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

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

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

Optical technology is poised to revolutionize short-reach interconnects. The leading candidate technology is silicon photonics, and the workhorse of such an interconnect is the optical modulator. Modulators have been improved dramatically in recent years, with a notable increase in bandwidth from the megahertz to the multigigahertz regime in just over half a decade. However, the demands of optical interconnects are significant, and many questions remain unanswered as to whether silicon can meet the required performance metrics. Minimizing metrics such as the device footprint and energy requirement per bit, while also maximizing bandwidth and modulation depth, is non-trivial. All of this must be achieved within an acceptable thermal tolerance and optical spectral width using CMOS-compatible fabrication processes. This Review discusses the techniques that have been (and will continue to be) used to implement silicon optical modulators, as well as providing an outlook for these devices and the candidate solutions of the future.

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Silicon optical modulators

An overview is presented of the current state-of-the-art in silicon nanophotonic ring resonators. Basic theory of ring resonators is discussed, and applied to the peculiarities of submicron silicon photonic wire waveguides: the small dimensions and tight bend radii, sensitivity to perturbations and the boundary conditions of the fabrication processes. Theory is compared to quantitative measurements. Finally, several of the more promising applications of silicon ring resonators are discussed: filters and optical delay lines, label-free biosensors, and active rings for efficient modulators and even light sources.

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Silicon microring resonators

Using reverse biased p-i-n diode structure, we efficiently reduced nonlinear absorption and achieved continuous-wave lasing in silicon waveguide cavities based on stimulated Raman scattering. We report here the lasing characteristics for different laser cavity configurations.

Continuous-wave silicon lasers based on stimulated Raman scattering

A semiconductor-based optical amplifier/attenuator is disclosed. An apparatus according to aspects of the present invention includes an optical waveguide disposed in semiconductor material. A first optical beam having a first wavelength is to be directed through the optical waveguide. The optical waveguide is optically coupled to receive a pump optical beam having a pump wavelength. The pump optical beam also has a power level sufficient to amplify the first optical beam via stimulated Raman scattering (SRS) in the optical waveguide. A diode structure is disposed in the optical waveguide. The diode structure coupled is to be selectively biased to selectively control free carrier lifetimes of the free carriers in the optical waveguide. This enables to selective amplification or attenuation of the first optical beam output from the optical waveguide.

Semiconductor Raman variable optical attenuator/amplifier and equalizer

A mode-locked silicon evanescent laser is used to generate over 100 wavelengths evenly spaced by 10 GHz. Optical injection locking reduces the linewidths of the remaining 30 modes to less than 100 kHz.

Silicon evanescent optical frequency comb generator

We demonstrate a silicon evanescent laser operating CW up to 60/spl deg/C. The laser operates at 1568 nm with a maximum fiber-coupled output power of 4.5 mW. Devices are fabricated with silicon waveguides bonded to AlGaInAs quantum wells.

An optically pumped silicon evanescent laser operating continuous wave at 60/spl deg/C

Single mode, polarisation independent filters with large free spectral range are basic requirements for optical networks. In this paper we discuss (with experimental results) the possibility of establishing such devices upon small strip waveguides in silicon-on-insulator (SOI)

Richard Jones

Papers

Continuous-wave silicon lasers based on stimulated Raman scattering

Semiconductor Raman variable optical attenuator/amplifier and equalizer

Silicon evanescent optical frequency comb generator

An optically pumped silicon evanescent laser operating continuous wave at 60/spl deg/C

Small Optical Filters in Silicon-on-Insulator