We study the charge density distribution, the electric field profile, and the resistance of an electrostatically created lateral p-n junction in graphene. We show that the electric field at the interface of the electron and hole regions is strongly enhanced due to limited screening capacity of Dirac quasiparticles. Accordingly, the junction resistance is lower than estimated in previous literature.

http://absimage.aps.org/image/MAR08/MWS_MAR08-2007-005702.pdf

Nonlinear screening and ballistic transport in a graphene p-n junction

Photonic artificial neural networks have garnered enormous attention due to their potential to perform multiply-accumulate (MAC) operations at much higher clock rates and consuming significantly lower power and chip real-estate compared to digital electronic alternatives. Herein, we present a comprehensive power consumption analysis of photonic neurons, taking into account global design parameters and concluding to analytical expressions for the neuron's energy- and footprint efficiencies. We identify the optimal design-space and analyze the performance plateaus and their dependence on a range of physical parameters, highlighting the existence of an optimal data-rate for maximizing the energy efficiency. Following a survey of the best-in-class integrated photonic devices, including on-chip lasers, photodetectors, modulators and weighting elements, the mathematically calculated energy and footprint efficiencies are mapped into real photonic neuron deployment scenarios. We reveal that silicon photonics can compete with the best-performing currently available digital electronic neural network engines, reaching TMAC/s/mm2 footprint- and sub-pJ/MAC energy efficiencies. Simultaneously, neuromorphic plasmonics, plasmo-photonics and sub-wavelength photonics hold the credentials for 1 to 3 orders of magnitude improvements even when the laser requirements and a reasonable waveguide pitch are accounted for, promising performance at a few fJ/MAC and up to a few TMAC/s/mm2.

Femtojoule per MAC Neuromorphic Photonics: An Energy and Technology Roadmap

Electro-optic modulators are an indispensable part of photonic communication systems, largely dictating the achievable transmission rate. Recent advances in materials and fabrication/processing techniques have brought new elements and a renewed dynamic to research on optical modulation. Motivated by the new opportunities, this Perspective reviews the state of the art in integrated electro-optic modulators, covering a broad range of contemporary materials and integrated platforms. To provide a better overview of the status of current modulators, an assessment of the different material platforms is conducted on the basis of common performance metrics: extinction ratio, insertion loss, electro-optic bandwidth, driving voltage, and footprint. The main physical phenomena exploited for electro-optic modulation are first introduced, aiming to provide a self-contained reference to researchers in physics and engineering. Additionally, we take care to highlight topics that can be overlooked and require attention, such as the accurate calculation of carrier density distribution and energy consumption, the correct modeling of thin and two-dimensional materials, and the nature of contact electrodes. Finally, a future outlook for the different electro-optic materials is provided, anticipating the research and performance trends in the years to come.

Electro-optic modulation in integrated photonics

With its unique and exclusive linear and nonlinear optical characteristics, epsilon-near-zero (ENZ) photonics has drawn a tremendous amount of attention in the recent decade in the fields of nanophotonics, nonlinear optics, plasmonics, light-matter interactions, material science, applied optical science, etc. The extraordinary optical properties, relatively high tuning flexibility, and CMOS compatibility of ENZ materials make them popular and competitive candidates for nanophotonic devices and on-chip integration in all-optical and electro-optical platforms. With exclusive features and high performance, ENZ photonics can play a big role in optical communications and optical data processing. In this review, we give a focused discussion on recent advances of the theoretical and experimental studies on ENZ photonics, especially in the regime of nonlinear ENZ nanophotonics and its applications. First, we overview the basics of the ENZ concepts, mechanisms, and nonlinear ENZ nanophotonics. Then the new advancements in theoretical and experimental optical physics are reviewed. For nanophotonic applications, the recent decades saw rapid developments in various kinds of different ENZ-based devices and systems, which are discussed and analyzed in detail. Finally, we give our perspectives on where future endeavors can be made.

/pdf/epsilon-near-zero-photonics-infinite-potentials-2q2fyr9652.pdf

Epsilon-near-zero photonics: infinite potentials

Driven by machine-learning tasks neural networks have demonstrated useful capabilities as nonlinear hypothesis classifiers. The underlying technologies performing the dot product multiplication, the summation, and the nonlinear thresholding on the input data in electronics, however, are limited by the same capacitive challenges known from electronic integrated circuits. The optical domain, in contrast, provides low delay interconnectivity suitable for such node distributed non Von Neumann architectures relying on dense node to node communication. Thus, once the neural network's weights are set, the delay of the network is just given by the time of flight of the photon, which is in the picosecond range for photonic integrated circuits. However, the functionality of memory for storing the trained weights does not exists in optics, thus demanding a fresh look to explore synergies between photonics and electronics in neural networks. Here we provide a roadmap to pave the way for emerging hybridized photonic electronic neural networks by taking a detailed look into a single node's perceptron, discussing how it can be realized in hybrid photonic electronic heterogeneous technologies. We show that a set of materials exist that exploit synergies with respect to a number of constrains including electronic contacts, memory functionality, electrooptic modulation, optical nonlinearity, and device packaging. We find that the material ITO, in particular, could provide a viable path for both the perceptron weights and the nonlinear activation function, while simultaneously being a foundry process near material. We finally identify a number of challenges that, if solved, could accelerate the adoption of such heterogeneous integration strategies of emerging memory materials into integrated photonics platforms for real time responsive neural networks.

Roadmap on Material-Function Mapping for Photonic-Electronic Hybrid Neural Networks

Graphene has extraordinary electro-optic properties and is therefore a promising candidate for monolithic photonic devices such as photodetectors. However, the integration of this atom-thin layer m...

Compact Graphene Plasmonic Slot Photodetector on Silicon-on-Insulator with High Responsivity

Electro-optic signal modulation provides a key functionality in modern technology and information networks. Photonic integration has enabled not only miniaturizing photonic components, but also provided performance improvements due to co-design addressing both electrical and optical device rules. However, the millimeter-to-centimeter large footprint of many foundry-ready photonic electro-optic modulators significantly limits on-chip scaling density. To address these limitations, here we experimentally demonstrate a coupling-enhanced electro-absorption modulator by heterogeneously integrating a novel dual-gated indium-tin-oxide (ITO) phase-shifting tunable absorber placed at a silicon directional coupler region. Our experimental modulator shows a 2 dB extinction ratio for a just 4 um short device at 4 volt bias. Since no material nor optical resonances are deployed, this device shows spectrally broadband operation as demonstrated here across the entire C-band. In conclusion we demonstrate a modulator utilizing strong index-change from both real and imaginary part of active material enabling compact and high-performing modulators using semiconductor foundry-near materials.

/pdf/coupling-enhanced-dual-ito-layer-electro-absorption-3ma8ha1cwv.pdf

Coupling-enhanced dual ITO layer electro-absorption modulator in silicon photonics

Abstract Topological insulators have shown great potential for future optoelectronic technology due to their extraordinary optical and electrical properties. Photodetectors, as one of the most widely used optoelectronic devices, are crucial for sensing, imaging, communication, and optical computing systems to convert optical signals to electrical signals. Here we experimentally show a novel combination of topological insulators (TIs) and transition metal chalcogenides (TMDs) based self-powered photodetectors with ultra-low dark current and high sensitivity. The photodetector formed by a MoS2/Sb2Te3 heterogeneous junction exhibits a low dark current of 2.4 pA at zero bias and 1.2 nA at 1V. It shows a high photoresponsivity of >150 mA W−1 at zero bias and rectification of 3 times at an externally applied bias voltage of 1V. The excellent performance of the proposed photodetector with its innovative material combination of TMDs and TIs paves the way for the development of novel high-performance optoelectronic devices. The TIs/TMDs transfer used to form the heterojunction is simple to incorporate into on-chip waveguide systems, enabling future applications on highly integrated photonic circuits.

Self-powered broadband photodetector based on MoS2/Sb2Te3 heterojunctions: a promising approach for highly sensitive detection

Here we present our latest PIC-integrated TMD-based slot-enhanced photodetector. The metallic slot enhances the light-matter-interaction and hence absorption into the semiconductor TMD layer. Unlike Graphene detectors, this device based on a 1+eV wide Eg detector (MoTe2) shows a low dark-current Of 100’s pA (i.e. 3 orders of magnitude lower than graphene). Utilizing the plasmonic slot allows to harness scaling effects known from FETs, and reduce carrier transit times. Thus, we demonstrate 10GHz roll-offs despite a rather low mobility. We further show that the short-channel allows for near-ballistic transport, and more importantly high gain-bandwidth-products (GPB), which scales with the source-drain distance squared. The combination of a TMD semiconductor with a slot for short transit times, enables we new class of efficient yet compact PIC-integrated detectors offering high GBP. 

20Gbps high-gain BW-product TMD slot-detector on PIC

Owing to the transition metal dichalcogenides strong light-matter interaction and distinctive materials properties. But low carrier mobilities limited the performance of the device. Here we demonstrate an efficient plasmonic slot photodetector based on integrating MoTe2 in silicon photonics. Narrowing the source-drain distance to tens of nm enables high gain-bandwidth-product detectors.

Hao Wang

Papers

Compact Graphene Plasmonic Slot Photodetector on Silicon-on-Insulator with High Responsivity

Coupling-enhanced dual ITO layer electro-absorption modulator in silicon photonics

Self-powered broadband photodetector based on MoS2/Sb2Te3 heterojunctions: a promising approach for highly sensitive detection

20Gbps high-gain BW-product TMD slot-detector on PIC

Compact Plasmonic Enhanced MoTe2 Photodetector based on Engineering Gain-Bandwidth-Product Scaling Laws