Recent years have witnessed many breakthroughs in research on graphene (the first two-dimensional atomic crystal) as well as a significant advance in the mass production of this material. This one-atom-thick fabric of carbon uniquely combines extreme mechanical strength, exceptionally high electronic and thermal conductivities, impermeability to gases, as well as many other supreme properties, all of which make it highly attractive for numerous applications. Here we review recent progress in graphene research and in the development of production methods, and critically analyse the feasibility of various graphene applications.

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A roadmap for graphene

The richness of optical and electronic properties of graphene attracts enormous interest. Graphene has high mobility and optical transparency, in addition to flexibility, robustness and environmental stability. So far, the main focus has been on fundamental physics and electronic devices. However, we believe its true potential lies in photonics and optoelectronics, where the combination of its unique optical and electronic properties can be fully exploited, even in the absence of a bandgap, and the linear dispersion of the Dirac electrons enables ultrawideband tunability. The rise of graphene in photonics and optoelectronics is shown by several recent results, ranging from solar cells and light-emitting devices to touch screens, photodetectors and ultrafast lasers. Here we review the state-of-the-art in this emerging field.

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Graphene photonics and optoelectronics

Graphene, the single-atom-thick form of carbon, holds promise for many applications, notably in electronics where it can complement or be integrated with silicon-based devices. Intense efforts have been devoted to develop a key enabling device, a broadband, fast optical modulator with a small device footprint. Now Liu et al. demonstrate an exciting new possibility for graphene in the area of on-chip optical communication: a graphene-based optical modulator integrated with a silicon chip. This new device relies on the electrical tuning of the Fermi level of the graphene sheet, and achieves modulation of guided light at frequencies over 1 gigahertz, together with a broad operating spectrum. At just 25 square micrometres in area, it is one of the smallest of its type. Integrated optical modulators with high modulation speed, small footprint and large optical bandwidth are poised to be the enabling devices for on-chip optical interconnects1,2. Semiconductor modulators have therefore been heavily researched over the past few years. However, the device footprint of silicon-based modulators is of the order of millimetres, owing to its weak electro-optical properties3. Germanium and compound semiconductors, on the other hand, face the major challenge of integration with existing silicon electronics and photonics platforms4,5,6. Integrating silicon modulators with high-quality-factor optical resonators increases the modulation strength, but these devices suffer from intrinsic narrow bandwidth and require sophisticated optical design; they also have stringent fabrication requirements and limited temperature tolerances7. Finding a complementary metal-oxide-semiconductor (CMOS)-compatible material with adequate modulation speed and strength has therefore become a task of not only scientific interest, but also industrial importance. Here we experimentally demonstrate a broadband, high-speed, waveguide-integrated electroabsorption modulator based on monolayer graphene. By electrically tuning the Fermi level of the graphene sheet, we demonstrate modulation of the guided light at frequencies over 1 GHz, together with a broad operation spectrum that ranges from 1.35 to 1.6 µm under ambient conditions. The high modulation efficiency of graphene results in an active device area of merely 25 µm2, which is among the smallest to date. This graphene-based optical modulation mechanism, with combined advantages of compact footprint, low operation voltage and ultrafast modulation speed across a broad range of wavelengths, can enable novel architectures for on-chip optical communications.

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A graphene-based broadband optical modulator

We present the science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems, targeting an evolution in technology, that might lead to impacts and benefits reaching into most areas of society. This roadmap was developed within the framework of the European Graphene Flagship and outlines the main targets and research areas as best understood at the start of this ambitious project. We provide an overview of the key aspects of graphene and related materials (GRMs), ranging from fundamental research challenges to a variety of applications in a large number of sectors, highlighting the steps necessary to take GRMs from a state of raw potential to a point where they might revolutionize multiple industries. We also define an extensive list of acronyms in an effort to standardize the nomenclature in this emerging field.

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Science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems

Metal interconnections are expected to become the limiting factor for the performance of electronic systems as transistors continue to shrink in size. Replacing them by optical interconnections, at different levels ranging from rack-to-rack down to chip-to-chip and intra-chip interconnections, could provide the low power dissipation, low latencies and high bandwidths that are needed. The implementation of optical interconnections relies on the development of micro-optical devices that are integrated with the microelectronics on chips. Recent demonstrations of silicon low-loss waveguides, light emitters, amplifiers and lasers approach this goal, but a small silicon electro-optic modulator with a size small enough for chip-scale integration has not yet been demonstrated. Here we experimentally demonstrate a high-speed electro-optical modulator in compact silicon structures. The modulator is based on a resonant light-confining structure that enhances the sensitivity of light to small changes in refractive index of the silicon and also enables high-speed operation. The modulator is 12 micrometres in diameter, three orders of magnitude smaller than previously demonstrated. Electro-optic modulators are one of the most critical components in optoelectronic integration, and decreasing their size may enable novel chip architectures.

Micrometre-scale silicon electro-optic modulator

We present an electrically pumped silicon evanescent laser that utilizes a silicon waveguide and offset AlGaInAs quantum wells. The silicon waveguide is fabricated on a Silicon-On-Insulator (SOI) wafer and is bonded with the AlGaInAs quantum well structure using low temperature O2 plasma-assisted wafer bonding. The optical mode in the hybrid waveguide is predominantly confined in the passive silicon waveguide and evanescently couples into the III-V active region providing optical gain via electrical current injection. The device lases continuous wave at 1577 nm with a threshold of 65 mA at 15 o C. The maximum single-sided fiber-coupled cw output power is 1.8 mW. The maximum operating temperature is 40 o C mainly limited by a high series resistance of the device. Operation up to 60 o C should be

High-temperature silicon evanescent lasers

According to one implementation an assembly is provided that facilitates a coupling of one or more light diffusing optical fibers to one or more light emitting diodes. According to one implementation the assembly includes a light emitting diode positioned inside a cavity of a frame that is equipped with means to directly or indirectly electrically couple the anode and cathode of the light emitting diode to a printed circuit board. A proximal end portion of the light diffusing optical fiber is supported inside a through opening of a lid positioned over a front side of the frame. The light diffusing optical fiber includes a core that is surround by a cladding. According to some implementations the proximal end of the light diffusing optical fiber is butt-coupled to the light emitting diode with there being no gap between the proximal end of the fiber and the light emitting side of the light emitting diode.

Light modules and devices incorporating light modules

As microprocessor technology advances toward multi-core and many-core architectures, optical interconnect is considered a promising way of meeting the associated demand for giga-scale and tera-scale input/output (I/O). While traditional optical communication systems demonstrate good performance, they are based on discrete components and are not suitable for computing applications, which require solutions with much lower cost and smaller size. Photonic integration, particularly when based on a silicon platform, has emerged as a key approach to realize the required low cost and small form factor optical transceivers. This chapter highlights a recent demonstration of a silicon photonic integrated chip that is capable of transmitting data at an aggregate rate of 200 Gb/s. It is based on wavelength division multiplexing where an array of eight high-speed silicon optical modulators is monolithically integrated with a demultiplexer and a multiplexer. This demonstration represents a key milestone on the way to fabricating terabit per second transceiver chips to meet the demand of future tera-scale I/O.

High-Speed Photonic Integrated Chip on a Silicon Platform

We present an integrated laser locked to an 8 mL microfabricated optical reference cavity with 1 Hz integrated linewidth, sub-10−14 fractional frequency instability and thermal noise limited performance to ~1 kHz offset. This work assembles the primary components of a fully integrated narrow-linewidth laser source.

Stabilization of an integrated laser with an 8 mL Fabry-Pérot optical reference cavity

We discuss integrated silicon photonic technologies that enable Tbit/s optical link for future VLSI interconnect applications. We also review recent advances in various fundamental building blocks, including 30 Gbit/s data transmission using silicon optical modulators.

Mario J. Paniccia

Papers

High-temperature silicon evanescent lasers

Light modules and devices incorporating light modules

High-Speed Photonic Integrated Chip on a Silicon Platform

Stabilization of an integrated laser with an 8 mL Fabry-Pérot optical reference cavity

Silicon Photonic Integrated Circuits for Optical Interconnect