Graphene Gives a Tremendous Boost to Future Terahertz Cameras

http://iconnect007.com/printfn/print.php?cdrID=116692

A study in Nano Letters reports on the development of a graphene-enabled detector for terahertz light that is faster and more sensitive than existing room-temperature technologies.

Detecting terahertz (THz) light is extremely useful for two main reasons. Firstly, THz technology is becoming a key element in applications regarding security (such as airport scanners), wireless data communication, and quality control, to mention just a few. However, current THz detectors have shown strong limitations in terms of simultaneously meeting the requirements for sensitivity, speed, spectral range, being able to operate at room temperature, etc.

Secondly, it is a very safe type of radiation due to its low-energy photons, with more than a hundred times less energy than that of photons in the visible light range.

Many graphene-based applications are expected to emerge from its use as material for detecting light. Graphene has the particularity of not having a bandgap, as compared to standard materials used for photodetection, such as silicon. The bandgap in silicon causes incident light with wavelengths longer than one micron to not be absorbed and thus not detected. In contrast, for graphene, even terahertz light with a wavelength of hundreds of microns can be absorbed and detected. Whereas THz detectors based on graphene have shown promising results so far, none of the detectors so far could beat commercially available detectors in terms of speed and sensitivity.

In a recent study, ICFO researchers Sebastián Castilla and Dr. Bernat Terrés, led by ICREA Prof. at ICFO Frank Koppens and former ICFO scientist Dr. Klaas-Jan Tielrooij (now Junior Group Leader at ICN2), in collaboration with scientists from CIC NanoGUNE, NEST (CNR), Nanjing University, Donostia International Physics Center, University of Ioannina and the National Institute for Material Sciences, have been able to overcome these challenges. They have developed a novel graphene-enabled photodetector that operates at room temperature, and is highly sensitive, very fast, has a wide dynamic range and covers a broad range of THz frequencies.

In their experiment, the scientists were able to optimize the photoresponse mechanism of a THz photodetector using the following approach. They integrated a dipole antenna into the detector to concentrate the incident THz light around the antenna gap region. By fabricating a very small (100 nm, about one thousand times smaller than the thickness of a hair) antenna gap, they were able to obtain a great intensity concentration of THz incident light in the photoactive region of the graphene channel. They observed that the light absorbed by the graphene creates hot carriers at a pn-junction in graphene; subsequently, the unequal Seebeck coefficients in the p- and n-regions produce a local voltage and a current through the device generating a very large photoresponse and, thus, leading to a very high sensitivity, high speed response detector, with a wide dynamic range and a broad spectral coverage.

The results of this study open a pathway towards the development a fully digital low-cost camera system. This could be as cheap as the camera inside the smartphone, since such a detector has proven to have a very low power consumption and is fully compatible with CMOS technology.

Abstract-Fast and sensitive terahertz detection using an antenna-integrated graphene pn-junction

Sebastian Castilla, Bernat Terres, Marta Autore, Leonardo Viti, Jian Li, Alexey Nikitin, Ioannis Vangelidis, Kenji Watanabe, Takashi Taniguchi, Elefterios Lidorikis, Miriam Serena Vitiello, Rainer Hillenbrand, Klaas-Jan Tielrooij, and Frank H.L. Koppens


https://pubs.acs.org/doi/10.1021/acs.nanolett.8b04171

Although the detection of light at terahertz (THz) frequencies is important for a large range of applications, current detectors typically have several disadvantages in terms of sensitivity, speed, operating temperature, and spectral range. Here, we use graphene as photoactive material to overcome all of these limitations in one device. We introduce a novel detector for terahertz radiation that exploits the photo-thermoelectric effect, based on a design that employs a dual-gated, dipolar antenna with a gap of ~100 nm. This narrow-gap antenna simultaneously creates a pn-junction in a graphene channel located above the antenna, and strongly concentrates the incoming radiation at this pn-junction, where the photoresponse is created. We demonstrate that this novel detector has excellent sensitivity, with a noise-equivalent power of 80 pW/√Hz at room temperature, a response time below 30 ns (setup-limited), a high dynamic range (linear power dependence over more than 3 orders of magnitude) and broadband operation (measured range 1.8 - 4.2 THz, antenna-limited), which fulfils a combination that is currently missing in the state of the art. Importantly, based on the agreement we obtain between experiment, analytical model, and numerical simulations, we have reached a solid understanding of how the PTE eect gives rise to a THz-induced photoresponse, which is very valuable for further detector optimization.

Analyzing Intersubband Transitions of 2D Materials by Scattering Scanning Near-Field Optical Microscopy

Schematic illustration of charge carriers confined within a TMD flake comprising different thicknesses. Charge carriers in the ground state (blue) can be excited upon resonant light excitation to a higher state (pink). (Image credit: ICFO/Fabien Vialla)

Semiconducting heterostructures have been vital to the advancement of electronics and opto-electronics. Various applications in the terahertz and infrared frequency range make use of transitions, known as intersubband transitions, between quantized states in semiconductor quantum wells.

https://www.azonano.com/news.aspx?newsID=36319

Such intraband transitions demonstrate remarkably large oscillator strengths, close to unity. The discovery of these transitions in III-V semiconductor heterostructures had a large influence on the field of condensed matter physics and initiated the development of quantum well infrared photodetectors and also quantum cascade lasers.

In general, quantum wells of the highest quality are produced by employing molecular beam epitaxy (sequential growth of crystalline layers), which is a well-established method. Yet, it has two major drawbacks: It requires lattice-matching, thereby constraining the freedom in choosing the materials, and the thermal growth leads to atomic diffusion and increases interface roughness.

Since 2D materials naturally form a quantum well that has atomically sharp interfaces, they have the ability to overcome these drawbacks. They offer atomically sharp and defect-free interfaces, thereby allowing the formation of perfect quantum well, free of diffusive inhomogeneities. They eliminated the need for epitaxial growth on a matching substrate and hence can be easily isolated and coupled to other electronic systems such as optical or Si CMOS systems such as waveguides and cavities.

In their experiment, the group of scientists applied scattering scanning near-field optical microscopy (s-SNOM) as a novel strategy for performing spectral absorption measurements with a spatial resolution less than 20 nm. They exfoliated TMDs consisting of terraces of different layer thicknesses over lateral sizes of around a few microns. The researchers directly observed the intersubband resonances for these distinctive quantum well thicknesses inside a single device. They also tuned the charge carrier density in an electrostatic manner and illustrated intersubband absorption in both the conduction and valence bands. Detailed theoretical calculations were performed to complement and support these observations, which revealed non-local and many-body effects.Astonishingly, theoretical or experimental analysis of intersubband transitions in few-layer 2D materials has never been performed. As a result, in a research recently reported in the Nature Nanotechnology journal, ICFO researchers Peter Schmidt, Fabien Vialla, Mathieu Massicotte, Klaas-Jan Tielrooij, and Gabriele Navickaite, headed by ICREA Professor at ICFO Frank Koppens, in collaboration with the Institut Lumière Matière–CNRS, Technical University of Denmark, Max Planck Institute for the Structure and Dynamics of Matter, CIC nanoGUNE, and the National Graphene Institute, report on the first theoretical calculations and first experimental observation of intersubband transitions in quantum wells of few-layer semiconducting 2D materials (TMDs).

The outcomes of this research open the door for an unexplored area in this new category of materials and provide a preview of the physics and technology facilitated by intersubband transitions in 2D materials, such as infrared sources, detectors, and lasers with the capability for compact integration with Si CMOS.

Graphene Paves the Way to Faster High-speed Optical Communications

http://www.sciencenewsline.com/news/2018052118050028.html

Graphene, among other materials, can capture photons, combine them, and produce a more powerful optical beam. This is due to a physical phenomenon called the optical harmonic generation, which is characteristic of nonlinear materials. Nonlinear optical effects can be exploited in a variety of applications, including laser technology, material processing and telecommunications.

Although all materials should present this behaviour, the efficiency of this process is typically small and cannot be controlled externally. Now, partners of the Graphene Flagship project in Cambridge (UK), Milan, and Genova (Italy) have demonstrated for the first time that graphene not only shows a good optical response, but also how to control the strength of this effect using an electric field.

Researches envision the creation of new graphene optical switches, which could also harness new optical frequencies to transmit data along optical cables, increasing the amount of data that can be transmitted. Currently, most commercial devices using nonlinear optics are only used in spectroscopy. Graphene could pave the way towards the fabrication of new devices for ultra-broad bandwidth applications.

"Our work shows that the third harmonic generation efficiency in graphene can be increased by over 10 times by tuning an applied electric field," explains Giancarlo Soavi, lead author of the paper and researcher at the Cambridge Graphene Centre (University of Cambridge, UK).

"The authors found again something unique about graphene: tuneability of THG over a broad wavelength range. As more and more applications are all-optical, this work paves the way to a multitude of technologies," said said ICREA Professor Frank Koppens from ICFO (The Institute of Photonic Sciences), Barcelona, Spain, who is the leader of the Photonics and Optoelectronics Work Package within the Graphene Flagship.

Professor Andrea C. Ferrari, Science and Technology Officer of the Graphene Flagship, and Chair of its Management Panel, added how "graphene never ceases to surprise us when it comes to optics and photonics." He also highlights that "the Graphene Flagship has put significant investment to study and exploit the optical properties of graphene. This collaborative work could lead to optical devices working on a range of frequencies broader than ever before, thus enabling a larger volume of information to be processed or transmitted."

Imaging visible and IR light

Frank Koppens, Stijn Goossens, Gerasimos Konstantatos

http://spie.org/newsroom/imaging-visible-and--ir-light?highlight=x2400&ArticleID=x128462&SSO=1

By placing quantum dots on top of graphene, a highly sensitive photodetector is constructed that works simultaneously for visible and IR light.

26 January 2018, SPIE Newsroom. DOI: 10.1117/2.2201801.04

Human vision is only capable of converting a tiny fraction of light into an image. Although our eyes can see visible light, they are completely blind to 99% of the infrared light, a range of light that our bodies can only perceive as heat.

The imaging technologies we commonly use to capture images, such as digital cameras, are not able to do any better than human eyes. This is to some extent a coincidence, but can be explained by the fact that most common imaging systems are based on silicon chips, which use silicon electronic integrated circuits and silicon photodetectors. These silicon photodetectors can only convert visible light into electrical signals.

Integrating non-silicon semiconductors for IR light into imaging systems is a challenging process since these types of semiconductors perform poorly when combined with manufactured circuits based on the well-established Si-CMOS technology circuits. This limitation has resulted in IR imaging sensors costing as much as three orders of magnitude more than visible-wavelength Si-based image sensors, not to mention that they also have limited pixel resolution.

Flagship researchers from ICFO integrate graphene and quantum dots with CMOS technology to create an array of photodetectors, producing a high resolution image sensor. Courtesy Fabien Vialla

Photodetector ‘sees' IR light
This major drawback has now been overcome, thanks to graphene and semiconducting nanoparticles known as quantum dots. Graphene is an atomically flat material, consisting of a crystalline lattice of carbon atoms, used in this case as an electrical conductor with extraordinary high electronic mobility. Colloidal quantum dots offer high absorption and bandgap tunability from UV to short-wavelength IR.

By placing these quantum dots on top of graphene, a highly sensitive photodetector is constructed that works simultaneously for visible and IR light. These hybrid phototransistors are entirely compatible with silicon technologies, drastically reducing the cost for the development and production of the detection system as well as the electronics.

Equally important is the fact that these detectors can maintain a very high sensitivity while operating at room temperature, despite their spectral coverage into the infrared and unlike IR photodiodes that require cooling to reach compelling sensitivity. Because no cooling is required, the device can be rather slim and lightweight, with low power consumption, features that are essential for applications that require portability, mobility, and seamless integration.

110,000 hybrid photodetectors
At the Institute of Photonics Sciences (ICFO), we have been able, for the first time, to integrate graphene with silicon-integrated circuits in order to build an imaging array of 288 x 388 pixels. This array is considered the most broadband CMOS-monolithic image sensor in the world.

The sensor was fabricated by connecting more than 110,000 graphene-quantum dot photodetectors to the CMOS electronics (containing thousands of transistors) inside a microchip. This configuration permits the conversion of incoming light into electronic signals for each of the pixels, building up an image through their read-out. All this happens automatically and instantaneously inside the chip.

Silicon CMOS wafer with image sensor read-out circuitry, integrated with graphene/quantum dot photodetectors. Courtesy Fabien Vialla

As a result, we obtained the core chip for a digital camera that can sense UV, visible, and IR at the same time.

Since the graphene can be grown on large scale using chemical-vapor deposition and integrated with the CMOS circuitry by a simple transfer technique, this technology is fully compatible with silicon and CMOS technologies, which considerably reduces its production cost.

Optoelectronic applications
The many application areas for this new type of imaging system are highly diverse. First, it can be used as a night-vision camera since the atmosphere at night is always emitting IR light. We cannot see this light with our eyes or with the use of a conventional camera. However, with our newly created graphene-quantum dot camera, it is possible to sense this light and therefore obtain images and videos even when it is pitch black.

Secondly, IR light can propagate through fog, and with the help of this new camera, we can actually "see" objects through it.

Another example could be food inspection. By distinguishing different IR wavelengths, such technology would allow us to see the inside of fruits and vegetables and measure their decomposition state, even before seeing it with your own eyes.

In addition to these safety and security applications, this technology has caught the attention of the automotive, medical imaging, pharmaceutical inspection, and environmental monitoring sectors, and it may bring forward future applications that could include consumer products such as smartphone cameras, wearables, or smart glasses.

The integration of graphene with Si-CMOS electronics is also a promising platform for a broad range of applications, including integrated photonics (for the next generation of data communication infrastructure) and sensor systems (for the Internet-of-Things). For this reason, significant efforts are currently being dedicated to the production of low-cost, high-quality graphene and its integration with silicon CMOS manufacturing lines.

This large-scale integration is one of the main goals of the graphene flagship program, the largest EU research initiative with €1 billion in funding over 10 years. It is expected that wafer-scale production will be ready within a few years, giving support to a bright outlook for the first graphene-based optoelectronic technologies to enter the marketplace.

-Frank Koppens is an ICREA professor at the Institute of Photonic Sciences (ICFO) in Barcelona where he leads the Quantum Nano-Optoelectronics Research Group. Gerasimos Konstantatos is also an ICREA professor at ICFO and leads the Functional Optoelectronic Nanomaterials Research Group. Stijn Goossens is senior research fellow and project leader. The three are among 16 authors of a 2017 paper published in Nature Photonics, "Broadband image sensor array based on graphene-CMOS integration." Koppens is scheduled to describe this work 30 January at an industry session during SPIE Photonics West.

This article was originally published in the January 2018 edition of SPIE Professional magazine.

Graphene based terahertz absorbers-(Printable graphene inks enable ultrafast lasers in the terahertz range)

            Graphene Flagship researches create a terahertz saturable absorber using printable graphene    inks with an order of magnitude higher absorption modulation than other devices produced to date.
Credit: Graphene Flagship

https://www.sciencedaily.com/releases/2017/09/170912102759.htm

Graphene Flagship researches from CNR-Istituto Nanoscienze, Italy and the University of Cambridge, UK have shown that it is possible to create a terahertz saturable absorber using graphene produced by liquid phase exfoliation and deposited by transfer coating and ink jet printing. The paper, published in Nature Communications, reports a terahertz saturable absorber with an order of magnitude higher absorption modulation than other devices produced to date.

A terahertz saturable absorber decreases its absorption of light in the terahertz range (far infrared) with increasing light intensity and has great potential for the development of terahertz lasers, with applications in spectroscopy and imaging. These high-modulation, mode-locked lasers open up many prospects in applications where short time scale excitation of specific transitions are important, such as time-resolved spectroscopy of gasses and molecules, quantum information or ultra-high speed communication.

"We started working on saturable terahertz absorbers to solve the problem of producing a miniaturized mode-locked terahertz laser with thin and flexible integrated components that also had good modulation" said Graphene Flagship researcher Miriam Vitiello from CNR-Istituto Nanoscienze in Italy.

Graphene is a promising saturable absorber as it has intrinsic broadband operations and ultrafast recovery time along with an ease of fabrication and integration, as first demonstrated in ultra-fast infra-red lasers by Flagship partner University of Cambridge. In the terahertz range, the present paper exploits graphene produced by liquid phase exfoliation, a method ideally suited to mass production, to prepare inks, easily deposited by transfer coating or ink jet printing

"It was important to us to use a type of graphene that could be integrated into the laser system with flexibility and control" said Vitiello "Ink jet printing along with transfer coating achieved that."

Using mode-locked lasers to produce ultra fast pulses in the terahertz range can have interesting and exciting uses. "These devices could have applications in medical diagnostics when time of flight topography is of importance -- you could see a tumour inside a tissue" said Vitiello.

Frank Koppens, of the Institute of Photonic Sciences in Spain, is the leader of the Graphene Flagship's Photonics and Optoelectronics Work Package, which focuses on developing graphene-based technologies for imaging and sensing, data transfer and other photonics applications. "This is a new discovery with immediate impact on applications. Clearly, this is a case where graphene beats existing materials in terms of efficiency, scalability, compactness and speed" he said.

Andrea C. Ferrari, Science and Technology Officer of the Graphene Flagship, and Chair of its Management Panel added "It is an important milestone to have demonstrated that easily produced and printable graphene inks can also serve to enable ultrafast lasers in the terahertz range. Since the Flagship's inception, a variety of lasers have been made covering the visible to IR spectral range, but now the important THz range, with applications in security and medical diagnostic, is finally made accessible by graphene, starting yet another possible application field."

---

Story Source:

Materials

provided by Graphene Flagship. Note: Content may be edited for style and length.

---

Journal Reference:

1. Vezio Bianchi, Tian Carey, Leonardo Viti, Lianhe Li, Edmund H. Linfield, A. Giles Davies, Alessandro Tredicucci, Duhee Yoon, Panagiotis G. Karagiannidis, Lucia Lombardi, Flavia Tomarchio, Andrea C. Ferrari, Felice Torrisi, Miriam S. Vitiello. Terahertz saturable absorbers from liquid phase exfoliation of graphite. Nature Communications, 2017; 8: 15763 DOI: 10.1038/ncomms15763

Abstract-Polaritons in layered 2D materials

Tony Low, Andrey Chaves, Joshua D. Caldwell, Anshuman Kumar, Nicholas X. Fang, Phaedon Avouris, Tony F. Heinz, Francisco Guinea, Luis Martin-Moreno, Frank Koppens

(Submitted on 14 Oct 2016 (v1), last revised 7 Jul 2017 (this version, v3))

https://arxiv.org/abs/1610.04548

In recent years, enhanced light-matter interactions through a plethora of dipole-type polaritonic excitations have been observed in two-dimensional (2D) layered materials. In graphene, electrically tunable and highly confined plasmon-polaritons were predicted and observed, opening up opportunities for optoelectronics, bio-sensing and other mid-infrared applications. In hexagonal boron nitride (hBN), low-loss infrared-active phonon-polaritons exhibit hyperbolic behavior for some frequencies, allowing for ray-like propagation exhibiting high quality factors and hyperlensing effects. In transition metal dichalcogenides (TMDs), reduced screening in the 2D limit leads to optically prominent excitons with large binding energy, with these polaritonic modes having been recently observed with scanning near field optical microscopy (SNOM). Here, we review recent progress in state-of-the-art experiments, survey the vast library of polaritonic modes in 2D materials, their optical spectral properties, figures-of-merit and application space. Taken together, the emerging field of 2D material polaritonics and their hybrids provide enticing avenues for manipulating light-matter interactions across the visible, infrared to terahertz spectral ranges, with new optical control beyond what can be achieved using traditional bulk materials.

A graphene-based digital camera

An imaging chip that replaces silicon with a novel pixel material is sensitive to more than just visible light.

http://physicstoday.scitation.org/do/10.1063/PT.6.1.20170612a/full/

Steven K. Blau

At the heart of a smartphone camera is CMOS circuitry that registers the electrons produced when visible light strikes a silicon wafer. Other semiconductors coupled to CMOS circuits could enable cameras to image in the UV, IR, and terahertz bands; such detectors could see applications for night vision, food inspection, environmental monitoring, and more. Now a research team led by Gerasimos Konstantatos and Frank Koppens of the Institute of Photonic Sciences (ICFO) in Barcelona, Spain, has taken the first steps toward that possible future: It has coupled a graphene–quantum dot photodetector to a CMOS circuit to create an imaging chip sensitive to wavelengths ranging from 300 nm to 1850 nm. (The group’s earlier related work was discussed in Physics Today, July 2012, page 15.)

The researchers’ device, whose surface is roughly 15 mm × 15 mm, has almost 120 000 active pixels, plus a row of insensitive “blind” pixels. The left side of the figure shows a side-view schematic of a single pixel. A graphene layer lies atop the CMOS; the photosensitive material, lead sulfide quantum dots, is deposited on the graphene. When light hits a PbS quantum dot it creates electron–hole (e–h) pairs. The holes enter the graphene layer, where they flow due to a voltage applied across each pixel. The device measures light intensity by comparing the current in an active pixel with that in the blind pixels. The right side of the figure shows the result: a view of a pear and apple illuminated with IR light. (S. Goossens et al., Nat. Photonics11, 366, 2017.)

Researchers explore new 2D materials that could make devices faster, smaller, and efficient

http://www.nanowerk.com/nanotechnology-news/newsid=45205.php

(Nanowerk News) A new study by an international team of researchers led by the University of Minnesota highlights how manipulation of 2D materials could make our modern day devices faster, smaller, and better.

The findings are now online and will be published in Nature Materials ("Polaritons in layered two-dimensional materials").

Two-dimensional materials allow strong light-matter interactions through polaritons.

Two-dimensional materials are a class of nanomaterials that are only a few atoms in thickness. Electrons in these materials are free to move in the two-dimensional plane, but their restricted motion in the third direction is governed by quantum mechanics. Research on these nanomaterials is still in its infancy, but 2D materials such as graphene, transition metal dichalcogenides and black phosphorus have garnered tremendous attention from scientists and engineers for their amazing properties and potential to improve electronic and photonic devices.

In this study, researchers from the University of Minnesota, MIT, Stanford, U.S. Naval Research Laboratory, IBM, and universities in Brazil, UK and Spain, teamed up to examine the optical properties of several dozens of 2D materials. The goal of the paper is to unify understanding of light-matter interactions in these materials among researchers and explore new possibilities for future research.

They discuss how polaritons, a class of quasiparticles formed through the coupling of photons with electric charge dipoles in solid, allow researchers to marry the speed of photon light particles and the small size of electrons.

“With our devices, we want speed, efficiency, and we want small. Polaritons could offer the answer,” said Tony Low, a University of Minnesota electrical and computer engineering assistant professor and lead author of the study.

By exciting the polaritons in 2D materials, electromagnetic energy can be focused down to a volume a million times smaller compared to when its propagating in free space.

“Layered two-dimensional materials have emerged as a fantastic toolbox for nano-photonics and nano-optoelectronics, providing tailored design and tunability for properties that are not possible to realize with conventional materials,” said Frank Koppens, group leader at the Institute of Photonic Sciences at Barcelona, Spain, and co-author of the study.

 “This will offer tremendous opportunities for applications.”Others on the team from private industry also recognize the potential in practical applications.“

The study of the plasmon-polaritons in two-dimensions is not only a fascinating research subject, but also offers possibilities for important technological applications,” said Phaedon Avoruris, IBM Fellow at the IBM T. J. Watson Research Center and co-author of the study. “For example, an atomic layer material like graphene extends the field of plasmonics to the infrared and terahertz regions of the electromagnetic spectrum allowing unique applications ranging from sensing and fingerprinting minute amounts of biomolecules, to applications in optical communications, energy harvesting and security imaging.

”The new study also examined the possibilities of combining 2D materials. Researchers point out that every 2D material has advantages and disadvantages. Combining these materials create new materials that may have the best qualities of both.

“Every time we look at a new material, we find something new,” Low said. “Graphene is often considered a ‘wonder’ material, but combining it with another material may make it even better for a wide variety of applications.”

In addition to Low, Avoruris and Koppens, other researchers involved in the study include Andrey Chaves, Universidade Federal do Cearán (Brazil) and Columbia University; Joshua D. Caldwell, U.S. Naval Research Laboratory; Anshuman Kumar, University of Minnesota and Massachusetts Institute of Technology; Nicholas X.Fang, MIT; Tony Heinz, Stanford University; Francisco Guinea, IMDEA Nanociencia and University of Manchester; and Luis Martin-Moreno, University of Zaragoza (Spain).

Communications among most promising applications for game-changing graphene

Experts hold 'great optimism' with caution against hype; niche markets to be addressed first

14 April 2016

http://spie.org/about-spie/press-room/press-releases/all-press-releases/communications-among-most-promising-applications-for-game-changing-graphene-14-apr-2016

Chairs Frank Koppens and Nathalie Vermeulen open a workshop featuring graphene research and industry leaders at SPIE Photonics Europe.

BRUSSELS, Belgium and BELLINGHAM, Washington, USA -- The unique and promising properties of graphene -- a one-atom-thick sheet of carbon atoms arranged in a honeycomb-shaped lattice -- and its game-changing potential in numerous applications were explored in a well-attended and highly interactive daylong workshop on 5 April during SPIE Photonics Europe 2016 in Brussels.

At the end of the day, the consensus was that the most promising nearer-term applications are in communications.

Workshop chairs were Frank Koppens of ICFO -- Institute of Photonic Sciences, and Nathalie Vermeulen of B-PHOT -- Brussels Photonics Team, Vrije Universiteit Brussel.

"The workshop clearly showed that bridges are being built between scientists, companies working on applications, and institutions developing the production processes," Koppens said. "In general, great optimism was expressed on the expectations that graphene-based applications will make it to the market. But there was also a warning for too much hype creation, and acknowledgement that niche markets will be addressed first."

The opening session focused on graphene's role in integrated photonics devices for data communications, with speakers Marco Romagnoli (National Laboratory for Photonics Networks, CNIT), Wolfgang Templ (Alcatel-Lucent Bell Labs), Tingyi Gu (Princeton University), and Klaas-Jan Tielrooij, ICFO.

Among graphene's intriguing properties for data communications are 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.

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.

This is well worth study, speakers agreed, in the face of exponentially growing consumer demand for bandwidth, higher performance, and faster delivery of content as well as the increasing importance of energy efficiency.

"We already have suitable technologies for datacoms systems. Integrated photonics is a promising technology for improving communications as data rates rise," Romagnoli said. "There is the simultaneous need for reduced power and costs."

In the second session, Hartmut Roskos (Physikalisches Institut, Goethe-Universität), Alan Colli(Nokia Technologies), Valerio Pruneri (ICFO), Tom Constant (University of Exeter), and Ilya Goykhman (Cambridge Graphene Centre, University of Cambridge) focused on infrared and terahertz applications in detection and sensing.

Applications such as metal detection, quality assurance, medical spectroscopy, integrity checks, breath-gas analysis, temperature sensing, and biosensing require very sensitive technologies. 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.

The third session, on wafer scale processing and integration, featured talks by Grzegorz Lupina (IHP, Leibniz-Institut für innovative Mikroelektronik), Cedric Huyghebaert (IMEC), and Christoph Stampfer(RWTH Aachen University).

Graphene has become more popular in integrated electronics and photonics, where mass manufacturability is key. However, the traditional exfoliation process using cellophane tape is not suitable for mass production.

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.

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 sheets.

The day's final session targeted the bottom line in brief presentations followed by lively discussion on next steps and challenges in commercialization. Broadband light detectors, gas sensors, and EUV pellicles could be among the first devices made from graphene, the panel agreed.

Rapid presentation participants and panelists were:

• Stijn Goossens, ICFO
• Cedric Huygebaert, IMEC
• Bjarke Jørgensen, Newtec
• Daniel Neumaier, AMO GmbH
• Paul Hedges, Applied Nanolayers
• Richard van Rijn, Applied Nanolayers
• Wolfgang Templ, Alcatel.

See also:

'Graphene has to show maturity before CMOS integration, say panel at Photonics Europe'
Electo Optics, 7 April 2016

'Graphene shows promise for next-generation optical communications'
optics.org, 6 April 2016

About SPIE

SPIE is the international society for optics and photonics, an educational not-for-profit organization founded in 1955 to advance light-based science, engineering, and technology. The Society serves nearly 264,000 constituents from approximately 166 countries, offering conferences and their published proceedings, continuing education, books, journals, and the SPIE Digital Library. In 2015, SPIE provided more than $5.2 million in support of education and outreach programs. www.spie.org

Ultrafast photodetectors with graphene

http://graphene-flagship.eu/?news=ultrafast-photodetectors-with-graphene

Scientists affiliated with Europe’s Graphene Flagship develop a photodetector that converts incident light into electrical signals on femtosecond timescales, enabling ultrafast operation speeds for electronic circuits in optical communications and various other applications.

The conversion of light into electricity underpins a range of technologies that includes solar cells, digital cameras and optical-fibre communications, and in most cases operation speed is critical. For example, lasers currently used in optical communications, medical imaging and surgery can generate light pulses a picosecond (10-12 s) and less in length. Shorten the pulse length by three orders of magnitude, and you have a femtosecond (10-15 s) laser, the fastest currently available. To give you a conceptual idea of the timescale involved, a femtosecond is a thousandth of a millionth of a millionth of a second.

Illustration of ultrafast photovoltage creation following infrared light absorption at the interface between two graphene areas with different Fermi energies. (image copyright © 2015 Achim Woessner).

Photovoltage generation through the photo-thermoelectric effect occurs in this case when incident light is focused at the interface between graphene layers with different doping. The process begins with excitation and electron-hole pair generation, and is followed by electron heating from scattering between the charge carriers. The electrons then cool by thermal equilibrium with the atomic lattice, and this takes place on a picosecond timescale, limiting switching rates to a few hundred gigahertz. Much faster, terahertz switching (equating to femtosecond durations) requires the exploitation of carrier heating.

Graphene has long been considered a promising material for ultrafast, broadband photodetectors, the performance of which is dependent on switching speed. Efficient carrier heating is what distinguishes the photodetector developed by physicists at the Institute for Photonic Sciences (ICFO) in Barcelona, together with colleagues in the US and Spain. The results of the study are detailed in a paper published in the journal Nature Nanotechnology, the first author of which is Klaas-Jan Tielrooij.

Led by ICFO professor Frank Koppens, the researchers use graphene to directly measure the duration of a laser pulse less than 50 femtoseconds 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 1,500 nanometres. This is consistent with efficient electron heating.

“Graphene photodetectors show fascinating performance and properties, enabling a wide range of applications,”says Koppens. “Ranging from multi-spectral imaging to ultra-fast communications, such applications are being actively developed within the Graphene Flagship programme.”

The Graphene Flagship is an international academic-industrial consortium, funded in part by the European Commission. One of Europe’s first Future and Emerging Technologies flagship initiatives, the Graphene Flagship focuses on the development of graphene and related two-dimensional materials.

Francis Sedgemore is the science writer for the Graphene Flagship.

Further reading

Tielrooij et al., Generation of photovoltage in graphene on a femtosecond timescale through efficient carrier heating, Nature Nanotech.(2015); doi:10.1038/nnano.2015.54.