On-chip Optical Communications Research Articles

High-speed CMOS-compatible III-V on Si membrane photodetectors

The monolithic integration of power-efficient optoelectronic devices with CMOS circuits is critical for future on-chip optical communication. In such platforms, ultra-high-speed photodetectors operating at datacom wavelengths are essential to convert optical signals in the electrical domain. Here, we demonstrate ultra-compact high-speed III-V/Si photodetectors integrated on silicon photonics and exclusively fabricated with processes and materials compatible with CMOS foundries. The sub-femtofarad capacitance, high responsivity photodetectors demonstrate a bandwidth around 65 GHz and data reception at 100 GBd OOK. Their thickness and efficiency enable an integration in the back-end-of-line without using an amplifier, further reducing the power consumption of the link.

Design of a graphene-based silicon nitride multimode waveguide-integrated electro-optic modulator

The mode-division (de) multiplexing (MDM) technology is promising to scale the transmission capacity of on-chip optical communications by carrying information on different spatial modes of a multimode waveguide. For the current MDM technology, different spatial modes are first converted to the fundamental mode and then modulated in parallel by individual electro-optic modulators, which may result in a large device footprint and high-power consumption. Here we designed a graphene-based silicon nitride waveguide-integrated spatial mode modulator to individually modulate different spatial modes in a multimode waveguide. Specifically, we utilized the coplanar interaction between patterned graphene layers and propagating light in the multimode waveguide to introduce contrasting phase shifts to different spatial modes. Based on the optimized waveguide configuration, a Mach–Zehnder interferometer modulator is designed. Our study paves the way for the development of high-density on-chip MDM systems for optical interconnects and on-chip optical networks.

On-chip metalenses based on one-dimensional gradient trench in the broadband visible.

Metasurfaces are composed of flat, ultrathin subwavelength nanoantennas with strong capability in manipulating light propagation by modulations on its phase, amplitude, and polarization. For instance, the invention of two-dimensional (2D) metalenses has enabled light focusing and imaging in three-dimensional (3D) free space with miniaturized thickness and device size at a planar surface. However, such inherent form of 2D arrays and focusing functionality at 3D optical free-space limits the degree of freedom for light propagation and manipulation along a 2D planar surface and eventually the possibility of on-chip photonic system integration. Here, we theoretically study and demonstrate a new type of planar on-chip metalens, which enables light focusing and strong localization at a 2D surface. The planar on-chip architecture design is based on the one-dimensional (1D) length or width gradient trench metalens (GTM), which could yield the elaborately engineered phase shift for propagating light within the on-chip waveguide at the visible wavelength of 500 nm. By generating 1D phase arrangement at the nanoscale, a miniature on-chip metalens with ∼3×0.5µm dimension could achieve light focusing on a 2D waveguide surface with the flexibility to design scalable focal lengths and ultra-high numerical aperture of up to ∼0.99. Additionally, GTM metalens designs could also exhibit overlapped high depth-of-focus, which consequently could behave as achromatic-like lensing at the selected focal plane. Furthermore, we manifest that the focusing functionality can also be subject to dynamically tuning and switching on-and-off with TE/TM polarization change or waveguide index alteration. We believe this new form of on-chip 1D metalens holds potential applications including on-chip light manipulation functionality of focusing and diverging, optical on-chip sensing, next-generation on-chip optical communication, signal processing as well as imaging devices, etc.

Tuneable red, green, and blue single-mode lasing in heterogeneously coupled organic spherical microcavities

Tuneable microlasers that span the full visible spectrum, particularly red, green, and blue (RGB) colors, are of crucial importance for various optical devices. However, RGB microlasers usually operate in multimode because the mode selection strategy cannot be applied to the entire visible spectrum simultaneously, which has severely restricted their applications in on-chip optical processing and communication. Here, an approach for the generation of tuneable multicolor single-mode lasers in heterogeneously coupled microresonators composed of distinct spherical microcavities is proposed. With each microcavity serving as both a whispering-gallery-mode (WGM) resonator and a modulator for the other microcavities, a single-mode laser has been achieved. The colors of the single-mode lasers can be freely designed by changing the optical gain in coupled cavities owing to the flexibility of the organic materials. Benefiting from the excellent compatibility, distinct color-emissive microspheres can be integrated to form a heterogeneously coupled system, where tuneable RGB single-mode lasing is realized owing to the capability for optical coupling between multiple resonators. Our findings provide a comprehensive understanding of the lasing modulation that might lead to innovation in structure designs for photonic integration.

A hybrid structure light-emitting device based on a CsPbBr3 nanoplate and two-dimensional materials

High-crystalline halide perovskite nanostructures [such as nanowires and nanoplates (NPs)] provide good potential in realizing nanoscale solid light sources for on-chip optical communication, high-density storage, and life science applications. However, it remains a great challenge to fabricate nanoscale perovskite light-emitting devices using traditional fabrication methods because the perovskite nanomaterials will be dissolved in polar solvents. Developing new device configurations to enhance radiative recombination efficiency as well as device stability is one of the most important research topics in nanoscale perovskite light-emitting devices. Here, we demonstrate nanoscale perovskite electroluminescence (EL) using a single-crystalline CsPbBr3 NP as the active layer. The device is based on a hybrid capacitance structure, where an underlying few-layer graphene (FLG) electrode, a single-crystalline CsPbBr3 NP, a thin hexagonal boron nitride (hBN) flake, and another FLG top electrode are stacking in sequence, forming a van der Waals heterostructure. A strong EL emission peak with a narrow linewidth (∼1.09 nm) is observed at 2 K. Alternating current voltage/frequency-dependent EL spectra are studied in detail. We attribute the superior EL behavior of the as-fabricated nanoscale perovskite light-emitting devices to (1) the high-quality single-crystalline CsPbBr3 NPs synthesized, (2) the hBN encapsulation, which enhances the device stability by providing a large heat dissipation pathway for CsPbBr3 NP and protecting it from the polar solvents, (3) the capacitance structure, which facilitates the injection of both electrons and holes. Our work demonstrates a method to construct nanoscale perovskite (with well-defined geometry) light sources, providing an opportunity for realizing a nanoscale electrically driven perovskite laser.

High-dimensional communication on etchless lithium niobate platform with photonic bound states in the continuum

Photonic bound states in the continuum (BICs) have been exploited in various systems and found numerous applications. Here, we investigate high-order BICs and apply BICs on an integrated photonic platform to high-dimensional optical communication. A four-channel TM mode (de)multiplexer using different orders of BICs on an etchless lithium niobate (LiNbO3) platform where waveguides are constructed by a low-refractive-index material on a high-refractive-index substrate is demonstrated. Low propagation loss of the TM modes in different orders and phase-matching conditions for efficient excitation of the high-order TM modes are simultaneously achieved. A chip consisting of four-channel mode (de)multiplexers was fabricated and measured with data transmission at 40 Gbps/channel. All the channels have insertion loss <4.0 dB and crosstalk <−9.5 dB in a 70-nm wavelength band. Therefore, the demonstrated mode (de)multiplexing and high-dimensional communication on LiNbO3 platform can meet the increasing demand for high capacity in on-chip optical communication.

Broadband graphene-on-silicon modulator with orthogonal hybrid plasmonic waveguides

Abstract Graphene, a two-dimensional nanomaterial, possess unique photoelectric properties that have potential application in designing optoelectronic devices. The tunable optical absorption is one of the most exciting properties that can be used to improve the performance of silicon modulators. However, the weak light–matter interaction caused by the size mismatch between the optical mode fields and graphene makes the graphene-on-silicon modulator (GOSM) has large footprint and high energy consumption, limiting the enhancement of modulation efficiency. Here, we propose a broadband GOSM with orthogonal hybrid plasmonic waveguides (HPWs) at near-infrared wavelengths. The orthogonal HPWs are designed to compress the interaction region of optical fields and enhance the light-graphene interaction. The results show that the GOSM has a modulation depth of 26.20 dB/μm, a footprint of 0.33 μm2, a 3 dB modulation bandwidth of 462.77 GHz, and energy consumption of 2.82 fJ/bit at 1.55 μm. Even working at a broad wavelength band ranging from 1.3 to 2 μm, the GOSM also has a modulation depth of over 8.58 dB/μm and energy consumption of below 4.97 fJ/bit. It is anticipated that with the excellent modulation performance, this GOSM may have great potential in broadband integrated modulators, on-chip optical communications and interconnects, etc.

Surface-passivated high-Q GaAs photonic crystal nanocavity with quantum dots

Photonic crystal (PhC) nanocavities with high quality (Q) factors have attracted much attention because of their strong spatial and temporal light confinement capability. The resulting enhanced light–matter interactions are beneficial for diverse photonic applications, ranging from on-chip optical communications to sensing. However, currently achievable Q factors for active PhC nanocavities, which embed active emitters inside, are much lower than those of the passive structures because of large optical loss, presumably originating from light scattering by structural imperfections and/or optical absorptions. Here, we demonstrate a significant improvement of Q factors up to ∼160 000 in GaAs active PhC nanocavities using a sulfur-based surface passivation technique. This value is the highest ever reported for any active PhC nanocavities with semiconductor quantum dots. The surface-passivated cavities also exhibit reduced variation in both Q factors and cavity resonant wavelengths. We find that the improvement in the cavity performance presumably arises from suppressed light absorption at the surface of the PhC’s host material by performing a set of PL measurements in spectral and time domains. With the surface passivation technique, we also demonstrate a strongly coupled single quantum dot-cavity system based on a PhC nanocavity with a high Q factor of ∼100 000. These results will pave the way for advanced quantum dot-based cavity quantum electrodynamics and GaAs micro/nanophotonic applications containing active emitters.

Modern architecture for photonic networks-on-chip

Development in photonic integrated circuits (PICs) provides a promising solution for on-chip optical computation and communication. PICs provides the best alternative to traditional networks-on-chip (NoC) circuits which face serious challenges such as bandwidth, latency and power consumption. Integrated optics have substantiated the ability to accomplish low-power communication and low-power data processing at ultra-high speeds. In this work, we propose a new architecture for NoC, which might improve overall on-chip network performance by reducing its power consumption, providing large channel capacity for communication, decreasing latency among nodes and reducing hop count. Some of the key features of the proposed architecture are to reduce the waveguide network for communication among nodes, and this architecture can be used as a brick to construct other architectures. In this architecture, we use micro-ring resonator (MRR) and it is used to provide a high bandwidth connection among nodes with a lesser number of waveguide networks. Furthermore, results show that this architecture of PICs provides better performance in terms of low communication latency, low power consumption, high bandwidth. It also provides acceptable FSR value, FWHR value, finesse value and Q-factor of micro-ring resonators used for the design of MRR in this architecture.

Optically Pumped Monolayer MoSe2 Excitonic Lasers from Whispering Gallery Mode Microcavities.

Developing integrable, nanoscale, and low-energy-consumption lasers is a crucial step toward on-chip optical communications and computing technologies. The strong exciton-photon interaction that emerged in monolayer transition metal dichalcogenides (TMDs) holds promise for engineering and integration. Herein, we prepare the MoSe2/microsphere cavities excitonic lasers by placing SiO2 microspheres on top of a monolayer MoSe2 film. By virtue of continuous-wave exciting MoSe2/microsphere whispering gallery mode (WGM) cavities, we realize multiple excitonic WGM lasing in the emission wavelength range of ∼750-875 nm at room temperature with tunable properties of free spectral range (FSR) and full width at half-maximum (fwhm) by varying the microsphere size. Theoretical calculations based on the finite element method (FEM) using COMSOL software were utilized to identify lasing modes and reveal the corresponding electric field distribution. These findings help to deepen fundamental understanding of excitonic WGM lasing and provide a promising research platform for integrable, scalable, and low-cost laser devices.

Superior optical (λ ∼ 1550 nm) emission and detection characteristics of Ge microdisks grown on virtual Si0.5Ge0.5/Si substrates using molecular beam epitaxy

We report the optical characteristics of relatively large sized (∼7.0–8.0 μm) but low aspect ratio Ge microdisks grown on a virtual Si0.5Ge0.5 substrate using molecular beam epitaxy following the Stranski–Krastanov growth mechanism. Grown microdisks with very low aspect ratio Ge islands exhibit direct band gap (∼0.8 eV) photoluminescence emission sustainable up to room temperature, enabled by the confinement of carriers into the microdisks. p–i–n diodes with an intrinsic layer containing Ge microdisks have been fabricated to study their emission and photoresponse characteristics at an optical communication wavelength of ∼1550 nm. A strong electroluminescence at 1550 nm has been achieved at low temperatures in the device for a very low threshold current density of 2.56 μA cm−2 due to the strong confinement of injected holes. The emission characteristics of the fabricated device with respect to the injected current density and temperature have been studied. Novel emission and optical modulation characteristics at 1550 nm of the fabricated p–i–n device containing Ge microdisks grown on a virtual SiGe substrate indicate its potential for Si CMOS compatible on-chip optical communications.

Electron-Beam-Driven III-Nitride Plasmonic Nanolasers in the Deep-UV and Visible Region.

Plasmonic nanolasers based on wide bandgap semiconductors are presently attracting immense research interests due to the breaking in light diffraction limit and subwavelength mode operation with fast dynamics. However, these plasmonic nanolasers have so far been mostly realized in the visible light ranges, or most are still under optical excitation pumping. In this work, III-nitride-based plasmonic nanolasers emitting from the green to the deep-ultraviolet (UV) region by energetic electron beam injection are reported, and a threshold as low as 8 kW cm-2 is achieved. A fast decay time as short as 123 ps is collected, indicating a strong coupling between excitons and surface plasmon. Both the spatial and temporal coherences are observed, which provide a solid evidence for exciton-plasmon coupled polariton lasing. Consequently, the achievements in III-nitride-based plasmonic nanolaser devices represent a significant step toward practical applications for biological technology, computing systems, and on-chip optical communication.

Strong optical response and light emission from a monolayer molecular crystal

Excitons in two-dimensional (2D) materials are tightly bound and exhibit rich physics. So far, the optical excitations in 2D semiconductors are dominated by Wannier-Mott excitons, but molecular systems can host Frenkel excitons (FE) with unique properties. Here, we report a strong optical response in a class of monolayer molecular J-aggregates. The exciton exhibits giant oscillator strength and absorption (over 30% for monolayer) at resonance, as well as photoluminescence quantum yield in the range of 60–100%. We observe evidence of superradiance (including increased oscillator strength, bathochromic shift, reduced linewidth and lifetime) at room-temperature and more progressively towards low temperature. These unique properties only exist in monolayer owing to the large unscreened dipole interactions and suppression of charge-transfer processes. Finally, we demonstrate light-emitting devices with the monolayer J-aggregate. The intrinsic device speed could be beyond 30 GHz, which is promising for next-generation ultrafast on-chip optical communications.

Controllable Polarization of Lasing Emission From a Polymer Microfiber Laser

Microlasers with controllable polarization of output emission are vital for on-chip optical communications, optical sensors and optical switches. In this work, we report a high quality (Q) factor, low-threshold polymer microfiber laser and the possibility of achieving laser emission with a desired polarization. The microfiber is fabricated by direct drawing from a dye-doped polymer solution and it can generate whispering gallery mode (WGM) lasing under optical pulse excitation. When the microfiber is pumped from the side with pumping direction perpendicular to the microfiber’s axis, the polarization direction of the output laser is found to be the same as that of the pump laser. Lasing emission with either transverse electric (TE) or transverse magnetic (TM) modes can be obtained and these two polarization states can be switched over by tuning the pumping laser. Furthermore, emission with both TE and TM modes can also be observed by changing the orientation of the microfiber relatively to pumping direction. Our finding provides an effective approach for achieving microlasers that have high Q lasing modes with anticipated polarization.

Ferroelectric [formula omitted] thin film based broadband waveguide coupled surface plasmon electro-optic modulator

Present article demonstrates the enhanced dynamically tuneable EO modulation characteristics in ferroelectric Strontium Barium Niobate SBN,Sr0.6Ba0.4Nb2O6 thin film modulator based on waveguide coupled surface plasmon resonance. The alternating modulating signals having broad frequency range (1 kHz to 10 MHz) with varying amplitude (40kV/cm to 400kV/cm) was applied to estimate the resonance frequency (~1 MHz) using differential modulated intensity interrogation signal. A high value of EO coefficient r=310pm/V leading to Modulation index of 56% was achieved with sensitivity of about 0.33 at resonance possessing a characteristics length of 54.64 μm. These waveguide modes coupled with the plasmon modes paves a way for development of EO modulators utilizing materials with high losses. Therefore, a thin film based WCSPR-EO modulator with compact footprint, low operation voltage, broad operation spectrum and faster speed of modulation was fabricated which paves the way for on-chip optical communications.

Quantum physics applied to modern optical metal oxide semiconductor transistor

In recent years, traditional complementary metal–oxide–semiconductor (CMOS) scaling techniques have begun to reach the technological limits of available materials. A revolution in block-to-block communication is necessary to meet the ever-growing demand for microprocessor computational power. On-chip optical communication has been designated as a promising solution to circumvent the CMOS scaling bottlenecks: second-order phenomenon, which causes significant interconnect delays, and the nonscalability of the thermal voltage, which becomes significant in submicron CMOS technology. The metal oxide semiconductor quantum well transistor, a silicon-on-insulator metal–oxide–semiconductor field-effect transistor device, with a channel thickness reduced to the single nanometer scale is examined. The nanometric gate oxide, silicon, and buried oxide heterostructure in the channel forms a quantum potential well, creating discrete sub-bands within the silicon layer. Inter-sub-band-transitions within the quantum well may allow for radiative recombination in indirect band gap materials.

Fiber-Integrated Reversibly Wavelength-Tunable Nanowire Laser Based on Nanocavity Mode Coupling.

As an ideal miniaturized light source, wavelength-tunable nanolasers capable of emitting a wide spectrum stimulate intense interests for on-chip optoelectronics, optical communications, and spectroscopy. However, realization of such devices remains a major challenge because of extreme difficulties in achieving continuously reversibly tunable gain media and high quality (Q)-factor resonators on the nanoscale simultaneously. Here, exploiting single bandgap-graded CdSSe NWs and a Fabry-Pérot/whispering gallery mode (FP/WGM) coupling cavity, a free-standing fiber-integrated reversibly wavelength-tunable nanolaser covering a 42 nm wide spectrum at room temperature with high stability and reproducibility is demonstrated. In addition, a 1.13 nm tuning spectral resolution is realized. The substrate-free device design enables integration in optical fiber communications and information. With reversible and wide, continuous tunability of emission color and precise control per step, our work demonstrates a general approach to nanocavity coupling affording high Q-factors, enabling an ideal miniaturized module for a broad range of applications in optics and optoelectronics, with optical fiber integration.

Broadband on-Chip Terahertz Asymmetric Waveguiding via Phase-Gradient Metasurface

As fundamental elements, optical diodes are required for on-chip optical communications and computing. However, it is still a challenge to realize ultracompact devices working in the terahertz (THz...

Low-energy high-speed graphene modulator for on-chip communication

In this work, we have proposed two different designs to improve the efficiency of a graphene-based electro-absorption modulator. The efficiency of the modulator is enhanced by increasing the overlap between the graphene layer and the optical mode of the waveguide. This is achieved by introducing the graphene layer between the silicon nitride cap and silicon waveguide. Using a single monolayer graphene configuration, a switching energy of 3.22 fJ/bit and maximum operating bandwidth of 10.89 GHz is achieved whereas the double monolayer graphene configuration allowed us to achieve a switching energy of 4.26 fJ/bit with a large operating bandwidth of 34 GHz at λ=1550 nm. Low energy consumption coupled with high bandwidth makes these modulator designs a potential candidate for use in on-chip optical communication.

Variable-Ratio Mode-Insensitive 1 × 2 Power Splitter Based on MMI Couplers and Phase Shifters

Mode-division multiplexing (MDM) has been considered as a low-cost and high-density means for high-bandwidth on-chip optical communications in a high-performance computer and a data center. However, simultaneously controlling the power of multiple modes in the MDM systems is difficult due to the complex mode coupling for high-order modes (HOMs). Here we propose a novel schematic structure of variable-ratio mode-insensitive 1 × 2 power splitter, which is based on multimode interference (MMI) couplers cascaded with phase shifters. We theoretically reveal the phase shifting relations between the fundamental mode and the HOMs for the phase shifter formed by two unequal-width waveguides. Then, the operating principles for the proposed structure are presented. Numerical simulations are carried out and it is found that the structure has the ability to realize variable-ratio dual-mode mode-insensitive 1 × 2 power splitter by designing the number and lengths of the phase shifters, which shows a good agreement with our theoretical analysis. The structural parameters of the MMI couplers and the phase shifters are optimized to minimize the transmittance losses and the differences of mode power splitting ratio. The simulation results also show that the optimized power splitters have the advantages of high tolerance to fabrication errors and wide operating bandwidths, which indicates that the proposed structure can operate on multiple modes and would be suited to potential applications in on-chip optical communications.