Cloud computing allows application software and services to be delivered from large server farms in data centers over the Internet. As more and more people use Cloud services, more and more data are created that need to be stored, transmitted and processed. This explosion of data requires larger and larger data centers — as big as 1 square mile.
Traditional semiconductor technologies are approaching their physical limitations. Therefore, innovators must explore new technologies to realize the most ambitious visions of a networked global society. Graphene promises a major step forward in performance for data communications in the area of photonics and optoelectronics. The wonderful optical properties of graphene afford multiple functions of signal emitting, transmitting, modulating, and detection to be realized in one material which are the key components of telecommunications.
Researchers have demonstrated how properties of graphene enable ultra-wide bandwidth communications coupled with low power consumption to radically change the way data is transmitted across the optical communications systems. This could make graphene-integrated devices the key ingredient in the evolution of 5G, the Internet-of-Things (IoT), and Industry 4.0.
“The size of current and future data centers is challenging the fundamental speed limits of today’s fiber optics technology,” said Bill Beifuss, CEO of Carbon Sciences. “The speed of data movement between servers is bottlenecked by these fundamental limitations. By exploiting the natural breakthrough optical and electrical properties of graphene, we plan to develop next generation fiber optics components that are ultrafast, low power and low cost.” Mr. Beifuss concluded, “We believe graphene-based components will unclog the bottlenecks in the Cloud and unleash a global era of high resolution video on demand, high fidelity music streaming, high volume e-commerce and many more Cloud-based services.”
Carbon Sciences Inc have announced its plans to developing graphene-based devices to enable ultrafast fiber optics communication in Cloud computing infrastructure. Researchers are developing Graphene-based fiber optics components, such as photodetectors, fiber lasers , optical switches and processing devices that are expected to unclog the existing bottlenecks and enable ultrafast communication in data centers for Cloud computing. Wolfgang Templ, Alcatel has estimated that commercialization is five to ten years away, and urged researchers to look carefully at early stages of development of the technology, to avoid getting hung up in dead-end directions.
Graphene can also enhance the situational awareness for military, a critical capability in modern warfare which requires threat detection, recognition and identification, during day and night and under all weather and visibility conditions. Night Vision has become essential capability for ground forces in modern warfare as well as for counter terrorist operations. Infrared imaging enables the spotting of targets, intruders and hidden bombs by detecting their heat signatures thereby protecting troops and making the application of force more discriminating. The performance of Night vision devices is constantly being improved while driving down the size, weight and power consumption in order to maintain an edge over adversaries. Graphene based photodetectors are capable of detecting the entire spectrum, from infrared to visible to ultraviolet and in future would lead to ultrasensitive detectors and that too at room temperature.
Graphene’s unique properties for photonics
Silicon has been widely hailed as suitable for monolithic integration for photonics. However, increasing the speed and reducing the power and footprint of key components of silicon photonics technology has not been achieved in a single chip, to date. But graphene—with its capacity for signal emission, modulation and detection—can be the next disruptive technology to achieve this.
It has unique combination of properties, its high carrier mobility and short excited state lifetimes enable ultrafast extraction of photo-generated carriers, which leads to high-bandwidth operation. It has a wide spectral absorption range from the ultraviolet to the infrared and the ability to modulate the absorption.
Graphene has several properties excelling for data communications including its excellent electrical conductivity, capability for strong light-matter interactions, and high optical nonlinearity. These allow all-optical signal processing, avoiding the need for conversion of the optical signal to the electrical, and consequently the need to design high-speed interfaces between optical and electronic parts.
In addition its mechanical strength, lightness, flexibility and compatibility with silicon technology makes it an ideal candidate for photonic and optoelectronic applications. The optical and electrical properties are tunable, making graphene an attractive candidate to complement silicon photonics in the near future, potentially transforming the future of data communication devices.
Graphene’s spectacular performance in high-speed optical communications
Light modulation and detection are key operations in photonic integrated circuits. Lacking a bandgap, graphene makes broadband light detection with a single material possible as it absorbs uniformly across a broad range in the visible and infrared spectrum. The 2-D material also displays electro-absorption and electro-refraction effects that can be used for ultrafast modulation.
“Graphene offers an all-in-one solution for optoelectronic technologies,” notes Daniel Neumaier from AMO GmbH, Leader of the Graphene Flagships Division on Electronics and Photonics Integration. Its tuneable optical properties, high electrical mobility, spectrally broadband operation and compatibility with silicon photonics allow monolithic integration of phase and absorption modulators, switches and photodetectors. Integration on a single chip can increase device performance and substantially reduce its footprint and fabrication cost.
The Graphene Flagship programme aims to act as a catalyst for the development of groundbreaking applications by bringing together academia and industry to take this versatile material into society within 10 years. The importance of integrating graphene in silicon photonics was evident in the joint results produced by the collaboration between Flagship partners AMO GmbH (Germany), the National Inter-University Consortium for Telecommunications (CNIT) (Italy), Ericsson (Sweden), Ghent University (Belgium), the Institute of Photonic Sciences (ICFO) (Spain), imec (Belgium), Nokia (Germany and Italy), the Vienna University of Technology (TU Wien) (Austria) and the University of Cambridge (UK).
Instead of relying on the expensive silicon-on-insulator wafer technology widely used in silicon photonics, Graphene Flagship researchers proposed a more convenient configuration. This consisted of a pair of single-layer graphene (SLG) layers, a capacitor consisting of an SLG-insulator-SLG stack on top of a passive waveguide. “Such an arrangement boasts several advantages compared to silicon photonic modulators,” explains Neumaier. As he further outlines, modulator fabrication does not rely on the waveguide material or the electro-absorption and electro-refraction modulation mechanisms. In addition, replacing germanium photodetectors with SLG removes the need for the fairly costly modules of germanium epitaxy and the accompanying specialised doping processes.
Silicon nitride (SiN) provided a good substrate for synthesising graphene, enabling high carrier mobility, transparency over the visible and infrared regions and perfect compatibility with silicon and complementary metal-oxide semiconductor (CMOS) technologies. As a passive waveguide platform, SiN facilitates laser integration and fibre coupling to the waveguide, thereby enabling the design of miniaturised devices.
Graphene datacom demo
Tapping into the potential of graphene, researchers successfully demonstrated data communication with graphene photonic components up to a data rate of 50 Gb/s. A graphene-based modulator processed the data on the transmitter side of the network, encoding an electronic data stream to an optical signal. On the receiver side, a graphene-based photodetector converted the optical modulation into an electronic signal. “These results are a promising start for using graphene-based photonic devices in next-generation data communications,” Neumaier concludes.
In the 2018’s Mobile World Congress, a gathering of some of the world’s biggest mobile and networking technology companies, a consortium of European telecom giants known collectively as The Graphene Flagship introduced an all-graphene communication link capable of operating at a data rate of 25 Gb/s per channel and a new set of graphene-based photonic switches that operate at never-before-seen speeds. These two devices used in concert may represent the next step in data transmission and mobile networking.
The key aspect of this demonstration is that all active electro-optical operations are performed by graphene devices. On the transmitter side a graphene modulator encodes an electronic data stream onto an optical carrier, then transferred to the receiver through an optical fibre. The receiver contains a graphene photodetector that converts the incoming optical data signal back into an electronic signal.
Daniel Schall from AMO commented, “The current challenge for graphene photonics is to develop a fabrication technology that enables not only single devices with superior performance but mass production of millions of devices. With the all-graphene datalink we are taking a significant step towards this goal because we are demonstrating that devices made using volume production techniques are functional in a real configuration.”
His colleague Daniel Neumaier, leader of the Graphene Flagships Division on Electronics & Photonics Integration, added, “This demonstration of a state-of-the-art photonic data link is a significant step forward, as it demonstrates the system compatibility of an all graphene based solution.”
If Graphene Flagship can bring costs down significantly and extend the reach of higher communication data rates throughout the network, it could provide the bandwidth-immersive virtual reality, the pervasive Internet of Things, and artificial intelligence that all will demand. The addition of multiple channels to boost bandwidth even further could help the data center keep up with the overall demand for increased bandwidth.
Light Sources: World’s thinnest light bulb
Creating light in small structures on the surface of a chip is crucial for developing fully integrated “photonic” circuits that do with light what is now done with electric currents in semiconductor integrated circuits. Researchers have developed many approaches to do this, but have not yet been able to put the oldest and simplest artificial light source—the incandescent light bulb—onto a chip. This is primarily because light bulb filaments must be extremely hot—thousands of degrees Celsius—in order to glow in the visible range and micro-scale metal wires cannot withstand such temperatures. In addition, heat transfer from the hot filament to its surroundings is extremely efficient at the microscale, making such structures impractical and leading to damage of the surrounding chip.
Led by Young Duck Kim, a postdoctoral research scientist in James Hone’s group at Columbia Engineering, a team of scientists from Columbia, Seoul National University (SNU), and Korea Research Institute of Standards and Science (KRISS) reported that they have demonstrated an on-chip visible light source using graphene. They attached small strips of graphene to metal electrodes, suspended the strips above the substrate, and passed a current through the filaments to heat them, when the temperature above 2500 degrees Celsius is reached the visible light from atomically thin graphene is so intense that it is visible even to the naked eye.
‘This new type of ‘broadband’ light emitter can be integrated into chips and will pave the way towards the realization of atomically thin, flexible, and transparent displays, and graphene-based on-chip optical communications.’ The team also demonstrated the scalability of their technique by realizing large-scale of arrays of chemical-vapor-deposited (CVD) graphene light emitters. The group is currently working to further characterize the performance of these devices — for example, how fast they can be turned on and off to create ‘bits’ for optical communications — and to develop techniques for integrating them into flexible substrates.
Graphene-Based Sensor Can Be Tuned to Detect Substances
A new sensor design makes use of the optoelectronic properties of graphene to enable simultaneous detection of multiple substances, including bacteria and other pathogens. The design is based on 2D graphene sheets, each just one atom thick, and was created by researchers at China Jiliang University.
Using theoretical calculations and simulations, the researchers designed an array of nanoscale graphene disks. They introduced a small circular defect — an off-center hole — into each nanodisk. When light hit the disk array, the interaction between the disks and their holes created a plasmon hybridization effect, increasing the sensitivity of the device. The hole and the disk create different wavelength peaks, which can be used simultaneously to detect the presence of different substances.
The graphene disks were installed between an ion-gel layer and a silicon layer. These layers were used to apply voltage to the sensor, tuning the graphene’s properties for detection of various substances. Simulations performed by the researchers using MIR wavelengths showed that their new sensor platform was more sensitive to substances present in gases, liquids, and solids than a sensor platform comprised of graphene disks alone, with no circular defects.
The researchers are now working to improve the process that will be used to make the array of nanoscale disks. According to the team, the degree of precision with which these structures are fabricated could greatly impact the performance of the sensor. “We also want to explore whether the graphene plasmon hybridization effect could be used to aid the design of dual-band mid-infrared optical communication devices,” said researcher Bing-Gang Xiao.
Because of its desirable properties, graphene has been used by various research teams to create sensors and materials for a range of applications. However, the China Jiliang team said few previous research efforts have demonstrated sensitive graphene sensors that work with the IR wavelengths necessary to detect bacteria and biomolecules. In addition to improving food safety, the new sensor design could be used to detect gases and chemicals for a wide range of applications. Its dual-band resonance structure feature could support the development of multisubstance detection.
High-speed and on-silicon-chip graphene blackbody emitters
Graphene-based blackbody emitters are also promising light emitters on silicon chip in NIR and mid-infrared region. However, although graphene-based blackbody emitters have been demonstrated under steady-state conditions or relatively slow modulation (100 kHz), the transient properties of these emitters under high-speed modulation have not been reported to date. Also, the optical communications with graphene-based emitters have never been demonstrated.
Japanese Researchers have demonstrated a highly integrated, high-speed and on-chip blackbody emitter based on graphene in NIR region including telecommunication wavelength. A fast response time of ~ 100 ps, which is ~ 105 higher than the previous graphene emitters, has been experimentally demonstrated for single and few-layer graphene, the emission responses can be controlled by the graphene contact with the substrate depending on the number of graphene layers.
In addition, first real-time optical communication with graphene-based light emitters was experimentally demonstrated, indicating that graphene emitters are novel light sources for optical communication. Furthermore, they fabricated integrated two-dimensional array emitters with large-scale graphene grown by chemical vapour deposition (CVD) method and capped emitters operable in air, and carried out the direct coupling of optical fibers to the emitters owing to their small footprint and planar device structure.
Graphene light emitters are greatly advantageous over conventional compound semiconductor emitters because they can be highly integrated on silicon chip due to simple fabrication processes of graphene emitters and direct coupling with silicon waveguide through an evanescent field. Because graphene can realize high-speed, small footprint and on-Si-chip light emitters, which are still challenges for compound semiconductors, the graphene-based light emitters can open new routes to highly integrated optoelectronics and silicon photonics.
South Korea’s Electronics and Telecommunications Research Institute announced that they’ve developed transparent electrodes for organic light emitting diode displays out of graphene in cooperation with Hanwha Techwin. The technology could support second generation flexible and foldable smartphones and tablets.
ETRI and Hanwha said their joint research team succeeded in using graphene to replace indium tin oxide, known as ITO, which has been widely used to make transparent electrodes in OLED panels. ITO is highly fragile, but graphene is flexible and resistant to chipping. “It is meaningful that we are the first to apply graphene to OLED panels,” Cho said. “The technology would help elevate Korea’s OLED panel technology by widening the technology gap with Chinese rivals.”
Korea Advanced Institute of Science and Technology and Pohang University of Science and Technology have developed a new architecture to develop highly flexible OLEDs with excellent efficiency by using graphene as a transparent electrode. They fabricated a transparent anode in a composite structure in which graphene electrodes were sandwiched between a TiO layer with a high refractive index (high-n), and a hole-injection layer (HIL) of conducting polymers with a low refractive index (low-n).
Graphene-based OLEDs that were developed using this approach exhibited ultrahigh external quantum efficiency (EQE) of 40.8 and 62.1 percent (64.7 and 103 percent with a half-ball lens) for single- and multi-junction devices, respectively. Further, the OLEDs remained intact and operational even after 1,000 bending cycles at a radius of curvature as small as 2.3 mm, partly due to the TiO layers withstanding flexural strain up to 4 percent.
Graphene-based OLEDs have emerged as a key element in next-generation displays and lighting, mainly due to their promise as highly flexible light sources. Graphene’s atomic thinness leads to a high degree of flexibility and transparency, making it an ideal candidate for TEs. Nonetheless, the efficiency of graphene-based OLEDs reported to date has been, at best, about the same level of indium tin oxidebased OLEDs. POSTECH professor Tae-Woo Lee said, “We expect that our technology will pave the way to develop an OLED light source for highly flexible and wearable displays, or flexible sensors that can be attached to the human body for health monitoring, for instance.”
Ultrafast mode-locked lasers
Mid-infrared ultrafast fiber lasers are valuable for various applications, including free-space optical communication due to atmospheric transparency window , material processing and military applications like long-range radar, chemical and biomedical sensing .
Ultrafast light pulses in the femtosecond range are needed for advanced photonics applications. Fiber lasers are attractive platforms for short pulse generation due to their simple and compact designs, efficient heat dissipation, and alignment-free operation. These characteristics, combined with advances in glass technology and nonlinear optics, resulted in systems working from the visible to the mid-infrared (MIR).
In order to generate ultrashort pulse at wavelength above 1.55 μm and even up to 3.9 μm32,33, erbium (Er), thulium (Tm), holmium (Ho), or Tm/Ho-co-doped active fibers are utilized to offer appropriate gain region for fiber laser applications. Very recently, graphene ultrafast fiber lasers have been demonstrated at the mid-infrared spectral range.
In fiber oscillators, ultrashort pulses can be obtained by passive mode-locking. This typically requires the aid of a non-linear component called a saturable absorber (SA). Graphene and (CNTs) have emerged as promising SAs for ultrafast lasers. In Graphene, the broadband operation is an intrinsic property of graphene. This, along with the ultrafast recovery time, low saturation fluence, and ease of fabrication and integration, makes graphene an excellent broadband SA. Graphene-mode locked laser make them attractive for applications such as optical frequency comb generation and high resolution laser spectroscopy.
Graphene optical lens a billionth of a meter thick breaks the diffraction limit
Researchers from Swinburne University of Technology has developed a graphene microlens one billionth of a meter thick weighing just a microgram and that can take sharper images of objects of the size of a single bacterium. At the same time, it has a precise and adjustable three-dimensional focus that allows a detailed view of objects at wavelengths ranging from visible to near infrared.
Ultrafast optical switches
In a paper published in Physical Review Letters, researchers from the Centre for Graphene Science at the Universities of Bath and Exeter have observed the response rate of an optical switch using a few layers of graphene to be around one hundred femtoseconds — nearly 100 times quicker than current materials. That should allow for building optoelectronic and photonic devices with modulation speeds up to 200 GHz, the researchers suggest and therefore the use of graphene in telecoms could increase internet speeds 100 times.
Researcher Enrico Da Como explained: ‘We’ve seen an ultrafast optical response rate, using few-layer graphene, which has exciting applications for the development of high speed optoelectronic components based on graphene. This fast response is in the infrared part of the electromagnetic spectrum, where many applications in telecommunications, security and also medicine are currently developing and affecting our society.’
At the GSMA Mobile World Congress, 2018, Ericsson stand discover the first ultrafast graphene-based photonic switches in an Ericsson testbed. The device works in an Ericsson testbed equipped with commercial 100 Gb/s transceivers cards and Dense Wavelength Division Multiplexing (DWDM) network interconnection, operative in the C band and performing over 50 GHz fixed grid.
These graphene-based photonic devices are the potential building blocks of the next generation of mobile networks, leading to ultrafast data streams with extreme bandwidth which are essential for a data-driven future
Graphene Plasmonic Antennas
Spanish researchers at CIC nanoGUNE outside of San Sebastian, the Institute of Photonic Sciences (ICFO) near Barcelona, and the company Graphenea located at the CIC nanoGUNE research center have demonstrated that an optical antenna made from graphene can capture infrared light and transform it into graphene plasmons. These graphene plasmons as they’ve come to be know have an advantage over the surface plasmons that metal surfaces produce because they can be tuned and controlled by a voltage gate.
The Spanish researchers demonstrated that a metal rod placed on graphene can serve as an antenna for infrared light and transform it into graphene plasmons in much the same way a radio antenna converts radio waves into electromagnetic waves in a metal cable.
“We introduce a versatile platform technology based on resonant optical antennas for launching and controlling of propagating graphene plasmons, which represents an essential step for the development of graphene plasmonic circuits”, said team leader Rainer Hillenbrand in a press release.
High speed, small footprint, and high bandwidth modulators are highly desirable for optical communications. The ability to modulate the Fermi level of graphene by a gate field naturally leads to its application as a fast electro-absorption modulator. A research team from IMEC and Ghent University has first time demonstrated high-quality optical modulation in a hybrid graphene-silicon modulator, at bit rates up to 10Gb/s.
“Combining low insertion loss, low drive voltage, high thermal stability, broadband operation and compact footprint, the device marks an important milestone in the realization of next-generation, high-density low-power integrated optical interconnects,” announced IMEC.
They are now aiming to further refine the devices to improve their broadband operation to 25 Gb/s, which would make them more directly competitive with existing SiGe modulators.
Optical nonlinearities play a central role in the operation of a wide variety of integrated photonic devices, such as on-chip broadband light sources, electro-optic modulators, optical switches and optical transistors.
With its strong affinity for interacting with photons and its unique electronic properties, graphene is an exciting new candidate for providing nonlinear optical functionalities in on-chip optical devices. In addition, the integration onto silicon chips of this revolutionary material, consisting of a one-atom thick layer of carbon, is rapidly maturing. After pioneering nonlinear optical experiments, various optical experiments on graphene have demonstrated optical nonlinearities orders of magnitude higher than those of other nonlinear materials
Researchers at Swinburne University of Technology in Melbourne, Australia have discovered that graphene oxide (GO) possesses a record-breaking optical nonlinearity. Optical nonlinearity is the ability of a medium to have its optical properties (transmission, refraction, etc.) manipulated by changing the intensity of the light traveling through it. Optical nonlinearity makes it possible to use light to control light so we can operate fiber optic networks.
GO’s high optical nonlinearity opens up its use in high-performance integrated photonic devices for all-optical communications, biomedicine, and photonic computing, according to the Australian researchers. In the long term this research could also lead to the development of quantum cascade lasers based on graphene. Quantum cascade lasers are semiconductor lasers used in pollution monitoring, security and spectroscopy. Few-layer graphene could emerge as a unique platform for this interesting application.
Graphene is an ideal material for optical communications systems. A new, waveguide-integrated photodetector from AMO, Germany sets a record high bandwidth for ultrafast, high data rate graphene devices. Researchers from the Graphene Flagship working at TU Vienna, Austria and AMO, Germany, have demonstrated ultrafast photodetectors that have the highest reported bandwidth for graphene-based devices, enabling data rates of up to 100 Gbit/s.
Simone Schuler, a researcher at TU Vienna, explained the importance of increasing data capabilities. “These kinds of photodetectors are typically used in optical data links, which form the back-bone of the internet. The maximum operation speed of a photodetector defines the maximum data rate the detector can receive. So, the faster the photodetector the more data it can receive.”
In the new photodetectors, light is guided into a slot waveguide that is covered with graphene. Under specific electrical conditions in the graphene, in which the graphene acts as semiconductor junction, the light in the waveguide generates a current in the graphene via the photothermoelectric effect, converting light into an electrical signal. The sensitivity of the detector can be tuned electrically without compromising the speed, enabling the high bandwidth and ultrafast data rate.
Speaking about this new photodetector design, another of the paper’s authors, Daniel Neumaier of AMO, Germany said “This is an important step towards high performance on-chip photo-detectors, demonstrating that competitive speed and sensitivity can be achieved in graphene photodetectors in a highly controlled way.” On-chip integration of different graphene-enable technologies is an important focus of the Graphene Flagship.
Earlier a new ultrafast graphene-based photodetector device was developed at the Institute for Photonic Sciences (ICFO) in Barcelona, that is capable of converting incident light into electrical signals on femtosecond timescales, enabling ultrafast operation speeds for electronic circuits in optical communications and various other applications.
Led by ICFO professor Frank Koppens, the researchers are using graphene to directly measure the duration of a laser pulse less than 50fs in length. In doing so, they show that energy from incident photons can be transferred efficiently to charge carrier heat, with a constant spectral response between visible and infrared wavelengths of 500 and 1,500nm. This is consistent with efficient electron heating.
Koppens commented, “Graphene photodetectors show fascinating performance and properties, enabling a wide variety of applications. These range from multi-spectral imaging to ultra-fast communications, and such applications are now being actively developed within the Graphene Flagship programme. These photodetectors generate photovoltage via the photo-thermoelectric effect that occurs when incident light is focused at the interface between graphene layers doped differently. Light causes an excitation leading to the generation of electron-hole pairs and a photovoltage.
In fact, last year, with great fan fare, the fastest photon switch was measured at 500 gigahertz. Now with this latest research having shortened the cooling times from picoseconds to femtoseconds, the potential for terahertz switching speeds appears to be within the grasp of graphene-based photodetectors.
Researchers develop the first broadband image sensor array based on graphene-CMOS integration
IFCO researchers have for the first time carried out the monolithic integration of a CMOS integrated circuit with graphene, resulting in a high-resolution image sensor consisting of hundreds of thousands of photodetectors based on graphene and quantum dots (QD). They incorporated it into a digital camera that is highly sensitive to UV, visible and infrared light simultaneously. This has never before been achieved with existing imaging sensors. In general, this demonstration of monolithic integration of graphene with CMOS enables a wide range of optoelectronic applications, such as low-power optical data communications and compact and ultra sensitive sensing systems.
The graphene-QD image sensor was fabricated by taking PbS colloidal quantum dots, depositing them onto the CVD graphene and subsequently depositing this hybrid system onto a CMOS wafer with image sensor dies and a read-out circuit. As Stijn Goossens comments, “No complex material processing or growth processes were required to achieve this graphene-quantum dot CMOS image sensor. It proved easy and cheap to fabricate at room temperature and under ambient conditions, which signifies a considerable decrease in production costs. Furthermore, because of its properties, it can be easily integrated on flexible substrates as well as CMOS-type integrated circuits.
Graphene for Terahertz
Terahertz has many applications that require very sensitive technologies such as metal detection, quality assurance, medical spectroscopy, integrity checks, breath-gas analysis, temperature sensing, and biosensing Much progress has been made with terahertz and infrared sensors, each of which employ physical effects — thermoelectrics in semiconductors and plasmonics in noble metals such as gold and silver. Performance is limited by unfavorable physical properties of the sensor materials, and cost and scalability remain challenging. Graphene, however, excels in both of these physical effects. Current technologies would benefit from combination with graphene, making the outlook of creating a highly scalable and cost-effective device very promising
DARPA Awards $1.3M for Graphene IR Detectors
DARPA has awarded a $1.3 million grant to a team led by University of Central Florida researcher Debashis Chanda to fund the development of a next-generation IR detector that could be used in night vision, meteorology and space exploration.
The team is working on an entirely new type of detector that relies on thin graphene, a one-atomic-layer thick, 2D material. Chanda envisions an infrared detector that is small, portable, doesn’t need to be cooled and produces high-resolution images. Unlike current technologies, which can detect only one band of light, the next-gen detector would be tunable and able to see a range of bands.
Portable infrared cameras that can see invisible wavelengths of light have long been used by law enforcement, soldiers, firefighters and others to see in the dark or locate people by the body heat they emit. But the blurry images those devices produce are sometimes nothing more than indistinct colored blobs.
More powerful infrared detectors that produce more detailed images – ones typically used by NASA and defense agencies – are large, expensive and can only function at ultra-low temperatures. “The biggest problem is that most infrared detectors need cryogenic cooling, and in most cases you can’t carry a big cooling tank with you,” Chanda said. “That is a big barrier.”
“We came up with the idea that one can make graphene to strongly absorb light in the infrared domain and we showed that we can also tune the response electronically,” Chanda said. “If you can take an infrared image in different spectral bands, you can extract much more information.”
The team intends to collaborate with defense majors such as Northrop Grumman, Lockheed Martin and St. Johns Optical Systems for integration and packaging.
“New methods to grow graphene on a large scale have emerged; of which chemical vapor deposition on copper is by far the most popular. A wet transfer technique is used to deposit graphene on electronic or photonic chips. However, this results in graphene sheets of relatively low quality,” says Graphene Photonic.
“In principle, the use of a single-crystal copper substrate and a dry transfer technique can tackle these issues, but has not been widely applied because of the difficulty of harvesting graphene from the copper substrate. Defects can be avoided by growing graphene on other substrate materials such as boron nitride, which yield perfectly flat substrate surfaces.” These new substrate materials are expected to pave the way for mass fabrication of graphene sheets with the same excellent quality as the exfoliated graphene sheet
Seeding Large Single-Crystal Graphene for Photonics Applications
Researchers have discovered a novel approach to developing graphene that has potential in photonics, optoelectronics and electronics applications, specifically data communications. Typically, polycrystalline graphene is used in applications which need to be integrated over wafer scale, however, this material presents grain boundaries between the different crystals that lower the exceptional electrical properties that have been measured in the exfoliated single crystals that made graphene famous.
The method—developed by Graphene Flagship researchers Camilla Coletti from Istituto Italiano di Tecnologia (IIT) and Marco Romagnoli from Consorzio Nazionale Interuniversitario per le Telecomunicazioni (CNIT) both working in Italy—is designed to grow single graphene crystals by chemical vapor deposition on copper “seeds” deposited using optical lithography. It results in graphene that is more mobile than graphene grown in a continuous film and also exhibits electric properties that are comparable to those achieved from pristine graphene exfoliated from flakes.
How it works
This flexible “seeding approach” produces high quality graphene to be grown in arrays with different spacing and dimensions. Metallic “seeds,” from which the graphene crystals grow, are deposited with optical lithography. This growth approach allows graphene to grow only where it is needed, reducing the amount of the material necessary. For example, on optoelectronic/photonic devices it is known where a device will be placed, so the crystals are grown just in the areas where they are needed. Working with “patches” of graphene rather than a continuous film also reduces the difficulty associated with transferring a 12-inch, one-atom-thin wafer, including adhesion issues, strain and wrinkles.
The end result is a process that is simpler and yields higher-quality graphene crystals that display the same conductive properties found in graphene that has been mechanically exfoliated from a flake. Furthermore, the process is fast, scalable, consistent and safe (utilizing less than 1.25 percent explosive gases), making it ideal for larger scale industrial production in a robust and low-cost manufacturing line.
Large, single crystal graphene vs polycrystalline graphene
Single-crystal has various advantages over traditional polycrystalline graphene. While the grain size for polycrystalline graphene is 5-20 µm, single crystals can have a grain of up to 4cm, and while 95 percent of polycrystalline graphene is a monolayer, single crystal graphene allows a great deal of control over the thickness. Carrier mobility for single crystal graphene has been measured at RT 300 000 cm2/ Vs, higher than for polycrystalline graphene which is limited to 1000-3000 cm2/ Vs by the presence of grain boundaries. The positioning of the single crystals, however, is not controlled.
The research team, a collaboration between IIT and CNIT, has demonstrated that it is possible to create graphene-based detectors and modulators with performance that is promising and indicate that graphene integrated photonics have the potential to solve present limitations.
The work has yielded detectors and modulators that demonstrate top performance for transmission and reception entirely through graphene devices, reaching world-record data rates for graphene devices (50 Gb/s for an electro-absorption modulator on SOI). Furthermore, the technology integrates at the back end of existent platforms. This, combined with the outstanding results for data transfer, have generated interest from data communications companies.
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