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New graphene-based metasurface capable of independent amplitude and phase control of light

Posted By Graphene Council, Tuesday, February 25, 2020
Researchers described a new strategy of designing metamolecules that incorporates two independently controllable subwavelength meta-atoms. This two-parametric control of the metamolecule secures the complete control of both amplitude and the phase of light.

A KAIST research team in collaboration with the University of Wisconsin-Madison theoretically suggested a graphene-based active metasurface capable of independent amplitude and phase control of mid-infrared light. This research gives a new insight into modulating the mid-infrared wavefront with high resolution by solving the problem of the independent control of light amplitude and phase, which has remained a long-standing challenge.

Light modulation technology is essential for developing future optical devices such as holography, high-resolution imaging, and optical communication systems. Liquid crystals and a microelectromechanical system (MEMS) have previously been utilized to modulate light. However, both methods suffer from significantly limited driving speeds and unit pixel sizes larger than the diffraction limit, which consequently prevent their integration into photonic systems.

The metasurface platform is considered a strong candidate for the next generation of light modulation technology. Metasurfaces have optical properties that natural materials cannot have, and can overcome the limitations of conventional optical systems, such as forming a high-resolution image beyond the diffraction limit. In particular, the active metasurface is regarded as a technology with a wide range of applications due to its tunable optical characteristics with an electrical signal.

However, the previous active metasurfaces suffered from the inevitable correlation between light amplitude control and phase control. This problem is caused by the modulation mechanism of conventional metasurfaces. Conventional metasurfaces have been designed such that a metaatom only has one resonance condition, but a single resonant design inherently lacks the degrees of freedom to independently control the amplitude and phase of light.

The research team made a metaunit by combining two independently controllable metaatoms, dramatically improving the modulation range of active metasurfaces. The proposed metasurface can control the amplitude and phase of the mid-infrared light independently with a resolution beyond the diffraction limit, thus allowing complete control of the optical wavefront.

The research team theoretically confirmed the performance of the proposed active metasurface and the possibility of wavefront shaping using this design method. Furthermore, they developed an analytical method that can approximate the optical properties of metasurfaces without complex electromagnetic simulations. This analytical platform proposes a more intuitive and comprehensively applicable metasurface design guideline.

The proposed technology is expected to enable accurate wavefront shaping with a much higher spatial resolution than existing wavefront shaping technologies, which will be applied to active optical systems such as mid-infrared holography, high-speed beam steering devices that can be applied for LiDAR, and variable focus infrared lenses.

Professor Min Seok Jang commented, "This study showed the independent control amplitude and phase of light, which has been a long-standing quest in light modulator technology. The development of optical devices using complex wavefront control is expected to become more active in the future."

PhD candidate Sangjun Han and Dr. Seyoon Kim of the University of Wisconsin-Madison are the co-first authors of the research, which was published and selected as the front cover of the January 28 edition of ACS Nano titled "Complete complex amplitude modulation with electronically tunable graphene plasmonic metamolecules".

Tags:  Graphene  KAIST  Min Seok Jang  photonics  plasmonics  Sangjun Han  Seyoon Kim 

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A novel formulation to explain heat propagation

Posted By Graphene Council, Thursday, February 13, 2020
Researchers at EPFL and MARVEL have developed a novel formulation that describes how heat spreads within crystalline materials. This can explain why and under which conditions heat propagation becomes fluid-like rather than diffusive. Their equations will make it easier to design next-generation electronic devices at the nanoscale, in which these phenomena can become prevalent.

Fourier's well-known heat equation describes how temperatures change over space and time when heat flows in a solid material. The formulation was developed in 1822 by Joseph Fourier, a French mathematician and physicist hired by Napoleon to increase a cannon's rate of fire, which was limited by overheating.

Fourier's equation works well to describe conduction in macroscopic objects (several millimeters in size or larger) and at high temperatures. However, it does not describe hydrodynamic heat propagation, which can appear in electronic devices containing materials such as graphite and graphene.

One of these heat-propagation phenomena is known as Poiseuille heat flow. This is where heat propagates within a material as a viscous-fluid flow. Another phenomenon, called "second sound," takes place when heat propagates in a crystal like a wave, similar to the way in which sound spreads through the air.

Since these phenomena are not described by Fourier's equation, until now researchers have analyzed them using explicit microscopic models, such as the Boltzmann transport equation. However, the complexity of these models means that they cannot be used to design complex electronic devices.

This problem has now been solved by Michele Simoncelli, a PhD student at EPFL, together with Andrea Cepellotti, a former EPFL PhD student now at Harvard, and Nicola Marzari, the chair of Theory and Simulation of Materials in the Institute of Materials at EPFL's School of Engineering and the director of NCCR MARVEL. They showed how heat originating from the atomic vibrations in a solid can be described rigorously by two novel "viscous heat equations", which extend Fourier's law to cover any heat propagation that is not diffusive.

"These viscous heat equations explain why and under which conditions heat propagation becomes fluid-like rather than diffusive. They show that heat conduction is governed not just by thermal conductivity, as described by Fourier's law, but also by a second parameter, thermal viscosity," says Simoncelli.

This breakthrough, published in Physical Review X, will help engineers design next-generation devices, particularly those that feature materials such as graphite or diamond in which hydrodynamic phenomena are prevalent. Overheating is the main limiting factor for the miniaturization and efficiency of electronic devices, and in order to maximize efficiency and predict whether a device will work - or simply melt - it is crucial to have the right model.

The results obtained by EPFL's team are timely. From the 1960s until recently, hydrodynamic heat phenomena had only been observed at cryogenic temperatures (around -260oC) and were therefore thought to be irrelevant for everyday applications. Already in 2015 Marzari and his colleagues predicted that this would be very different in two-dimensional and layered materials - a prediction that was confirmed with the publication in Science of pioneering experiments that found second-sound (or wavelike heat propagation) in graphite at temperatures around -170oC.

The formulation presented by the EPFL researchers yields results that line up closely with those experiments. Most important, they also predict that hydrodynamic heat propagation can also happen at room temperature, depending on the size and type of material.

Through their work, the EPFL researchers are providing new and original insight into heat transport, but also laying the groundwork for an understanding of shape and size effects - not only in next-generation electronic devices but also in "phononic" devices that control cooling and heating through engineered superstructures. Finally, the novel formulation can also be adapted to describe viscous phenomena involving electrons discovered in 2016 by Philip Moll, now a professor at EPFL's Institute of Materials.

Tags:  Andrea Cepellotti  Electronics  EPFL  Graphene  Michele Simoncelli  Nicola Marzari  Philip Moll  photonics 

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Chemists have managed to stabilize the 'capricious' phosphorus

Posted By Graphene Council, Tuesday, January 21, 2020
An international team of Russian, Swedish and Ukrainian scientists has identified an effective strategy to improve the stability of two-dimensional black phosphorus, which is a promising material for use in optoelectronics.

The most effective mechanism of fluorination has been revealed. In addition to increased stability compared to previously proposed structures, the materials predicted by the researchers showed high antioxidative stability. The main results of the work have been presented in The Journal of Physical Chemistry Letters.

Black phosphorus is obtained from white phosphorus under conditions of high pressure and elevated temperature. The material has a layered structure and resembles graphite in appearance and properties. However, unlike graphite, it is a good semiconductor.

"Phosphorene is a monolayer of black phosphorus with interesting physical properties (high anisotropic electrical and thermal conductivity, flexible band gap variability depending on the number of layers), which makes it a promising material for use in various fields of optoelectronics (transistors, inverters, flexible electronics, solar panels). Unfortunately, one of its main problems is instability in the environment. Unlike its volumetric analogue, which is almost immune to external conditions, phosphorene quickly begins to attach oxygen from the air and degrades within a few hours. As one of the strategies for improving the stability of phosphorene, mechanism of fluorination was proposed. Over the past five years, scientists have proposed several theoretically possible options for such a "coupling". An experiment was conducted that showed a significant increase in the stability of phosphorus in ambient conditions after fluorination. However, the features of the obtained material structure remained unexplained.

Using various theoretical approaches, my colleagues and I showed that the previously proposed structures of "stabilized" phosphorus were actually unstable. It is known that phosphorus is able to form compounds with 3 or 5 fluorine atoms. Our calculations also confirmed that the characteristic coordination of the phosphorus atom in the PF system is 3 or 5. By sequential addition of atoms, it was possible to identify the most effective and really working mechanism by which fluorine atoms should attach to the surface of phosphorene. Thus, we have determined the type of structures that are likely to have been obtained by our predecessors in the above-mentioned experiment," -- said Artem Kuklin, a research fellow of SibFU.

Scientists note that the materials formed by the predicted mechanism are really stable and have increased antioxidant ability (that is, they are not quickly degradable) and their electronic properties, which do not differ much from the properties of pure phosphorus, provide the possibility of their practical application in optoelectronic devices, i.e. transistors, solar panels, flexible electronics, LEDs, photosensors, biomedical devices, optical devices for storing and transmitting information, etc.

Tags:  Artem Kuklin  Graphene  optoelectronics  photonics  Semiconductor  Siberian Federal University 

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Graphene nanoarchitectures for diverse applications

Posted By Graphene Council, Wednesday, January 1, 2020

Graphene is an exceptional material with many potential applications. The on-surface synthesis of covalent architectures with atomic precision has emerged as one of the most promising methods for providing new functionalities to graphene.

Researchers from the ICN2 Atomic Manipulation and Spectroscopy Group and the DIPC discuss it in an article published in the Revista Española de Física.

This method allows creating a wide range of graphenic architectures from precursor molecules that are designed practically à la carte.

ICN2 researcher César Moreno and ICREA Prof. Aitor Mugarza (Leader of the Atomic Manipulation and Spectroscopy Group), together with 

Ikerbasque researcher Aran Garcia-Lekue (DIPC) have written an article for the Revista Española de Física discussing these topics.

They present the milestones achieved and the challenges and opportunities ahead regarding the top-down and the bottom-up approaches to build graphene nanoarchitectures. They focus on the potential applications of graphene nanostrips for nanoelectronics and photonics and of nanoporous graphene for advanced filtering.

Tags:  Aitor Mugarza  Aran Garcia-Lekue  César Moreno  Graphene  ICN2  nanoelectronics  photonics 

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Graphene and layered materials boost silicon technologies

Posted By Graphene Council, Saturday, November 16, 2019
Updated: Friday, November 8, 2019
Silicon semiconductor technology has done marvels for the advancement of our society, which has benefited tremendously from its versatile use and amazing capabilities. The development of electronics, automation, computers, digital cameras and smartphones based on this material and its underpinning technology has reached skyrocket limits, downscaling the physical size of devices and wires to the nanometre regime. 

Although this technology has been growing since the late 1960s, the miniaturization of circuits seems to have reached a possible halt, since transistors can only be shrunk down to a certain size and not further beyond. Thus, there is a pressing need to complement Si CMOS technology with new materials and fulfil the future computing requirements as well as the needs for diversification of applications.

Graphene and related materials offer prospects of advances in device performance at the atomic limit.  They provide a possible solution to overcome the limitations of silicon technology, where the combination of layered materials with silicon chips promises to surpass the current technological limitations.

A team of researchers including Stijn Goossens and Frank Koppens, based at Graphene Flagship partner ICFO, and industrial leaders from Graphene Flagship partner IMEC and TSMC provided an in-depth and thorough review of opportunities, progress and challenges of integrating atomically thin materials with Si-based technology. They give insights on how and why layered materials could overcome current challenges posed by the existing technology and how they can enhance both device component function and performance, to boost the features of future technologies, in the areas of computational and non-computational applications.

For non-computational applications, they review the possible integration of these materials for future cameras, low power optical data communications and gas and bio-sensors. In particular, in image sensors and photodetectors, graphene and related materials could enable new vision in the infrared and terahertz range in addition to the visible range of the spectrum. These can serve for example in autonomous vehicles, security at airports and augmented reality.

For computational systems, and in particular in the field of transistors, they show how challenges such as doping, contact resistance and dielectrics/encapsulation can be diminished when integrating layered materials with Si technology. Layered materials could also improve memory and data storage devices with novel switching mechanisms for meta-insulator-metal structures, avoid sneak currents in memory arrays, or even push the performance gains of copper wire-based circuitry by adhering graphene to the ultrathin copper barrier materials and thus reduce resistance, scattering and self-heating.

The review provides a roadmap of layered material integration and CMOS technology, pinpointing the stage at which all challenges regarding growth, transfer, interface, doping, contacting, and design are currently standing today and what possible processes are expected to be resolved to achieve such goals of moving from a research laboratory environment to a pilot line for production of the first devices that combine both technologies. The layered materials-CMOS roadmap, as presented in this review, gives an exciting glimpse into the future, with pilot production expected to be just a few years from now.

Frank Koppens, Graphene Flagship Work Package Leader for Photonics and Optoelectronics and lead author of the study, says: "Now we have a clear industry-driven roadmap on layered material-silicon technologies and manufacturing. Complementing the established silicon technology with layered materials is key to combine the best of both worlds and enable a plethora of large volume and low-cost applications."

Marco Romagnoli, Graphene Flagship Work Package Leader for Wafer-Scale System Integration, comments: "This is an interesting paper complementing a previous one focused on graphene photonics for telecommunications that completes the range of applications in which graphene can be exploited for large scale production in CMOS environments. Also interesting is the type of application, in which graphene can best exploit its characteristics, from IR/THz cameras to low-power electronic switching and memories.

Andrea C. Ferrari, Science and Technology Officer of the Graphene Flagship and Chair of its Management Panel, adds: "The integration of graphene and related materials with silicon and CMOS technology is the next goal for the Flagship. For this reason, we will fund the first foundry focussed on the integration of layered materials. This work clearly spells out the vision for the transformative technology that integration will enable."

Tags:  Andrea C. Ferrari  Frank Koppens  Graphene  Graphene Flagship  ICFO  Marco Romagnoli  optoelectronics  photonics  Semiconductor  Stijn Goossens  transistor 

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Graphene layer enables advance in super-resolution microscopy

Posted By Graphene Council, Thursday, September 12, 2019
Updated: Friday, September 13, 2019
Researchers at the University of Göttingen have developed a new method that takes advantage of the unusual properties of graphene to electromagnetically interact with fluorescing (light-emitting) molecules. This method allows scientists to optically measure extremely small distances, in the order of 1 ångström (one ten-billionth of a meter) with high accuracy and reproducibility for the first time. This enabled researchers to optically measure the thickness of lipid bilayers, the stuff that makes the membranes of all living cells. The results were published in Nature Photonics.

Researchers from the University of Göttingen led by Professor Enderlein used a single sheet of graphene, just one atom thick (0.34 nm), to modulate the emission of light-emitting (fluorescent) molecules when they came close to the graphene sheet. The excellent optical transparency of graphene and its capability to modulate through space the molecules' emission made it an extremely sensitive tool for measuring the distance of single molecules from the graphene sheet. 

The accuracy of this method is so good that even the slightest distance changes of around 1 ångström (this is about the diameter of an atom or half a millionth of a human hair) can be resolved. The scientists were able to show this by depositing single molecules above a graphene layer. They could then determine their distance by monitoring and evaluating their light emission. This graphene-induced modulation of molecular light emission provides an extremely sensitive and precise "ruler" for determining single molecule positions in space. They used this method to measure the thickness of single lipid bilayers which are constituted of two layers of fatty acid chain molecules and have a total thickness of only a few nanometers (1 billionth of a meter).

"Our method has enormous potential for super-resolution microscopy because it allows us to localise single molecules with nanometre resolution not only laterally (as with earlier methods) but also with similar accuracy along the third direction, which enables true three-dimensional optical imaging on the length scale of macromolecules," says Arindam Ghosh, the first author of the paper.

"This will be a powerful tool with numerous applications to resolve distances with sub-nanometer accuracy in individual molecules, molecular complexes, or small cellular organelles," adds Professor Jörg Enderlein, the publication's corresponding author and head of the Third Institute of Physics (Biophysics) where the work took place.

Tags:  Arindam Ghosh  Graphene  Jörg Enderlein  photonics  University of Göttingen 

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Light-induced active ion transport in graphene oxide membranes

Posted By Graphene Council, Wednesday, April 10, 2019
Updated: Thursday, March 21, 2019

Nanofluidic channels feature a unique unipolar ionic transport when properly designed and constructed. Recent research in nanofluidics has adopted reconstructed layered two-dimensional (2D) sheets – such as graphene oxide or clay – as a promising material platform for nanofluidics. These membranes contain a high volume fraction of interconnected 2D nanochannels.

Compared to other materials used for nanofluidic devices, such as anodized aluminum oxide membrane, block copolymer membrane and nanofluidic crystals, a unique feature of layered membranes is that the channels are horizontally aligned and the channel height (i.e., the spacing between the layers), which is responsible for confinement of the electrolyte, remains uniform throughout the entire thin film.

"However, mass and charge transport in existing membrane materials follows their concentration gradient," Wei Guo, a professor at the Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing, tells Nanowerk. "Attaining anti-gradient transport as effective as natural counterparts remains a great challenge in fully abiotic nanosystems."

In new work led by Guo, reported in Nature Communications ("Photo-induced ultrafast active ion transport through graphene oxide (GO) membranes"), the researchers demonstrate a coupled photon-electron-ion transport phenomenon through graphene oxide membranes.

It shows a straightforward way on how to power the transport in 2D layered materials using the energy of light.
"Using the energy of light, cations are able to move thermodynamically uphill over a broad range of concentrations, at rates orders of magnitude faster than that via simple diffusion," Guo explains. "Based on this mechanism, we developed photonic ion switches, photonic ion diodes, and photonic ion transistors as the fundamental elements for active ion sieving and artificial photosynthesis on synthetic nanofluidic circuits."

This is the first discovery of photo-induced active (anti-gradient) ion transport in 2D layered materials with extraordinarily high pumping rates. It provides a completely new way for remote, non-invasive, and active control of the transport behaviors in synthetic membrane materials.

"Using light to control the mass and charge transportation in fully synthetic membranes is the dream of a materials scientist, like me," says Guo. "As far as I know, many research groups currently are engaged in this field. However, their findings are restricted to use the light as a gate, allowing or prohibiting the transport. In contrast, we use the light as a motive force to realize active transport."

Upon asymmetric light illumination, a net cationic flow through the layered graphene oxide membrane is generated from the non-illuminated region to the illuminated region. This phenomenon is reported for the first time.

Against a concentration gradient, the pumping rates for cations can be five orders of magnitude higher than that via simple diffusion.

The team established a theoretical model and performed molecular dynamics simulations to unveil the mechanism. Light irradiation reduces the local electric potential on the graphene oxide membrane following a carrier diffusion mechanism. When the illumination is applied to an off-center position, an electric potential difference is built across the GO strip that can drive the transport of ionic species.

Superior to existing molecular transport systems, the light-induced active ion transport reported in this work does not rely on lipid or liquid membranes, which significantly improves its robustness and compatibility. In addition, it does not hinge on specific ion-binding shuttle molecules to achieve the transmembrane ion transport. Thus, its transport range can be at the scale of centimeters.

This work provides a new route for remote, non-invasive, and active control of the transport behaviors in synthetic membrane materials. It demonstrates a way to fabricate innovative membrane materials for active ionic sieving, artificial photosynthesis, and modular computation on integrated nanofluidic circuits.

Following the mechanism proposed in this work, as shown in the figure below, the researchers constructed photonic ion switches (PIS), photonic ion diodes (PID), and photonic ion transistors (PIT) as the fundamental elements for light-controlled nanofluidic circuits.

"So far in our lab, the photo-induced active ion transport systems has been developed to the third generation," notes Guo. "The photo-induced active ion transport phenomenon can be also found in almost the whole family of 2D semiconductors. There is tremendous room to further exploit their unique photo-responsiveness in liquid processable colloidal 2D materials. The present work opens up exciting new possibilities."

"Now, we are trying to amplify the generation of photocurrent and voltage, and scale up the membrane materials with, for example, printing techniques," he concludes. "Also, we intend to further extend the scope of the materials with which the active transport behaviors can take place."

Tags:  2D materials  Beijing  Chinese Academy of Sciences  Graphene  graphene oxide  photonics  Wei Guo 

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How Quantum Dots and Graphene Combined to Change the Landscape for Optoelectronics

Posted By Dexter Johnson, IEEE Spectrum, Wednesday, November 1, 2017

Last June, we covered research that brought graphene, quantum dots and CMOS all together into one to change the future of both optoelectronics and electronics. 

That research was conducted at the Institute of Photonics (ICFO) located just outside of Barcelona, Spain. The Graphene Council has been speaking to Frank Koppens at ICFO since 2015 about how graphene was impacting photonics and optoelectronics.

Now, in a series of in-person interviews with several researchers at ICFO (the first of which you can find here),  we are gaining better insight into how these technologies came to be and where they ultimately may lead.

Gerasimos Konstantatos - group leader at ICFO

The combination of graphene with quantum dots for use in optoelectronics stems in large part from the contributions of Gerasimos Konstantatos, a group leader at ICFO, who worked with Ted Sargent at the University of Toronto, whose research group has been at the forefront of exploiting colloidal quantum dots for use in a range of applications, most notably high-efficiency photovoltaics.

“Our initial expertise and focus was on actually exploiting the properties of solution-process materials particularly colloidal quantum dots as optoelectronic materials for solar cells and photodetectors,” explained Konstantatos. “The uniqueness of these materials is that they give us access to a spectrum that is very rarely reached in the shortwave and infrared and they can do it at a much lower cost than any other technology.”

Konstantatos and his group were able to bring their work with quantum dots to the point of the near-infrared wavelength spectrum, which falls in the wavelength size range of one to five microns. Konstantos is now developing these solution-based quantum dot materials to produce even more sensitive materials capable of getting to 10 microns, putting them squarely in the mid-infrared range.

“My group is now working with Frank Koppens to sensitize graphene and other 2D materials in order to make very sensitive photodetectors at a very low cost that are capable of accessing the entire spectrum, and this cannot be done with any other technology,” said Konstantatos.

What Konstantatos and Koppens have been able to do is to basically eliminate the junction between graphene and the quantum dots and in so doing have developed a way to control the charge transfer in a very efficient way so that they can exploit the very high mobility and transport conductance of graphene.

“We can re-circulate the charges through the materials so that with a single photon we have several billion charges re-circulating through the material and this constitutes the baseline of this material combination,” adds Konstantatos.

With that as their baseline technology, Konstantatos and his colleagues have engineered the quantum dot layer so instead of just having a passive quantum dot layer they have converted it into an electro-diode. In this way they can make much more complex detectors. In the combination of the graphene-based transistor with the quantum dots, it’s not just a collection of quantum dots but is a photodiode made from quantum dots.

“In this way, we kind of get the benefit of both kinds of detectors,” explains Konstantatos. “You have a phototransistor that has a very high sensitivity and a very high gain, but you also get the high quantum efficiency you get in photodiodes. It’s basically a quantum photodiode that activates a transistor.”

In addition to the use of graphene, the ICFO researchers are looking at other 2D materials in this combination, specifically the semiconductor molybdenum disulfide. While this material is a semiconductor and sacrifices somewhat on the electron mobility of graphene, it does make it possible to switch off the material to control the current. As a result, Konstantatos notes that you can have much lower noise in the detector with much lower power consumption.

In continuing research, Konstantatos hinted at yet to be published work on how all of this combination of quantum dots and graphene could be used in solar cell applications.

In the meantime, the work they have been doing with graphene and quantum dots is much further advanced than what they have yet been able to achieve with molybdenum disulfide, mainly because work has advanced much further in making large scale amounts of graphene. But as the processes for producing other 2D materials improves, there will be a real competition between all of the 2D materials to see which provides the best possible performance as well as manufacturability properties.

In any event, Konstantatos sees that the way forward with both quantum dots and 2D materials is using them together.

He adds: “I think we can explore the synergies in between different material platforms. There's no such thing as a perfect material that can do everything right. But there is definitely a group of materials with some unique properties. And if you can actually combine them in a smart way and make hybrid structures, then I think you can have significant added value.”

Tags:  2D materials  graphene  optoelectronics  photodetectors  photonics  photovoltaics  quantum dots 

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Plasmonics Without Light Just Flipped Nanophotonics on its Head

Posted By Dexter Johnson, IEEE Spectrum, Monday, October 23, 2017

The use of graphene in the growing field known as plasmonics—in which the waves of electrons known as surface plasmons that are generated when photons strike a metallic structure—has been transforming the world of photonics and optoelectronics, enabling the possibility of much smaller devices operated by photons rather than electrons.

The Graphene Council has covered the work being performed at one of the leading research institutes in the world in this field of plasmonics, the Institute of Photonic Sciences (ICFO) in Barcelona. 

We had the opportunity to visit ICFO last week and speak to a number of their researchers, which we will be sharing in the coming weeks. In particular, we spoke to F. Javier García de Abajo from the Nanophotonics Theory research group at ICFO,  who has proposed a revolutionary approach of exploiting graphene for plasmonics.

It’s worth providing a bit of background on the field of plasmonics before jumping to this latest research. The use of photons instead of electrons for something like an integrated circuit has the clear benefit that photons travel much faster than electrons, promising much faster devices. However, the use of light in these applications is limited by the relatively large size of wavelengths of light. Light is fast, but their wavelengths are much larger than nanometer-scale dimensions of most integrated circuits.

Plasmonics provides a way to convert that light—photons—into waves of electrons that can be tuned to have much smaller dimensions than those of light. The dimensions of these plasmon waves can be a hundred times smaller than the smallest wavelengths of light. This means that light can serve as the basis of photonic integrated circuits, but many more devices than that.

The field of plasmonics has really taken in off in the last half-decade, and ICFO has been at the forefront of a lot of that work, especially in using graphene to enable the effect. However, what Garcia de Abajo has proposed is a new theoretical approach to generate visible plasmons in graphene not from light but from tunneling electrons.

In research published in the journal ACS Photonics, Garcia de Abajo and his colleague Sandra de Vega have suggested that there are more efficient ways of generating surface plasmons on graphene than using an external light source and have instead shown through models that graphene plasmons can be efficiently excited via electron tunneling in a sandwich structure formed by two graphene monolayers separated by a few atomic layers of hexagonal boron nitride.

As mentioned, it’s possible to tune the size of the plasmon waves, especially graphene plasmons, which can be changed in size according to the amount of doping level (an addition of other materials). While high doping levels can push the wavelength of the graphene plasmons towards the visible range, these grpahene plasmons primarily reside in the mid-infrared region, which translates into a weak coupling between far-field light and graphene.

What de Vega and García de Abajo have proposed is a methodology for visible-plasmon generation in graphene that requires no light at all. Instead, plasmons are generated from tunneling electrons, which are electrons that are able to pass through a material on the quantum level that they could not otherwise pass through.

To achieve this photon-less plasmonics, the researchers propose a graphene–hexagonal boron nitride (hBN)–graphene sandwich structure. In their model, the hBN layer is 1-nm thick that is sandwiched between two graphene monolayers.

When the right amount of voltage (bias) is applied between the two graphene sheets, it produces tunneling electrons through the gap. The researchers discovered a particular voltage window in which the tunneling electrons lose energy through the excitation of a propagating optical plasmon rather than dissipate through coupling with the vibrations of the crystal lattice of hBN that carry heat, which are known as phonons, (low bias) or electron–electron interactions (high bias).

One of the side benefits of plasmonic devices that operate in this way—without the need for photons—can also be used in reverse as sensors. In this way when a change occurs in the graphene plasmon properties, that change could lead to a voltage readout.

Tags:  electrons  graphene  hexagonal boron nitride  ICFO  photonics  photons  plasmonics  sensors 

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Established Optical Society Sees the Light in Graphene

Posted By Dexter Johnson, IEEE Spectrum, Saturday, October 8, 2016
Updated: Thursday, October 6, 2016

SPIE—the international society for optics and photonics—has been a society set up to advance light-based technologies since 1955. In this role, it has offered its members conferences, news services and a range of different avenues for exchanging information on this quickly developing field.

As evidence of its commitment to staying ahead of the latest science and technologies in photonics and optics, SPIE has been offering conferences on the topic of graphene since 2009. SPIE has identified graphene and other two-dimensional materials as a key area of interest for its members because of the properties these new materials are offering in the field.

The Graphene Council certainly shares in SPIE’s interest in how two-dimensional materials, including graphene, will play a key role in optoelectronics and photonics, with our frequent coverage of these two fields. 

Now that SPIE has become one of our Corporate Members we took the opportunity to speak to Robert F. Hainsey, Ph.D., the Director of Science and Technology for SPIE to ask him about the role graphene is positioned to play in optics and photonics, how the market is developing and the role of SPIE as these developments evolve.

Q: Graphene has exhibited a number of appealing properties for applications within photonics and optoelectronics, so it’s clear to see why SPIE would become involved with the topic. But could you tell us a little bit about the evolution of how SPIE started getting involved in the topic of graphene? 

A:  SPIE has a long history of supporting the topic of graphene having launched a volunteer-inspired conference at our Optics + Photonics event held annually in San Diego as early as 2009.  The topic appears in a number of other SPIE conferences as well.  In 2014, Frank Koppens of ICFO delivered an excellent plenary talk on the subject at our Photonics Europe event in Brussels, and this led, in turn, to Frank Koppens and Nathalie Vermeulen of the B-PHOT team at Vrije Universiteit Brussel organizing and chairing a full-day workshop at this year’s Photonic Europe event on applications and commercialization of graphene.  We continue to look for methods to enable the community to best share results and exchange ideas in this rapidly evolving field.

Q: How is SPIE now approaching the topic, i.e. what sort of mediums are you using to get the message out about graphene? How do you see this information serving your members? 

A:  The information is disseminated in a number of ways.  Primary among these methods are our conferences which enable researchers to share and discuss the latest findings in the area of graphene and similar materials.  The work shared in those conferences is then packaged into proceedings and made part of the SPIE Digital Library so as to share the results with a wider audience.  We also have our journals where researchers can publish their results in a peer-reviewed medium.  The “SPIE Professional” magazine, the quarterly magazine for our members, has included articles in this area including one written by Frank Koppens earlier this year.  Naturally, we share news about graphene research on our News Room webpage, via Twitter and through our LinkedIn groups.  In terms of serving our members, we hope that this diverse set of methods of sharing information keeps our members informed on the latest work in the field and stimulates discussion among researchers to advance the field.

Q: There are a number of different applications within photonic and optoelectronics in which graphene has exhibited promise. In one of your more recent conferences on graphene, communication applications were identified as the most near-term. Has SPIE begun to get a better feel of how graphene applications within photonics and optoelectronics are developing commercially? And could you give us an outline of that development? 

A:  The workshop you refer to is a positive step towards moving graphene along the commercialization pipeline.  This workshop served to bring together academic and industrial researchers as well as entrepreneurs and start-up companies to discuss what is needed to move graphene from a laboratory to a production setting.  A look at the program for that event illustrates that large enterprises are investing in the research.  In addition, more start-up’s are appearing on the scene at various positions of the value chain.  Progress is being made on the road to full-scale production but there is still work to be done.

Q: Is SPIE involved with any of the standards bodies that are attempting to create industry standards for the material? Whether you are involved or not, does SPIE have a position on the role of materials standards as the material becomes increasingly commercialized?

A:  At this point we are not actively engaged in the work on developing standards outside of the presentations given in our conferences.  That said, one sign of research maturing and preparing to transition to a production environment is the discussion and adoption of standards.  Standards are oftentimes crucial since they provide a baseline for methods and performance by which the industry can determine capability and map progress.  SPIE supports standards development in other areas through methods such as providing meeting space for standards bodies at our events.  We would welcome dialogue with standards bodies in this area to determine if there is a way SPIE can more actively support that work.

Q: How do you see SPIE’s role in graphene education and providing information evolving as the field moves from the lab to the fab? Does the approach to disseminating information on a topic change as it moves from research to commercial interests? 

A:  Certainly the topic will continue to be a vibrant one in our conferences, our proceedings, the SPIE Digital Library, and our social media outlets.  SPIE events also include a set of industry sessions containing presentations, panel discussions, and networking opportunities focused on the commercial aspects of optics and photonics technologies.  This combination of conferences, publications, and industry sessions positions SPIE events to track the migration of the technology as it matures.  The flexibility we have within our events to include unique offerings such as the dedicated workshop on graphene commercialization at the SPIE Photonics Europe event earlier this year allows SPIE to tailor the forum to best serve the community.

Q: How does partnering with groups, such as The Graphene Council, help or contribute to your strategy in education and providing information on the topic of graphene?

A:  SPIE is an organization dedicated to serving the optics and photonics community.  Partnering with other organizations to further the sharing of information and enhancing the discussion around technologies not only helps SPIE meet its charter but, more importantly, enables the advancement of research, science, engineering and practical applications in these technologies.

Tags:  corporate members  optoelectronics  photonics  SPIE 

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