of sensing signals scaling with the comb-line number to result in unprecedented sensitivity enhancement, straightforward fading suppression, and high-power Brillouin-free transmission that can extend the detection distance considerably. Leveraging 10-line comb pairs, a world-class detection limit of 560 fε/√Hz@1 kHz with 5 m spatial resolution is achieved. Such a combination of dual-comb metrology and DAS technology may open an era of extremely sensitive DAS at the fε/√Hz level, leading to the creation of next-generation distributed geophones and sonars.

Resonant Transducers Consisting of Graphene Ribbons with Attached Proof Masses for NEMS Sensors

The unique mechanical and electrical properties of graphene make it an exciting material for nanoelectromechanical systems (NEMS). NEMS resonators with graphene springs facilitate studies of graphene's fundamental material characteristics and thus enable innovative device concepts for applications such as sensors. Here, we demonstrate resonant transducers with ribbon-springs made of double-layer graphene and proof masses made of silicon and study their nonlinear mechanics at resonance both in air and in vacuum by laser Doppler vibrometry. Surprisingly, we observe spring-stiffening and spring-softening at resonance, depending on the graphene spring designs. The measured quality factors of the resonators in a vacuum are between 150 and 350. These results pave the way for a class of ultraminiaturized nanomechanical sensors such as accelerometers by contributing to the understanding of the dynamics of transducers based on graphene ribbons with an attached proof mass.

Functionalizing nanophotonic structures with 2D van der Waals materials

The integration of two-dimensional (2D) van der Waals materials with nanostructures has triggered a wide spectrum of optical and optoelectronic applications. Photonic structures of conventional materials typically lack efficient reconfigurability or multifunctionality. Atomically thin 2D materials can thus generate new functionality and reconfigurability for a well-established library of photonic structures such as integrated waveguides, optical fibers, photonic crystals, and metasurfaces, to name a few. Meanwhile, the interaction between light and van der Waals materials can be drastically enhanced as well by leveraging micro-cavities or resonators with high optical confinement. The unique van der Waals surfaces of the 2D materials enable handiness in transfer and mixing with various prefabricated photonic templates with high degrees of freedom, functionalizing as the optical gain, modulation, sensing, or plasmonic media for diverse applications. Here, we review recent advances in synergizing 2D materials to nanophotonic structures for prototyping novel functionality or performance enhancements. Challenges in scalable 2D materials preparations and transfer, as well as emerging opportunities in integrating van der Waals building blocks beyond 2D materials are also discussed.

Very high-frequency, gate-tunable CrPS4 nanomechanical resonator with single mode

Two-dimensional (2D) antiferromagnetic semiconductor chromium thiophosphate (CrPS₄) has gradually become a major candidate material for low-dimensional nanoelectromechanical devices due to its remarkable structural, photoelectric characteristics and potentially magnetic properties. Here, we report the experimental study of a new few-layer CrPS₄ nanomechanical resonator demonstrating excellent vibration characteristics through the laser interferometry system, including the uniqueness of resonant mode, the ability to work at the very high frequency, and gate tuning. In addition, we demonstrate that the magnetic phase transition of CrPS₄ strips can be effectively detected by temperature-regulated resonant frequencies, which proves the coupling between magnetic phase and mechanical vibration. We believe that our findings will promote the further research and applications of the resonator for 2D magnetic materials in the field of optical/mechanical signal sensing and precision measurement.

Near-zero-dispersion soliton and broadband modulational instability Kerr microcombs in anomalous dispersion

The developing advances of microresonator-based Kerr cavity solitons have enabled versatile applications ranging from communication, signal processing to high-precision measurements. Resonator dispersion is the key factor determining the Kerr comb dynamics. Near the zero group-velocity-dispersion (GVD) regime, low-noise and broadband microcomb sources are achievable, which is crucial to the application of the Kerr soliton. When the GVD is almost vanished, higher-order dispersion can significantly affect the Kerr comb dynamics. Although many studies have investigated the Kerr comb dynamics near the zero-dispersion regime in microresonator or fiber ring system, limited by dispersion profiles and dispersion perturbations, the near-zero-dispersion soliton structure pumped in the anomalous dispersion side is still elusive so far. Here, we theoretically and experimentally investigate the microcomb dynamics in fiber-based Fabry-Perot microresonator with ultra-small anomalous GVD. We obtain 2/3-octave-spaning microcombs with ~10 GHz spacing, >84 THz span, and >8400 comb lines in the modulational instability (MI) state, without any external nonlinear spectral broadening. Such widely-spanned MI combs are also able to enter the soliton state. Moreover, we report the first observation of anomalous-dispersion based near-zero-dispersion solitons, which exhibits a local repetition rate up to 8.6 THz, an individual pulse duration <100 fs, a span >32 THz and >3200 comb lines. These two distinct comb states have their own advantages. The broadband MI combs possess high conversion efficiency and wide existing range, while the near-zero-dispersion soliton exhibits relatively low phase noise and ultra-high local repetition rate. This work complements the dynamics of Kerr cavity soliton near the zero-dispersion regime, and may stimulate cross-disciplinary inspirations ranging from dispersion-controlled microresonators to broadband coherent comb devices.

Graphene oxide for photonics, electronics and optoelectronics

Graphene oxide (GO) was initially developed to emulate graphene, but it was soon recognized as a functional material in its own right, addressing an application space that is not accessible to graphene and other carbon materials. Over the past decade, research on GO has made tremendous advances in material synthesis and property tailoring. These, in turn, have led to rapid progress in GO-based photonics, electronics and optoelectronics, paving the way for technological breakthroughs with exceptional performance. In this Review, we provide an overview of the optical, electrical and optoelectronic properties of GO and reduced GO on the basis of their chemical structures and fabrication approaches, together with their applications in key technologies such as solar energy harvesting, energy storage, medical diagnosis, image display and optical communications. We also discuss the challenges of this field, together with exciting opportunities for future technological advances.

Waveguide-Integrated MoTe2 p-i-n Homojunction Photodetector

Two-dimensional (2D) materials, featuring distinctive electronic and optical properties and dangling-bond-free surfaces, are promising for developing high-performance on-chip photodetectors in photonic integrated circuits. However, most of the previously reported devices operating in the photoconductive mode suffer from a high dark current or a low responsivity. Here, we demonstrate a MoTe₂ p-i-n homojunction fabricated directly on a silicon photonic crystal (PC) waveguide, which enables on-chip photodetection with ultralow dark current, high responsivity, and fast response speed. The adopted silicon PC waveguide is electrically split into two individual back gates to selectively dope the top regions of the MoTe₂ channel in p- or n-types. High-quality reconfigurable MoTe₂ (p-i-n, n-i-p, n-i-n, p-i-p) homojunctions are realized successfully, presenting rectification behaviors with ideality factors approaching 1.0 and ultralow dark currents less than 90 pA. Waveguide-assisted MoTe₂ absorption promises a sensitive photodetection in the telecommunication O-band from 1260 to 1340 nm, though it is close to MoTe₂'s absorption band-edge. A competitive photoresponsivity of 0.4 A/W is realized with a light on/off current ratio exceeding 10⁴ and a record-high normalized photocurrent-to-dark-current ratio of 10⁶ mW^-1. The ultrasmall capacitance of p-i-n homojunction and high carrier mobility of MoTe₂ promise a high dynamic response bandwidth close to 34.0 GHz. The proposed device geometry has the advantages of employing a silicon PC waveguide as the back gates to build a 2D material p-i-n homojunction directly and simultaneously to enhance light-2D material interaction. It provides a potential pathway to develop 2D material-based photodetectors, laser diodes, and electro-optic modulators on silicon photonic chips.

A Monolithic Graphene-Functionalized Microlaser for Multispecies Gas Detection

Optical-microcavity-enhanced light-matter interaction offers a powerful tool to develop fast and precise sensing techniques, spurring applications in the detection of biochemical targets ranging from cells, nanoparticles, and large molecules. However, the intrinsic inertness of such pristine microresonators limits their spread in new fields such as gas detection. Here, a functionalized microlaser sensor is realized by depositing graphene in an erbium-doped over-modal microsphere. By using a 980 nm pump, multiple laser lines excited in different mode families of the microresonator are co-generated in a single device. The interference between these splitting mode lasers produce beat notes in the electrical domain (0.2-1.1 MHz) with sub-kHz accuracy, thanks to the graphene-induced intracavity backward scattering. This allows for lab-free multispecies gas identification from a mixture, and ultrasensitive gas detection down to individual molecule.

Ultra-narrow linewidth vertical-cavity surface-emitting laser based on external-cavity weak distributed feedback

We demonstrate an ultra-narrow linewidth vertical-cavity surface-emitting laser (VCSEL) based on external-cavity weak distributed feedback from Rayleigh backscattering (RBS). A single longitudinal mode VCSEL with the linewidth as narrow as 435 Hz and a contrast of 55 dB are experimentally achieved by RBS fiber with a feedback level of RBS signal of -27.6 dB. By adjusting the thermal resistance of the VCSEL from 4.5 kΩ to 7.0 kΩ, the laser wavelength can be tuned from 1543.324 nm to 1542.06 nm with a linear tuning slope of -0.506 nm/kΩ. In the tuning process, the linewidth fluctuates in the range of 553-419 Hz.

Coherent terahertz radiation with 2.8-octave tunability through chip-scale photomixed microresonator optical parametric oscillation

High-spectral-purity frequency-agile room-temperature sources in the terahertz spectrum are foundational elements for imaging, sensing, metrology, and communications. Here we present a chip-scale optical parametric oscillator based on an integrated nonlinear microresonator that provides broadly tunable single-frequency and multi-frequency oscillators in the terahertz regime. Through optical-to-terahertz down-conversion using a plasmonic nanoantenna array, coherent terahertz radiation spanning 2.8-octaves is achieved from 330 GHz to 2.3 THz, with ≈20 GHz cavity-mode-limited frequency tuning step and ≈10 MHz intracavity-mode continuous frequency tuning range at each step. By controlling the microresonator intracavity power and pump-resonance detuning, tunable multi-frequency terahertz oscillators are also realized. Furthermore, by stabilizing the microresonator pump power and wavelength, sub-100 Hz linewidth of the terahertz radiation with 10⁻¹⁵ residual frequency instability is demonstrated. The room-temperature generation of both single-frequency, frequency-agile terahertz radiation and multi-frequency terahertz oscillators in the chip-scale platform offers unique capabilities in metrology, sensing, imaging and communications.

High-spectral-purity frequency-agile room-temperature THz sources are foundational elements for imaging, sensing, metrology, and communications. Here a parametric oscillator-photomixer chip with coherent 2.8-octave tunable THz radiation is achieved.

Nonlinear absorption and integrated photonics applications of MoSSe

This study explores the wavelength-dependent and pulse-width-dependent nonlinear optical properties of liquid-phase exfoliated molybdenum sulfide selenide (MoSSe) nanosheets. The saturable absorption response of MoSSe nanosheets in the visible region is better than that in the near-infrared region, and the response under 6-ns pulse excitation is better than that of a 380-fs pulse. Furthermore, based on the first-principles calculations, we designed a phase modulator and optimized its structure by integrating a monolayer MoSSe into a silicon slot waveguide. The simulation results revealed that the phase shift could achieve a high optical extinction. Consequently, MoSSe exhibits satisfactory nonlinear optical properties and an excellent potential for applications in optoelectronic devices.

Engineering van der Waals Materials for Advanced Metaphotonics

The outstanding chemical and physical properties of 2D materials, together with their atomically thin nature, make them ideal candidates for metaphotonic device integration and construction, which requires deep subwavelength light-matter interaction to achieve optical functionalities beyond conventional optical phenomena observed in naturally available materials. In addition to their intrinsic properties, the possibility to further manipulate the properties of 2D materials via chemical or physical engineering dramatically enhances their capability, evoking new science on light-matter interaction, leading to leaped performance of existing functional devices and giving birth to new metaphotonic devices that were unattainable previously. Comprehensive understanding of the intrinsic properties of 2D materials, approaches and capabilities for chemical and physical engineering methods, the resulting property modifications and novel functionalities, and applications of metaphotonic devices are provided in this review. Through reviewing the detailed progress in each aspect and the state-of-the-art achievement, insightful analyses of the outstanding challenges and future directions are elucidated in this cross-disciplinary comprehensive review with the aim to provide an overall development picture in the field of 2D material metaphotonics and promote rapid progress in this fast emerging and prosperous field.

Dynamic and Active THz Graphene Metamaterial Devices

In recent years, terahertz waves have attracted significant attention for their promising applications. Due to a broadband optical response, an ultra-fast relaxation time, a high nonlinear coefficient of graphene, and the flexible and controllable physical characteristics of its meta-structure, graphene metamaterial has been widely explored in interdisciplinary frontier research, especially in the technologically important terahertz (THz) frequency range. Here, graphene’s linear and nonlinear properties and typical applications of graphene metamaterial are reviewed. Specifically, the discussion focuses on applications in optically and electrically actuated terahertz amplitude, phase, and harmonic generation. The review concludes with a brief examination of potential prospects and trends in graphene metamaterial.

Nonlinear co-generation of graphene plasmons for optoelectronic logic operations

Surface plasmons in graphene provide a compelling strategy for advanced photonic technologies thanks to their tight confinement, fast response and tunability. Recent advances in the field of all-optical generation of graphene’s plasmons in planar waveguides offer a promising method for high-speed signal processing in nanoscale integrated optoelectronic devices. Here, we use two counter propagating frequency combs with temporally synchronized pulses to demonstrate deterministic all-optical generation and electrical control of multiple plasmon polaritons, excited via difference frequency generation (DFG). Electrical tuning of a hybrid graphene-fibre device offers a precise control over the DFG phase-matching, leading to tunable responses of the graphene’s plasmons at different frequencies across a broadband (0 ~ 50 THz) and provides a powerful tool for high-speed logic operations. Our results offer insights for plasmonics on hybrid photonic devices based on layered materials and pave the way to high-speed integrated optoelectronic computing circuits.

Nano-photonic devices based on 2D materials offer a potential solution for the miniaturization of optical computing technologies. Here, the authors demonstrate the implementation of high-speed logic operations via the all-optical generation and electrical control of multiple plasmon polaritons in a hybrid graphene device.

Silicon Thermo-Optic Switches with Graphene Heaters Operating at Mid-Infrared Waveband

The mid-infrared (MIR, 2-20 μm) waveband is of great interest for integrated photonics in many applications such as on-chip spectroscopic chemical sensing, and optical communication. Thermo-optic switches are essential to large-scale integrated photonic circuits at MIR wavebands. However, current technologies require a thick cladding layer, high driving voltages or may introduce high losses in MIR wavelengths, limiting the performance. This paper has demonstrated thermo-optic (TO) switches operating at 2 μm by integrating graphene onto silicon-on-insulator (SOI) structures. The remarkable thermal and optical properties of graphene make it an excellent heater material platform. The lower loss of graphene at MIR wavelength can reduce the required cladding thickness for the thermo-optics phase shifter from micrometers to tens of nanometers, resulting in a lower driving voltage and power consumption. The modulation efficiency of the microring resonator (MRR) switch was 0.11 nm/mW. The power consumption for 8-dB extinction ratio was 5.18 mW (0.8 V modulation voltage), and the rise/fall time was 3.72/3.96 μs. Furthermore, we demonstrated a 2 × 2 Mach-Zehnder interferometer (MZI) TO switch with a high extinction ratio of more than 27 dB and a switching rise/fall time of 4.92/4.97 μs. A comprehensive analysis of the device performance affected by the device structure and the graphene Fermi level was also performed. The theoretical figure of merit (2.644 mW-1μs-1) of graphene heaters is three orders of magnitude higher than that of metal heaters. Such results indicate graphene is an exceptional nanomaterial for future MIR optical interconnects.

Fabrication Technologies for the On-Chip Integration of 2D Materials

With compact footprint, low energy consumption, high scalability, and mass producibility, chip-scale integrated devices are an indispensable part of modern technological change and development. Recent advances in 2D layered materials with their unique structures and distinctive properties have motivated their on-chip integration, yielding a variety of functional devices with superior performance and new features. To realize integrated devices incorporating 2D materials, it requires a diverse range of device fabrication techniques, which are of fundamental importance to achieve good performance and high reproducibility. This paper reviews the state-of-art fabrication techniques for the on-chip integration of 2D materials. First, an overview of the material properties and on-chip applications of 2D materials is provided. Second, different approaches used for integrating 2D materials on chips are comprehensively reviewed, which are categorized into material synthesis, on-chip transfer, film patterning, and property tuning/modification. Third, the methods for integrating 2D van der Waals heterostructures are also discussed and summarized. Finally, the current challenges and future perspectives are highlighted.

Boosting the SiN nonlinear photonic platform with transition metal dichalcogenide monolayers

In the past few years, we have witnessed increased interest in the use of 2D materials to produce hybrid photonic nonlinear waveguides. Although graphene has attracted most of the attention, other families of 2D materials such as transition metal dichalcogenides have also shown promising nonlinear performance. In this work, we propose a strategy for designing silicon nitride waveguiding structures with embedded MoS₂ for nonlinear applications. The transverse geometry of the hybrid waveguide is optimized for high third-order nonlinear effects using optogeometrical engineering and multiple layers of MoS₂. Stacking multiple monolayers results in an improvement of two orders of magnitude compared to standard silicon nitride waveguides. The hybrid waveguide performance is then investigated in terms of four-wave mixing enhancement in micro-ring resonator configurations. A signal/idler conversion efficiency of -6.3 dB is reached for a wavelength of around 1.55 µm with a 5 mW pumping level.

MoS2 hybrid integrated micro-ring resonator phase shifter based on a silicon nitride platform

We demonstrate a low-power, compact micro-ring phase shifter based on hybrid integration with atomically thin two-dimensional layered materials, and experimentally establish a low-loss silicon nitride platform. Using a wet transfer method, a large-area few-layer MoS₂ film is hybrid integrated with a micro-ring phase shifter, leading to a tuning efficiency of 5.8 pm V^-1 at a center wavelength of 1545.294 nm and a half-wave-voltage-length product as low as 0.09 V cm. Our device is designed to provide a hybrid-integration-based active phase modulation scheme for integrated optical communication networks with large-cross-section silicon nitride waveguides.

Optical ultrasound sensors for photoacoustic imaging: a narrative review

Optical ultrasound sensors have been increasingly employed in biomedical diagnosis and photoacoustic imaging (PAI) due to high sensitivity and resolution. PAI could visualize the distribution of ultrasound excited by laser pulses in biological tissues. The information of tissues is detected by ultrasound sensors in order to reconstruct structural images. However, traditional ultrasound transducers are made of piezoelectric films that lose sensitivity quadratically with the size reduction. In addition, the influence of electromagnetic interference limits further applications of traditional ultrasound transducers. Therefore, optical ultrasound sensors are developed to overcome these shortcomings. In this review, optical ultrasound sensors are classified into resonant and non-resonant ones in view of physical principles. The principles and basic parameters of sensors are introduced in detail. Moreover, the state of the art of optical ultrasound sensors and applications in PAI are also presented. Furthermore, the merits and drawbacks of sensors based on resonance and non-resonance are discussed in perspectives. We believe this review could provide researchers with a better understanding of the current status of optical ultrasound sensors and biomedical applications.

Multispecies and individual gas molecule detection using Stokes solitons in a graphene over-modal microresonator

Soliton frequency combs generate equally-distant frequencies, offering a powerful tool for fast and accurate measurements over broad spectral ranges. The generation of solitons in microresonators can further improve the compactness of comb sources. However the geometry and the material’s inertness of pristine microresonators limit their potential in applications such as gas molecule detection. Here, we realize a two-dimensional-material functionalized microcomb sensor by asymmetrically depositing graphene in an over-modal microsphere. By using one single pump, spectrally trapped Stokes solitons belonging to distinct transverse mode families are co-generated in one single device. Such Stokes solitons with locked repetition rate but different offsets produce ultrasensitive beat notes in the electrical domain, offering unique advantages for selective and individual gas molecule detection. Moreover, the stable nature of the solitons enables us to trace the frequency shift of the dual-soliton beat-note with uncertainty <0.2 Hz and to achieve real-time individual gas molecule detection in vacuum, via an optoelectronic heterodyne detection scheme. This combination of atomically thin materials and microcombs shows the potential for compact photonic sensing with high performances and offers insights toward the design of versatile functionalized microcavity photonic devices.

The integration of 2D materials on photonic devices provides advanced functionalities in sensing applications. The authors demonstrate a graphene functionalized microcomb sensor by exploiting spectrally trapped Stokes solitons. They obtain both multispecies gas identification and individual molecule sensitivity.

Electrically Controlled Thermal Radiation from Reduced Graphene Oxide Membranes

We demonstrate a fabrication procedure of hybrid devices that consist of reduced graphene oxide films supported by porous polymer membranes that host ionic solutions. We find that we can control the thermal radiation from the surface of reduced graphene oxide through a process of electrically driven reversible ionic intercalation. Through a comparative analysis of the structural, chemical, and optical properties of our reduced graphene oxide films, we identify that the dominant mechanism leading to the intercalation-induced reduction of light emission is Pauli blocking of the interband recombination of charge carriers. We inspect the capabilities of our devices to act as a platform for the electrical control of mid-infrared photonics by observing a bias-induced reduction of apparent temperature of hot surfaces visualized through an infrared thermal camera.

Loss-induced switching between electromagnetically induced transparency and critical coupling in a chalcogenide waveguide

Optical loss is generally perceived to be an adverse effect in integrated optics. Herein, in contrast, we propose a mechanism to harness the loss in a coupled ${\rm {high-}}{\! Q}$ resonators system to realize on-chip electromagnetically induced transparency (EIT). The increasing loss of one of the coupled resonators results in a difference in ${Q}$ factor, leading to EIT generation. This optical loss-induced EIT is studied analytically using the coupled-mode theory and demonstrated experimentally in chalcogenide coupled microring resonators. By taking advantage of the chalcogenide phase change materials that feature exceptional optical property contrasts, we further demonstrate the loss-induced mechanism to realize fast and nonvolatile responses between the EIT state and the critical coupling state in a monolithically integrated chip. Our results provide a new perspective to harvest the negative loss effect of coupled resonators for tunable photonic devices, which might shed new light on the design ideology for on-chip slow-light optical components.

Hot carriers in graphene - fundamentals and applications

Hot charge carriers in graphene exhibit fascinating physical phenomena, whose understanding has improved greatly over the past decade. They have distinctly different physical properties compared to, for example, hot carriers in conventional metals. This is predominantly the result of graphene's linear energy-momentum dispersion, its phonon properties, its all-interface character, and the tunability of its carrier density down to very small values, and from electron- to hole-doping. Since a few years, we have witnessed an increasing interest in technological applications enabled by hot carriers in graphene. Of particular interest are optical and optoelectronic applications, where hot carriers are used to detect (photodetection), convert (nonlinear photonics), or emit (luminescence) light. Graphene-enabled systems in these application areas could find widespread use and have a disruptive impact, for example in the field of data communication, high-frequency electronics, and industrial quality control. The aim of this review is to provide an overview of the most relevant physics and working principles that are relevant for applications exploiting hot carriers in graphene.

Kilometers Long Graphene-Coated Optical Fibers for Fast Thermal Sensing

The combination of optical fiber with graphene has greatly expanded the application regimes of fiber optics, from dynamic optical control and ultrafast pulse generation to high precision sensing. However, limited by fabrication, previous graphene-fiber samples are typically limited in the micrometer to centimeter scale, which cannot take the inherent advantage of optical fibers—long-distance optical transmission. Here, we demonstrate kilometers long graphene-coated optical fiber (GCF) based on industrial graphene nanosheets and coating technique. The GCF shows unusually high thermal diffusivity of 24.99 mm² s⁻¹ in the axial direction, measured by a thermal imager directly. This enables rapid thermooptical response both in optical fiber Bragg grating sensors at one point (18-fold faster than conventional fiber) and in long-distance distributed fiber sensing systems based on backward Rayleigh scattering in optical fiber (15-fold faster than conventional fiber). This work realizes the industrial-level graphene-fiber production and provides a novel platform for two-dimensional material-based optical fiber sensing applications.

Nanometric Precision Distance Metrology via Hybrid Spectrally Resolved and Homodyne Interferometry in a Single Soliton Frequency Microcomb

Laser interferometry serves a fundamental role in science and technology, assisting precision metrology and dimensional length measurement. During the past decade, laser frequency combs-a coherent optical-microwave frequency ruler over a broad spectral range with traceability to time-frequency standards-have contributed pivotal roles in laser dimensional metrology with ever-growing demands in measurement precision. Here we report spectrally resolved laser dimensional metrology via a free-running soliton frequency microcomb, with nanometric-scale precision. Spectral interferometry provides information on the optical time-of-flight signature, and the large free-spectral range and high coherence of the microcomb enable tooth-resolved and high-visibility interferograms that can be directly read out with optical spectrum instrumentation. We employ a hybrid timing signal from comb-line homodyne, microcomb, and background amplified spontaneous emission spectrally resolved interferometry-all from the same spectral interferogram. Our combined soliton and homodyne architecture demonstrates a 3-nm repeatability over a 23-mm nonambiguity range achieved via homodyne interferometry and over 1000-s stability in the long-term precision metrology at the white noise limits.

Graphene Oxide for Integrated Photonics and Flat Optics

With superior optical properties, high flexibility in engineering its material properties, and strong capability for large-scale on-chip integration, graphene oxide (GO) is an attractive solution for on-chip integration of 2D materials to implement functional integrated photonic devices capable of new features. Over the past decade, integrated GO photonics, representing an innovative merging of integrated photonic devices and thin GO films, has experienced significant development, leading to a surge in many applications covering almost every field of optical sciences such as photovoltaics, optical imaging, sensing, nonlinear optics, and light emitting. This paper reviews the recent advances in this emerging field, providing an overview of the optical properties of GO as well as methods for the on-chip integration of GO. The main achievements made in GO hybrid integrated photonic devices for diverse applications are summarized. The open challenges as well as the potential for future improvement are also discussed.

Electrically Tunable Four-Wave-Mixing in Graphene Heterogeneous Fiber for Individual Gas Molecule Detection

Detection of individual molecules is the ultimate goal of any chemical sensor. In the case of gas detection, such resolution has been achieved in advanced nanoscale electronic solid-state sensors, but it has not been possible so far in integrated photonic devices, where the weak light-molecule interaction is typically hidden by noise. Here, we demonstrate a scheme to generate ultrasensitive down-conversion four-wave-mixing (FWM) in a graphene bipolar-junction-transistor heterogeneous D-shaped fiber. In the communication band, the FWM conversion efficiency can change steeply when the graphene Fermi level approaches 0.4 eV. In this condition, we exploit our unique two-step optoelectronic heterodyne detection scheme, and we achieve real-time individual gas molecule detection in vacuum. Such combination of graphene strong nonlinearities, electrical tunability, and all-fiber integration paves the way toward the design of versatile high-performance graphene photonic devices.

On-demand inkjet-printed microdisk laser with air cladding by liquid flow microetching

We have novelly, to the best of our knowledge, developed the liquid flow microetching method that can treat a single microdisk in a microregion with precise position control for inkjet-printed microdisk lasers. The injection-drain wet etching setup consisted of two microneedles that successfully performed a formation of a fine undercut structure of an inkjet-printed microdisk on a pre-pedestal layer through the individual wet etching process. Then measurement of the undercut structure using scanning electron microscopy and lasing characteristics with whispering gallery modes were carried out to demonstrate performance of the etched microdisks. The measured lasing threshold decreased by half compared with that of the unetched microdisk directly printed on a fluorine-type film. A point to note is that this etching method exhibits an excellent undercut and lasing characteristics even when using a clad pre-pedestal layer having a refractive index higher than that of core microdisks. This technique, combined with inkjet printing, offers a powerful tool for individually designing a microdisk and can help develop novel devices that comprise several inkjet-printed microdisks being evanescently coupled.

Hybrid integrated photonics using bulk acoustic resonators

Integrated photonic devices based on Si3N4 waveguides allow for the exploitation of nonlinear frequency conversion, exhibit low propagation loss, and have led to advances in compact atomic clocks, ultrafast ranging, and spectroscopy. Yet, the lack of Pockels effect presents a major challenge to achieve high-speed modulation of Si3N4. Here, microwave-frequency acousto-optic modulation is realized by exciting high-overtone bulk acoustic wave resonances (HBAR) in the photonic stack. Although HBAR is ubiquitously used in modern communication and superconducting circuits, this is the first time it has been incorporated on a photonic integrated chip. The tight vertical acoustic confinement releases the lateral design of freedom, and enables negligible cross-talk and preserving low optical loss. This hybrid HBAR nanophotonic platform can find immediate applications in topological photonics with synthetic dimensions, compact opto-electronic oscillators, and microwave-to-optical converters. As an application, a Si3N4-based optical isolator is demonstrated by spatiotemporal modulation, with over 17 dB isolation achieved.

Stress-released Si3N4 fabrication process for dispersion-engineered integrated silicon photonics

We develop a stress-released stoichiometric silicon nitride (Si₃N₄) fabrication process for dispersion-engineered integrated silicon photonics. To relax the high tensile stress of a thick Si₃N₄ film grown by low-pressure chemical vapor deposition (LPCVD) process, we grow the film in two steps and introduce a conventional dense stress-release pattern onto a ∼400nm-thick Si₃N₄ film in between the two steps. Our pattern helps minimize crack formation by releasing the stress of the film along high-symmetry periodic modulation directions and helps stop cracks from propagating. We demonstrate a nearly crack-free ∼830nm-thick Si₃N₄ film on a 4" silicon wafer. Our Si₃N₄ photonic platform enables dispersion-engineered, waveguide-coupled microring and microdisk resonators, with cavity sizes of up to a millimeter. Specifically, our 115µm-radius microring exhibits an intrinsic quality (Q)-factor of ∼2.0×10⁶ for the TM₀₀ mode and our 575µm-radius microdisk demonstrates an intrinsic Q of ∼4.0×10⁶ for TM modes in 1550nm wavelengths.

2D Material Optoelectronics for Information Functional Device Applications: Status and Challenges

Graphene and the following derivative 2D materials have been demonstrated to exhibit rich distinct optoelectronic properties, such as broadband optical response, strong and tunable light-mater interactions, and fast relaxations in the flexible nanoscale. Combining with optical platforms like fibers, waveguides, grating, and resonators, these materials has spurred a variety of active and passive applications recently. Herein, the optical and electrical properties of graphene, transition metal dichalcogenides, black phosphorus, MXene, and their derivative van der Waals heterostructures are comprehensively reviewed, followed by the design and fabrication of these 2D material-based optical structures in implementation. Next, distinct devices, ranging from lasers to light emitters, frequency convertors, modulators, detectors, plasmonic generators, and sensors, are introduced. Finally, the state-of-art investigation progress of 2D material-based optoelectronics offers a promising way to realize new conceptual and high-performance applications for information science and nanotechnology. The outlook on the development trends and important research directions are also put forward.

Deterministic single-soliton generation in a graphene-FP microresonator

Dissipative Kerr solitons (DKS) in high-Q microresonators have attracted considerable attention for their broadband optical frequency combs and ultra-short pulse generation. Owing to thermal effects, complicated tuning strategies are required to generate and sustain the single-soliton state in microresonators. In this paper, we propose a novel microresonator scheme based on the Fabry-Pérot fiber resonator and single-layer graphene saturable absorber (SA) and demonstrate that this design allows deterministic single-soliton generation without frequency tuning and has strong robustness against pump perturbation. The soliton range and thermal instability of the proposed device are also discussed. This work facilitates a novel nonlinear platform connecting high-Q microresonators and conventional SA-assisted mode-locking operations.

Real-time transition dynamics and stability of chip-scale dispersion-managed frequency microcombs

Femtosecond mode-locked laser frequency combs have served as the cornerstone in precision spectroscopy, all-optical atomic clocks, and measurements of ultrafast dynamics. Recently frequency microcombs based on nonlinear microresonators have been examined, exhibiting remarkable precision approaching that of laser frequency combs, on a solid-state chip-scale platform and from a fundamentally different physical origin. Despite recent successes, to date, the real-time dynamical origins and high-power stabilities of such frequency microcombs have not been fully addressed. Here, we unravel the transitional dynamics of frequency microcombs from chaotic background routes to femtosecond mode-locking in real time, enabled by our ultrafast temporal magnifier metrology and improved stability of dispersion-managed dissipative solitons. Through our dispersion-managed oscillator, we further report a stability zone that is more than an order-of-magnitude larger than its prior static homogeneous counterparts, providing a novel platform for understanding ultrafast dissipative dynamics and offering a new path towards high-power frequency microcombs.

Interface Engineering-Assisted 3D-Graphene/Germanium Heterojunction for High-Performance Photodetectors

Three-dimensional graphene (3D-Gr) with excellent light absorption properties has received enormous interest, but in conventional processes to prepare 3D-Gr, amorphous carbon layers are inevitably introduced as buffer layers that may degrade the performance of graphene-based devices. Herein, 3D-Gr is prepared on germanium (Ge) using two-dimensional graphene (2D-Gr) as the buffer layer. 2D-Gr as the buffer layer facilitates the in situ synthesis of 3D-Gr on Ge by plasma-enhanced chemical vapor deposition (PECVD) by promoting 2D-Gr nucleation and reducing the barrier height. The growth mechanism is investigated and described. The enhanced light absorption as confirmed by theoretical calculation and 3D-Gr/2D-Gr/Ge with a Schottky junction improves the performance of optoelectronic devices without requiring pre- and post-transfer processes. The photodetector constructed with 3D-Gr/2D-Gr/Ge shows an excellent responsivity of 1.7 A W^-1 and detectivity 3.42 × 10¹⁴ cm Hz^1/2 W^-1 at a wavelength of 1550 nm. This novel hybrid structure that incorporates 3D- and 2D-Gr into Ge-based integrated circuits and photodetectors delivers excellent performance and has large commercial potential.

Efficient electro-optical tuning of an optical frequency microcomb on a monolithically integrated high-Q lithium niobate microdisk

We demonstrate efficient tuning of a monolithically integrated lithium niobate (LN) microdisk optical frequency microcomb. Utilizing the high optical quality (Q) factor (i.e., Q∼7.1×10⁶) of the microdisk, the microcomb spans over a spectral bandwidth of ∼200nm at a pump power as low as 20.4 mW. Combining the large electro-optic coefficient of LN and the optimum design of the geometry of microelectrodes, we demonstrate electro-optical tuning of the comb with a spectral range of 400 pm and a tuning efficiency of ∼38pm/100V.

Enhanced all-optical cavity-tuning using graphene

All-optical tuning of the resonance of an optical cavity is used to realise optical signal-processing including modulation, switching, and signal-routing. The tuning of optical resonance is dictated by the two primary effects induced by optical absorption: charge-carrier-generation and heat-generation. Since these two effects shift the resonance in opposite directions in a pure silicon-on-insulator (SOI) micro-ring resonator as well as in a graphene-on-SOI system, the efficiency and the dynamic range of all-optical resonance-tuning is limited. In this work, in a graphene-oxide-silicon waveguide system, we demonstrate an exceptional resonance-tuning-efficiency of 300 p m/m W (0.055 π/m W), with a large dynamic range of 1.2 n m (0.22 π) from linear resonance to optical bistability. The dynamics of the resonance-tuning indicates that the superior resonance-tuning is due to large linear-absorption-induced thermo-optic effect. Competing free-carrier dispersion is suppressed as a result of the large separation between graphene and the silicon core. This work reveals new ways to improve the performance of graphene-on-waveguide systems in all-optical cavity-tuning, low-frequency all-optical modulation, and switching.

Gain-through-filtering enables tuneable frequency comb generation in passive optical resonators

Optical frequency combs (OFCs), consisting of a set of phase-locked, equally spaced laser frequency lines, have enabled a great leap in precision spectroscopy and metrology since seminal works of Hänsch et al. Nowadays, OFCs are cornerstones of a wealth of further applications ranging from chemistry and biology to astrophysics and including molecular fingerprinting and light detection and ranging (LIDAR) systems, among others. Driven passive optical resonators constitute the ideal platform for OFC generation in terms of compactness and low energy footprint. We propose here a technique for the generation of OFCs with a tuneable repetition rate in externally driven optical resonators based on the gain-through-filtering process, a simple and elegant method, due to asymmetric spectral filtering on one side of the pump wave. We demonstrate a proof-of-concept experimental result in a fibre resonator, pioneering a new technique that does not require specific engineering of the resonator dispersion to generate frequency-agile OFCs.

Defect-Driven Heterogeneous Electron Transfer between an Individual Graphene Sheet and Electrode

Understanding the heterogeneous electron-transfer (ET) kinetics on graphene is essential for its extensive applications. Here, on the basis of the redox-induced fluorescence variation of monolayer graphene itself, the heterogeneous ET kinetics at the interface between the electrode and the monolayer graphene was studied label-freely at the single-sheet level. By tuning the defect density on graphene, an optimal heterogeneous ET rate was observed at a moderate defect density, indicating defect-driven ET kinetics. The heterogeneities of both the intrasheet and intersheet ET kinetics were revealed at the single-sheet level. With the optimal defective graphene sheets as a sensing material for oxygen gas, a cost-effective electrochemical oxygen sensor was obtained with high sensitivity, fast response/recovery, and remarkable durability. The results obtained here deepen our understanding of the electrochemical properties of graphene and imply that rational defect control can enhance the ET process between the electrode and graphene and then improve the performance of graphene-based functional materials or devices.

High-contrast optical switching using an epsilon-near-zero material coupled to a Bragg microcavity

Epsilon-near-zero (ENZ) materials have recently been suggested as excellent candidates for constructing all-optical and electro-optical switches in the infrared. The performance of previously reported ENZ material-based optical switches, however, has been greatly hampered by the low quality- (Q-) factor of the ENZ cavity, resulting in a large required optical pump fluence or applied voltage, a large insertion loss, or a small modulation depth. Here, we propose a solution by integrating the ENZ material into a Bragg microcavity, such that the Q-factor of the coupled cavity can be dramatically enhanced. Using high-mobility Dysprosium-doped cadmium oxide (CdO) as the prototype ENZ material, we numerically show an infrared all-optical switch with its reflectance modulated from near-zero to 94% under a pump fluence of only 7 μJ cm^-2, about a 59-time-reduction compared with a state-of-the-art Berreman-type cavity. Moreover, the high-Q coupled cavity can also be adopted to realize a reflective electro-optical switch. Its reflectance can be switched from near-zero to 89%, with a bias electric field well below the breakdown field of conventional gate dielectrics. The switching operation can further be extended to the transmission mode with a slightly modified cavity geometry, with its absolute transmittance modulated by 40%.