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

High-Q optical resonators offer access to nonlinear physics at low pumping powers attainable using non-amplified semiconductor lasers. Recent resonator advances offer Q factors over 200 million in platforms that are fully CMOS compatible. I will review these new systems and how they are making possible a new generation of frequency microcombs. 

Advanced high-Q resonators for next-generation frequency microcombs

In this paper, a chip-scale Raman silicon laser and amplifier based on a ring resonator architecture are presented. Lasing threshold and efficiency are significantly improved from the previous experiments. Much lower pump power and smaller foot print are needed for the ring resonator amplifier compared to Raman amplifier in linear configuration due to the resonance enhancement effect. The ring resonator based laser and amplifier architecture allows for on-chip integration with other silicon photonics components to provide a monolithic integrated photonic device.

Monolithic integrated Raman silicon lasers and amplifiers

A method and an apparatus for cooling a semiconductor die. In one embodiment, a C4 packaged semiconductor die (301) is thermally coupled to a cooling plate (313) having an opening (315). The opening of the cooling plate is disposed over a back side surface of the semiconductor die such that direct unobstructed access to the exposed back side surface of the semiconductor die is provided. A conformable thermal conductor, such as indium, is disposed between the semiconductor die and the cooling plate to improve the thermal coupling between the semiconductor and cooling plate. In one embodiment, the semiconductor die (501) is mounted on a circuit board (503) and a cooling block (519) is disposed on the opposite side of the circuit board. The cooling plate is thermally coupled to the cooling block with heat transfer conduits, such as thermal screws, that extend through the circuit board to transfer the heat from the semiconductor die through the cooling plate through the heat transfer conduits to the cooling block located on the opposite side of the circuit board. In one embodiment, coolant is circulated through the cooling block to remove heat from the cooling block.

Method and apparatus for cooling a semiconductor die

In this paper, the effects of asymmetric narrowband optical filtering are investigated in a 10-Gbit/s optical communication link using integrated electro-absorption modulated lasers (EML). We investigate the effect of EML chirp on link performance as well as the optimal filter bandwidth and wavelength detuning. We show that both the phase response and the spectral narrowing of the filter will enable a longer distance transmission by interacting with the EML transient chirp and compensating for the fiber chromatic dispersion. Experimentally, an 8.75-GHz filter is shown to improve the link distance by 40 km from 65 to 105 km, when transmitting over standard single mode fiber.

A 10-Gbit/s EML link using detuned narrowband optical filtering

This work demonstrates continuous (mode-hop free) tuning of a single-mode silicon Raman laser over more than 20 GHz range which is broad enough to cover typical molecular transitions in gasses at room temperature, and allows to perform infrared absorption spectroscopy measurements of methane. Wider mode-hop free tuning could be obtained by controlling the temperature of the laser chip in sync with the tuning of the pump laser wavelength. The coarse tuning range of the laser is only limited by the pump laser's wavelength.

Mario J. Paniccia

Papers

Advanced high-Q resonators for next-generation frequency microcombs

Monolithic integrated Raman silicon lasers and amplifiers

Method and apparatus for cooling a semiconductor die

A 10-Gbit/s EML link using detuned narrowband optical filtering

Continuous tuning of silicon Raman laser for molecular spectroscopy