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 report a wavelength conversion efficiency of -5.5 dB via four-wave-mixing in a low-loss 2.5 cm long sub-micron silicon-on-insulator rib waveguide. The impact of non-linear absorption and waveguide dimensions on the conversion efficiency is studied.

Efficient wavelength conversion via four-wave mixing in sub-micron silicon rib waveguides

A method and apparatus may include receiving data from a first device. The data may be received via a first protocol. The method can also include converting the data to be transmitted via a second protocol. The second protocol may be a high-speed proprietary or standard protocol. The method can also include transmitting the data via the second protocol to a second device.

Method and apparatus for implementing high-speed connections for logical drives

Disclosed herein are configurations and methods to produce very low loss waveguide structures, which can be single-layer or multi-layer. These waveguide structures can be used as a sensing component of a small-footprint integrated optical gyroscope. By using pure fused silica substrates as both top and bottom cladding around a SiN waveguide core, the propagation loss can be well below 0.1 db/meter. Low-loss waveguide-based gyro coils may be patterned in the shape of a spiral (circular or rectangular or any other shape), that may be distributed among one or more of vertical planes to increase the length of the optical path while avoiding the increased loss caused by intersecting waveguides in the state-of-the-art designs. Low-loss adiabatic tapers may be used for a coil formed in a single layer where an output waveguide crosses the turns of the spiraling coil.

Single-layer and multi-layer structures for integrated silicon photonics optical gyroscopes

Advancements in silicon photonics technology have resulted in significant progress toward tactical-grade chip-scale optical gyroscopes for applications such as inertial navigation for a range of self-driving vehicles. Our first generation of gyro, reported a year ago, was a resonant ring gyro fabricated with an ultra-low-loss silicon-nitride waveguide in a racetrack shape with a perimeter of 37 mm and a finesse of 1270. When the laser frequency was tuned to interrogate the resonance with the lowest backscattering coefficient, and balanced detection was implemented to reduce common noise in the two output signals, the angular random walk (ARW) was measured to be 80 deg/h/√Hz, and the gyro output was dominated by backscattering noise. The second-generation reported here utilizes a longer ring to further reduce backscattering noise. The ring resonator is a circular spiral with 33 turns, a length of 1.2 m, and a finesse of 29. When interrogated with a narrow-linewidth laser like the racetrack gyro, it has a measured ARW of 210 deg/h/√Hz dominated by laser-frequency noise. The ARW is higher than that of the racetrack gyro because the balanced detection was not as effective (13.2 dB of common noise rejection compared to 18 dB in the racetrack gyro). Tests in a vacuum indicate that environmental fluctuations do not contribute to the noise, and that most of the measured drift (3,500 deg/h) has an optical and/or electronic origin. We also report the noise performance of the racetrack gyro interrogated in a Sagnac interferometer probed with broadband light. This configuration was inspired by a recent publication from Shanghai Jiao Tong University that reports a resonant fiber optic gyroscope interrogated with broadband light with a measured ARW that meets tactical-grade requirements. The advantages of this interrogation technique are that it eliminates the need to stabilize the resonator, it reduces the component count, and by making use of incoherent light, it reduces the backscattering noise. The measured ARW of the racetrack gyro interrogated with broadband light was dominated by excess noise at large detected powers, and it was a factor of ~900 larger than the ARW of the same racetrack gyro interrogated with the laser. The reason for this increase in ARW is that the advantage of having a high-finesse resonator is lost when the ring is interrogated with broadband light, and the sensitivity is reduced by a factor of the finesse compared to the same ring resonator interrogated with a laser. This reduction in sensitivity is demonstrated experimentally. Achieving tactical-grade requirements will require returning to a laser interrogation, improving the balanced detection scheme to achieve a noise cancellation of 25 dB or better, and optimizing the laser linewidth to minimize both laser frequency noise and backscattering noise.

High-Q silicon nitride resonator gyroscope interrogated with broadband light

We propose a model for understanding dark solitons formation dynamics in self-injection-locking laser-resonator systems using dynamical instabilities and domain walls. Snapshots of solitons are captured with dual-comb imaging techniques and validate the model.

Mario J. Paniccia

Papers

Efficient wavelength conversion via four-wave mixing in sub-micron silicon rib waveguides

Method and apparatus for implementing high-speed connections for logical drives

Single-layer and multi-layer structures for integrated silicon photonics optical gyroscopes

High-Q silicon nitride resonator gyroscope interrogated with broadband light

Formation Dynamics and Snapshots of Self-injection-locking Dark Solitons