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

The adoption of optical technologies by high-volume consumer markets is severely limited by the cost and complexity of
manufacturing complete optical transceiver systems. This is in large part because "boutique" semiconductor fabrication
processes are required for III-V lasers, modulators, and photodetectors; furthermore, precision bonding and painstaking
assembly are needed to integrate or assemble such dissimilar devices and materials together. On the other hand, 200mm
and 300mm silicon process technology has been bringing ever-increasing computing power to the masses by relentless
cost reduction for several decades. Intel's silicon photonics program aims to marry this CMOS infrastructure and recent
developments in MEMS manufacturing with the burgeoning field of microphotonics to make low cost, high-speed
optical links ubiquitous. In this paper, we will provide an overview of several aspects of silicon photonics technology
development in a CMOS fabrication line. First, we will describe fabrication strategies from the MEMS industry for
micromachining silicon to create passive optical devices such as mirrors, waveguides, and facets, as well as alignment
features. Second, we will discuss some of the challenges of fabricating hybrid III-V lasers on silicon, including such
aspects as hybrid integration of InP-based materials with silicon using various bonding methods, etching of InP films,
and contact formation using CMOS-compatible metals.

CMOS-Compatible Fabrication, Micromachining, and Bonding Strategies for Silicon Photonics

According to some implementations a floor is provided that comprises a substrate having a top surface and a groove formed therein. The groove has an open end located at the top surface of the substrate. The groove further includes a bottom wall and side walls with the bottom wall located a first distance below the top surface of the substrate. The groove extends along a length of the substrate. A light diffusing optical fiber or other lighting means is supported inside the groove by a transparent or translucent fiber support that spaces the light diffusing optical fiber or other lighting means a distance away from the bottom wall, side walls and open end of the groove. According to some implementations the walls of the groove or the outer surfaces of the fiber support are provided with light reflectors to enhance the propagation of light toward the open end of the groove.

Apparatus and methods for lighting a floor using a light diffusing fiber

Silicon photonics has made rapid progress in recent years achieving several key breakthroughs. In particular, chip-scale silicon lasers and amplifiers based on stimulated Raman scattering have been successfully demonstrated in continuous wave operation

A Ring Cavity Raman Silicon Laser

The strong optical nonlinearity of silicon and tight optical field confinement in silicon waveguides, accompanied by
silicon's unique material properties such as high optical damage threshold and thermal conductivity, enable compact
nonlinear photonic devices to be fabricated in silicon using cost effective CMOS compatible fabrication technology.
By integrating a p-i-n diode into the silicon waveguide, the nonlinear optical loss due to two photon absorption
induced free carrier absorption in silicon waveguides can be dramatically reduced, and efficient nonlinear optical
devices can be realized on silicon chips for high speed optical communications. In this paper, we report recent
development of silicon p-i-n waveguide based nonlinear photonic chips for wavelength conversion and dispersion
compensation applications. Wavelength conversion efficiency of -8.5 dB can be achieved in an 8-cm long p-i-n
silicon waveguide by four-wave mixing in continuous-wave operation, and chromatic dispersion compensation by
mid-span spectral inversion is demonstrated experimentally using silicon spectral inverter at the mid-span of a fiber
optical link, achieving transmission of optical data at 40 Gb/s over 320 km of standard fiber with negligible power
penalty. The unique advantages of using silicon over previously proposed nonlinear optical media for dispersion
compensation are discussed.

Silicon-based nonlinear optical devices for high-speed optical communications

We present a monolithic integrated low-threshold Raman silicon laser based on silicon-on-insulator (SOI) rib
waveguide ring cavity with an integrated p-i-n diode. The laser cavity consists of a race-track shaped ring resonator
connected to a straight bus waveguide via a directional coupler which couples both pump and signal light into and
out of the cavity. Reverse biasing the diode with 25V reduces the free carrier lifetime to below 1 ns, and stable,
single-mode, continuous-wave (CW) Raman lasing is achieved with threshold of 20mW, slope efficiency of 28%,
and output power of 50mW. With zero bias voltage, a lasing threshold of 26mW and laser output power >10mW can
be obtained. The laser emission has high spectral purity with a side-mode suppression of >80dB and laser linewidth
of <100 kHz. The laser wavelength can be tuned continuously over 25 GHz. To demonstrate the performance
capability of the laser for gas sensing application, we perform absorption spectroscopy on methane at 1687 nm using
the CW output of the silicon Raman laser. The measured rotationally-resolved direct absorption IR spectrum agrees
well with theoretical prediction. This ring laser architecture allows for on-chip integration with other silicon
photonics components to provide an integrated and scaleable monolithic device. By proper design of the ring cavity
and the directional coupler, it is possible to achieve higher order cascaded Raman lasing in silicon for extending
laser wavelengths from near IR to mid IR regions.

Mario J. Paniccia

Papers

CMOS-Compatible Fabrication, Micromachining, and Bonding Strategies for Silicon Photonics

Apparatus and methods for lighting a floor using a light diffusing fiber

A Ring Cavity Raman Silicon Laser

Silicon-based nonlinear optical devices for high-speed optical communications

A monolithic integrated low-threshold Raman silicon laser