2D-3D integration of hexagonal boron nitride and a high-κ dielectric for ultrafast graphene-based electro-absorption modulators

Waveguide-integrated twisted bilayer graphene photodetectors

Graphene photodetectors have exhibited high bandwidth and capability of being integrated with silicon photonics (SiPh), holding promise for future optical communication devices. However, they usually suffer from a low photoresponsivity due to weak optical absorption. In this work, we have implemented SiPh-integrated twisted bilayer graphene (tBLG) detectors and reported a responsivity of 0.65 A W-1 for telecom wavelength 1,550 nm. The high responsivity enables a 3-dB bandwidth of >65 GHz and a high data stream rate of 50 Gbit s-1. Such high responsivity is attributed to the enhanced optical absorption, which is facilitated by van Hove singularities in the band structure of high-mobility tBLG with 4.1o twist angle. The uniform performance of the fabricated photodetector arrays demonstrates a fascinating prospect of large-area tBLG as a material candidate for heterogeneous integration with SiPh.

Button shear testing for adhesion measurements of 2D materials

Two-dimensional (2D) materials are considered for numerous applications in microelectronics, although several challenges remain when integrating them into functional devices. Weak adhesion is one of them, caused by their chemical inertness. Quantifying the adhesion of 2D materials on three-dimensional surfaces is, therefore, an essential step toward reliable 2D device integration. To this end, button shear testing is proposed and demonstrated as a method for evaluating the adhesion of 2D materials with the examples of graphene, hexagonal boron nitride (hBN), molybdenum disulfide, and tungsten diselenide on silicon dioxide and silicon nitride substrates. We propose a fabrication process flow for polymer buttons on the 2D materials and establish suitable button dimensions and testing shear speeds. We show with our quantitative data that low substrate roughness and oxygen plasma treatments on the substrates before 2D material transfer result in higher shear strengths. Thermal annealing increases the adhesion of hBN on silicon dioxide and correlates with the thermal interface resistance between these materials. This establishes button shear testing as a reliable and repeatable method for quantifying the adhesion of 2D materials.

Technology and Integration Roadmap for Optoelectronic Memristor

Optoelectronic memristors (OMs) have emerged as a promising optoelectronic Neuromorphic computing paradigm, opening up new opportunities for neurosynaptic devices and optoelectronic systems. These OMs possess a range of desirable features including minimal crosstalk, high bandwidth, low power consumption, zero latency, and the ability to replicate crucial neurological functions such as vision and optical memory. By incorporating large-scale parallel synaptic structures, OMs are anticipated to greatly enhance high-performance and low-power in-memory computing, effectively overcoming the limitations of the von Neumann bottleneck. However, progress in this field necessitates a comprehensive understanding of suitable structures and techniques for integrating low-dimensional materials into optoelectronic integrated circuit platforms. This review aims to offer a comprehensive overview of the fundamental performance, mechanisms, design of structures, applications, and integration roadmap of optoelectronic synaptic memristors. By establishing connections between materials, multilayer optoelectronic memristor units, and monolithic optoelectronic integrated circuits, this review seeks to provide insights into emerging technologies and future prospects that are expected to drive innovation and widespread adoption in the near future.

All-Solution-Processed High-Performance MoS2 Thin-Film Transistors with a Quasi-2D Perovskite Oxide Dielectric

Assembling solution-processed van der Waals (vdW) materials into thin films holds great promise for constructing large-scale, high-performance thin-film electronics, especially at low temperatures. While transition metal dichalcogenide thin films assembled in solution have shown potential as channel materials, fully solution-processed vdW electronics have not been achieved due to the absence of suitable dielectric materials and high-temperature processing. In this work, we report on all-solution-processedvdW thin-film transistors (TFTs) comprising molybdenum disulfides (MoS2) as the channel and Dion-Jacobson-phase perovskite oxides as the high-permittivity dielectric. The constituent layers are prepared as colloidal solutions through electrochemical exfoliation of bulk crystals, followed by sequential assembly into a semiconductor/dielectric heterostructure for TFT construction. Notably, all fabrication processes are carried out at temperatures below 250 °C. The fabricated MoS2 TFTs exhibit excellent device characteristics, including high mobility (>10 cm2 V-1 s-1) and an on/off ratio exceeding 106. Additionally, the use of a high-k dielectric allows for operation at low voltage (∼5 V) and leakage current (∼10-11 A), enabling low power consumption. Our demonstration of the low-temperature fabrication of high-performance TFTs presents a cost-effective and scalable approach for heterointegrated thin-film electronics.

Semi-embedded slot waveguide electro-optic modulator

Electro-optic modulators are essential devices on silicon photonic chips in modern optical communication networks. This paper presents a compact, low-loss electro-optic modulator. The modulation efficiency is greatly improved by embedding the lower half of the slot waveguide into the buried oxide layer and inserting graphene at the junction. The interaction of graphene with an optical field in a waveguide is studied using the finite element method. The functions of phase modulation and absorption modulation are realized by changing the gate voltage to change the chemical potential of graphene. The semi-embedded slot waveguide optical modulator has a length of 50 µm. After simulation verification, it can be used as an electro-absorption modulator and can achieve a modulation depth of 26.38 dB and an insertion loss of 0.60 dB. When used as an electro-refractive modulator, it can be realized with a linear change of phase from zero to π; the total insertion loss is only 0.59 dB. The modulator has a modulation bandwidth of 79.6 GHz, and the energy consumption as electro-absorption and electro-refraction modulation are 0.51 and 1.92 pj/bit, respectively. Compared with common electro-optic modulators, the electro-optic modulator designed in this paper has a higher modulation effect and also takes into account the advantages of low insertion loss and low energy consumption. This research is helpful for the design of higher-performance optical communication network devices.

High-speed mid-infrared graphene electro-optical modulator based on suspended germanium slot waveguides

The mid-infrared (MIR) region is attracting increasing interest for on-chip synchronous detection and free-space optical (FSO) communications. For such applications, a high-performance electro-optical modulator is a crucial component. In this regard, we propose and investigate a graphene-based electro-absorption modulator (EAM) and microring modulator (MRM) using the suspended germanium waveguide platform. The modulators are designed for the second atmospheric window (8 to 12 µm). The incorporation of double-layer graphene on the suspended slot waveguide structure allows for the significant enhancement of light-graphene interaction, theoretically achieving a 3-dB bandwidth as high as 78 GHz. The EAM shows a calculated modulation depth of 0.022-0.045 dB/µm for the whole operation wavelength range. The MRM exhibits a calculated extinction ratio as high as 68.9 dB and a modulation efficiency of 0.59 V·cm around 9 µm. These modulators hold promise for constructing high-speed FSO communication and on-chip spectroscopic detection systems in the MIR atmospheric window.

Ultra-compact on-chip meta-waveguide phase modulator based on split ring magnetic resonance

With the development of photonic integration technology, meta-waveguides have become a new research hotspot. They have broken through the theoretical diffraction limit by virtue of the strong electromagnetic manipulation ability of the metasurface and the strong electromagnetic field limitation and guidance ability of the waveguide. However, the reported meta-waveguides lack research on dynamic modulation. Therefore, we analyze the modulation effect of the metasurface on the optical field in the waveguide and design an ultra-compact on-chip meta-waveguide phase modulator using split ring magnetic resonance. It has a very short modulation length of only 3.65 µm, wide modulation bandwidth of 116.8 GHz, and low energy consumption of 263.49 fJ/bit. By optimizing the structure, the energy consumption can be further reduced to 90.69 fJ/bit. Meta-waveguides provide a promising method for the design of integrated photonic devices.

Graphene Photonics I/Q Modulator for Advanced Modulation Formats

Starting from its classical domain of long distance links, optical communication is conquering new application areas down to chip-to-chip interconnections in response to the ever-increasing demand for higher bandwidth. The use of coherent modulation formats, typically employed in long-haul systems, is now debated to be extended to short links to increase the bandwidth density. Next-generation transceivers are targeting high bandwidth, high energy efficiency, compact footprint, and low cost. Integrated photonics is the only technology to reach this goal, and silicon photonics is expected to play the leading actor. However, silicon modulators have some limits, in terms of bandwidth and footprint. Graphene is an ideal material to be integrated with silicon photonics to meet the requirements of next generation transceivers. This material provides optimal properties: high mobility, fast carrier dynamics and ultrabroadband optical properties. Graphene photonics for direct detection systems based on binary modulation formats have been demonstrated so far, including electro-absorption modulators, phase modulators, and photodetectors. However, coherent modulation for increased data-rates has not yet been reported for graphene photonics yet. In this work, we present the first graphene photonics I/Q modulator based on four graphene on silicon electro-absorption modulators for advanced modulation formats and demonstrate quadrature phase shift keying (QPSK) modulation up to 40 Gb/s.

Recent advances in density functional theory approach for optoelectronics properties of graphene

Graphene has received tremendous attention among diverse 2D materials because of its remarkable properties. Its emergence over the last two decades gave a new and distinct dynamic to the study of materials, with several research projects focusing on exploiting its intrinsic properties for optoelectronic devices. This review provides a comprehensive overview of several published articles based on density functional theory and recently introduced machine learning approaches applied to study the electronic and optical properties of graphene. A comprehensive catalogue of the bond lengths, band gaps, and formation energies of various doped graphene systems that determine thermodynamic stability was reported in the literature. In these studies, the peculiarity of the obtained results reported is consequent on the nature and type of the dopants, the choice of the XC functionals, the basis set, and the wrong input parameters. The different density functional theory models, as well as the strengths and uncertainties of the ML potentials employed in the machine learning approach to enhance the prediction models for graphene, were elucidated. Lastly, the thermal properties, modelling of graphene heterostructures, the superconducting behaviour of graphene, and optimization of the DFT models are grey areas that future studies should explore in enhancing its unique potential. Therefore, the identified future trends and knowledge gaps have a prospect in both academia and industry to design future and reliable optoelectronic devices.

Hexagonal Boron Nitride for Next-Generation Photonics and Electronics

Hexagonal boron nitride (h-BN), an insulating 2D layered material, has recently attracted tremendous interest motivated by the extraordinary properties it shows across the fields of optoelectronics, quantum optics, and electronics, being exotic material platforms for various applications. At an early stage of h-BN research, it is explored as an ideal substrate and insulating layers for other 2D materials due to its atomically flat surface that is free of dangling bonds and charged impurities, and its high thermal conductivity. Recent discoveries of structural and optical properties of h-BN have expanded potential applications into emerging electronics and photonics fields. h-BN shows a very efficient deep-ultraviolet band-edge emission despite its indirect-bandgap nature, as well as stable room-temperature single-photon emission over a wide wavelength range, showing a great potential for next-generation photonics. In addition, h-BN is extensively being adopted as active media for low-energy electronics, including nonvolatile resistive switching memory, radio-frequency devices, and low-dielectric-constant materials for next-generation electronics.

Mode-selective modulator and switch based on graphene-polymer hybrid waveguides

The mode-division multiplexing (MDM) is an effective technology with huge development potential to improve the transmission capacity of optical communication system by transmitting multiple modes simultaneously in a few-mode fiber. In traditional MDM technology, the fundamental modes of multiple channels are usually modulated by external individual arranged electro-optic modulators, and then multiplexed into the few-mode fiber or waveguide by a mode multiplexer. However, this is usually limited by large device footprint and high power consumption. Here, we report a mode-selective modulator and switch to individually modulate or switch the TE₁₁, TE₁₂ and TE₂₁ modes in a few-mode waveguide (FMW) to overcome this limitation. Our method is based on the graphene-polymer hybrid platform with four graphene capacitors buried in different locations of the polymer FMW by utilizing the coplanar interaction between the capacitors and spatial modes. The TE₁₁, TE₁₂ and TE₂₁ modes in the FMW can be modulated and switched separately or simultaneously by applying independent gate voltage to different graphene capacitor of the device. Our study is expected to make the selective management of the spatial modes in MDM transmission systems more flexible.

Linearity of a silicon-based graphene electro-absorption modulator

A silicon-based graphene modulator, holding the advantages of high modulation efficiency, high speed, and being ultra-compact, is regarded as a promising candidate for next-generation communication networks. Although the properties involved for optical communications have been widely studied, very few works evaluate the performance required for the microwave scenarios. Here, for the first time, to the best of our knowledge, the linearity of silicon-based graphene electro-absorption modulator (EAM) is analyzed and experimentally characterized through spurious free dynamic range (SFDR) with 82.5 dB·Hz^1/2 and 100.3 dB·Hz^2/3. Further calculations reveal that a higher SFDR value could be achieved through optimizing the bias voltage. Variations of capacitor structural parameters have little influence on the linearity. Such performance leads to the first, to the best of our knowledge, demonstration of a Gbps-level pulse-amplitude 4-level modulation scheme (PAM-4) eye diagram in a silicon-based graphene modulator.

Advances in integrated ultra-wideband electro-optic modulators [Invited]

Increasing data traffic and bandwidth-hungry applications require electro-optic modulators with ultra-wide modulation bandwidth for cost-efficient optical networks. Thus far, integrated solutions have emerged to provide high bandwidth and low energy consumption in compact sizes. Here, we review the design guidelines and delicate structures for higher bandwidth, applying them to lumped-element and traveling-wave electrodes. Additionally, we focus on candidate material platforms with the potential for ultra-wideband optical systems. By comparing the superiority and mechanism limitations of different integrated modulators, we design a future roadmap based on the recent advances.

Silicon-Based Graphene Electro-Optical Modulators

With the increasing demand for capacity in communications networks, the use of integrated photonics to transmit, process and manipulate digital and analog signals has been extensively explored. Silicon photonics, exploiting the complementary-metal-oxide-semiconductor (CMOS)-compatible fabrication technology to realize low-cost, robust, compact, and power-efficient integrated photonic circuits, is regarded as one of the most promising candidates for next-generation chip-scale information and communication technology (ICT). However, the electro-optic modulators, a key component of Silicon photonics, face challenges in addressing the complex requirements and limitations of various applications under state-of-the-art technologies. In recent years, the graphene EO modulators, promising small footprints, high temperature stability, cost-effective, scalable integration and a high speed, have attracted enormous interest regarding their hybrid integration with SiPh on silicon-on-insulator (SOI) chips. In this paper, we summarize the developments in the study of silicon-based graphene EO modulators, which covers the basic principle of a graphene EO modulator, the performance of graphene electro-absorption (EA) and electro-refractive (ER) modulators, as well as the recent advances in optical communications and microwave photonics (MWP). Finally, we discuss the emerging challenges and potential applications for the future practical use of silicon-based graphene EO modulators.

Towards Perfect Absorption of Single Layer CVD Graphene in an Optical Resonant Cavity: Challenges and Experimental Achievements

Graphene is emerging as a promising material for the integration in the most common Si platform, capable to convey some of its unique properties to fabricate novel photonic and optoelectronic devices. For many real functions and devices however, graphene absorption is too low and must be enhanced. Among strategies, the use of an optical resonant cavity was recently proposed, and graphene absorption enhancement was demonstrated, both, by theoretical and experimental studies. This paper summarizes our recent progress in graphene absorption enhancement by means of Si/SiO2-based Fabry-Perot filters fabricated by radiofrequency sputtering. Simulations and experimental achievements carried out during more than two years of investigations are reported here, detailing the technical expedients that were necessary to increase the single layer CVD graphene absorption first to 39% and then up to 84%. Graphene absorption increased when an asymmetric Fabry-Perot filter was applied rather than a symmetric one, and a further absorption increase was obtained when graphene was embedded in a reflective rather than a transmissive Fabry-Perot filter. Moreover, the effect of the incident angle of the electromagnetic radiation and of the polarization of the light was investigated in the case of the optimized reflective Fabry-Perot filter. Experimental challenges and precautions to avoid evaporation or sputtering induced damage on the graphene layers are described as well, disclosing some experimental procedures that may help other researchers to embed graphene inside PVD grown materials with minimal alterations.

Graphene on Silicon Photonics: Light Modulation and Detection for Cutting-Edge Communication Technologies

Graphene—a two-dimensional allotrope of carbon in a single-layer honeycomb lattice nanostructure—has several distinctive optoelectronic properties that are highly desirable in advanced optical communication systems. Meanwhile, silicon photonics is a promising solution for the next-generation integrated photonics, owing to its low cost, low propagation loss and compatibility with CMOS fabrication processes. Unfortunately, silicon’s photodetection responsivity and operation bandwidth are intrinsically limited by its material characteristics. Graphene, with its extraordinary optoelectronic properties has been widely applied in silicon photonics to break this performance bottleneck, with significant progress reported. In this review, we focus on the application of graphene in high-performance silicon photonic devices, including modulators and photodetectors. Moreover, we explore the trend of development and discuss the future challenges of silicon-graphene hybrid photonic devices.

Structural, electronic, and transport properties of Janus GaInX2(XS, Se, Te) monolayers: first-principles study

Two-dimensional Janus monolayers have outstanding electronic and transport properties due to their asymmetric atomic structures. In the present work, we systematically study the structural, electronic, and transport properties of the Janus GaInX₂(X= S, Se, Te) monolayers by using the first-principles calculations. The stability of the investigated monolayers is confirmed via the analysis of vibrational spectrum and molecular dynamics simulations. Our calculations demonstrate that while GaInS₂and GaInSe₂monolayers are direct semiconductors, GaInTe₂monolayer exhibits the characteristics of an indirect semiconductor. The band gap of GaInX₂decreases when the chalcogen elementXvaries from S to Te. Obtained results reveal that small spin-orbit splitting energy in the valence band is found around the Γ point of the Brillouin zone when the spin-orbit coupling is included. Interestingly, GaInS₂and GaInSe₂have high and directional isotropic electron mobility meanwhile the directional anisotropy of the electron mobility is found in the Janus GaInTe₂monolayer. Our findings not only present superior physical properties of GaInX₂monolayers but also show promising potential applications of these materials in nanoelectronic devices.

Extraordinary Optical Transmission by Hybrid Phonon-Plasmon Polaritons Using hBN Embedded in Plasmonic Nanoslits

Hexagonal boron nitride (hBN) exhibits natural hyperbolic dispersion in the infrared (IR) wavelength spectrum. In particular, the hybridization of its hyperbolic phonon polaritons (HPPs) and surface plasmon resonances (SPRs) induced by metallic nanostructures is expected to serve as a new platform for novel light manipulation. In this study, the transmission properties of embedded hBN in metallic one-dimensional (1D) nanoslits were theoretically investigated using a rigorous coupled wave analysis method. Extraordinary optical transmission (EOT) was observed in the type-II Reststrahlen band, which was attributed to the hybridization of HPPs in hBN and SPRs in 1D nanoslits. The calculated electric field distributions indicated that the unique Fabry–Pérot-like resonance was induced by the hybridization of HPPs and SPRs in an embedded hBN cavity. The trajectory of the confined light was a zigzag owing to the hyperbolicity of hBN, and its resonance number depended primarily on the aspect ratio of the 1D nanoslit. Such an EOT is also independent of the slit width and incident angle of light. These findings can not only assist in the development of improved strategies for the extreme confinement of IR light but may also be applied to ultrathin optical filters, advanced photodetectors, and optical devices.