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

A semiconductor-based tunable add/drop method and apparatus. In one aspect of the present invention, a method according to an embodiment of the present invention includes splitting a first optical beam having multiple wavelengths into second and third optical beams with a first 3 dB optical coupler disposed in a semiconductor substrate. Portions of the second and third optical beams having a tunable wavelength are reflected back to the first 3 dB optical coupler with first and second pluralities of silicon and polysilicon interfaces, respectively, disposed along the semiconductor substrate. In another embodiment, the portions of the second and third optical beams having a tunable wavelength are reflected back to the first 3 dB optical coupler with first and second pluralities of modulated charged regions. Portions of the second and third optical beams not reflected are directed to a second 3 dB optical coupler disposed in the semiconductor substrate. The portions of the second and third optical beams not reflected are combined into a fourth optical beam with the second 3 dB optical coupler.

Method and apparatus for adding/droping optical signals in a semiconductor substrate

An optical modulator that modulates light through the semiconductor substrate through the back side of a flip chip packaged integrated circuit. The optical modulator of the present invention enables integrated circuit signals to be extracted through the back side of the semiconductor substrate. In one embodiment, an optical modulator is disposed within a flip chip packaged integrated circuit die. The optical modulator includes a deflector and a diffraction grating. An infrared light beam is directed through the back side of a silicon substrate of the integrated circuit die, deflected off the deflector through the diffraction grating and back out the back side of the integrated circuit die. The diffraction grating modulates the phase of a portion of the deflected light beam in response to an integrated circuit signal. A resulting diffraction interference occurs between the phase modulated portions and nonphase modulated portions of the deflected light beam. The interference causes amplitude modulation of a zero order diffraction of the deflected light beam, from which the integrated circuit signal can be extracted. The zero order diffraction is allowed to pass through an optical passage to exit the semiconductor substrate of the flip chip packaged integrated circuit die while the non-zero order diffractions are blocked.

Higher order rejection method and apparatus for optical modulator

An apparatus and method for modulating a phase of optical beam in an electrically isolated active region of an optical waveguide. In one embodiment, an apparatus according to embodiments of the present invention includes an active region of an optical waveguide disposed in a semiconductor layer. The active region includes a p doped region and an n doped region. The apparatus further includes an insulating region disposed in the semiconductor layer surrounding the active region in the semiconductor layer. The insulating region electrically isolates the active region of the optical waveguide from a passive region of the optical waveguide disposed in the semiconductor layer. An optical beam is to be directed through the optical waveguide and through the active region to be phase shifted in response to a modulated charge region in the active region in the optical waveguide.

Method and apparatus for isolating an active region in an optical waveguide

We report a silicon photonic integrated circuit that contains a fast silicon optical modulator array and wavelength multiplexer/de-multiplexer. We demonstrate high-speed data transmission with an aggregate data rate of 200 Gbps on a single silicon chip.

200 Gbps photonic integrated chip on silicon platform

High-speed silicon optical modulator is one of key components for integrated silicon photonic chip aiming at Tb/s data
transmission for next generation communication networks as well as future high performance computing applications. In
this paper we review the recent development of the silicon modulator. In particular, we present a high-speed and highly
scalable silicon optical modulator based on the free carrier plasma dispersion effect. The fast refractive index modulation
of the device is due to electric-field-induced carrier depletion in a Silicon-on-Insulator waveguide containing a reverse
biased pn junction. To achieve high-speed performance, a traveling-wave design is employed to allow co-propagation of
electrical and optical signals along the waveguide. We demonstrate high-frequency modulator optical response with 3
dB bandwidth of ~20 GHz and data transmission up to 30 Gb/s. We also highlight the future device optimization for 40
Gb/s and beyond.

Mario J. Paniccia

Papers

Method and apparatus for adding/droping optical signals in a semiconductor substrate

Higher order rejection method and apparatus for optical modulator

Method and apparatus for isolating an active region in an optical waveguide

200 Gbps photonic integrated chip on silicon platform

Recent advances in high speed silicon optical modulator