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 recent results 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 are monolithically integrated with a demultiplexer and a multiplexer. The modulators, each capable of 25 Gb/s data transmission, are based on the carrier depletion effect of pn diodes, and the demultiplexer/multiplexer is based on a cascaded asymmetric Mach Zehnder interferometer design.

Silicon photonic modulator and integration for high-speed applications

Silicon photonic platforms have been of great interest for the integration of photonic and electronic circuits on a silicon substrate. However, efficient light generation inside a silicon crystal has been the major obstacle due to the indirect bandgap characteristics of silicon. To overcome this problem, recently we demonstrated optically pumped hybrid silicon evanescent lasers, in which III-V based quantum wells are bonded to silicon rib waveguides. This approach enables to build active silicon photonic devices by combining high optical gain of Ill-V materials with well developed silicon processing technology. The work presented here is a first step of the future development of the electrically driven silicon active photonic devices as well as the photonic integration with electrical circuitry on silicon-on-insulator (SOI) wafers.

Hybrid silicon evanescent lasers

We demonstrate a mode locked silicon evanescent laser (ML-SEL) which generates a 39.2 GHz pulse stream without requiring any RF drive signal. This passively mode locked laser outputs 3.7 ps pulses which are nearly transform limited and have 18 dB extinction ratio.

A 40 GHz Mode Locked Silicon Evanescent Laser

With a reverse biased p-i-n structure embedded in a silicon waveguide, we efficiently reduced the nonlinear loss due to two photon absorption induced free carrier absorption and achieved continuous-wave net Raman amplification and lasing in a silicon waveguide on a single chip. The low-loss p-i-n waveguides also enabled efficient wavelength conversion in the 1550 nm band via four-wave mixing in silicon. Here we report the performance characteristics of the silicon based laser, amplifier as well as wavelength converter for different device configurations. With a pump wavelength at 1550 nm, the laser output at 1686 nm is single mode with over 55 dB side mode suppression and has less than 80 MHz linewidth. At 25V reverse bias, the threshold pump power is ~180 mW. The slope efficiency is ~4.3% for a single side output and a total output power of >10 mW can be reached at a pump power of 500 mW. The laser wavelength can be tuned by adjusting the wavelength of the pump laser. A 3 dB on-chip amplification and -8.5 dB wavelength conversion efficiency is achieved in an 8-cm long waveguide at a pump powers of < 640 mW. We demonstrate that a high-speed pseudo-random bit sequence optical data at 10 Gb/s rate can be amplified or converted to a new wavelength channel with clear open eye diagram and no waveform distortion.

Mario J. Paniccia

Papers

Silicon photonic modulator and integration for high-speed applications

Hybrid silicon evanescent lasers

A 40 GHz Mode Locked Silicon Evanescent Laser

Silicon based laser, amplifier, and wavelength converter for optoelectronic integration

Silicon Photonic Applications