ICFO’s NEST program, supported by Fundació Cellex and Fundació Mir-Puig, allows the institute to offer outstanding opportunities for young scientists aiming to start and lead an independent research group. We are very pleased to announce a new member of the program, Dr F. Pelayo García de Arquer, who will join ICFO as a new faculty member and Group Leader, coming from the University of Toronto. Pelayo will lead a program seeking to reduce the growing CO2 emissions to revert global warming and climate change.
Dr. García de Arquer studied telecommunications engineering, mathematics, and photonics. He earned his PhD from ICFO, during which he investigated how the interaction between nanostructured semiconductors and metals could be manipulated dictating key optoelectronic properties such as absorption, charge transport and doping. He also explored new types of devices where highly energetic electrons in metals could be harnessed for sensing and energy harvesting. He applied his findings to make more efficient photodetectors and solar cells.
Pelayo joined the University of Toronto as a Connaught Postdoctoral Fellow in Bioinspired Ideas for Sustainable Energy. In his postdoctoral work, he expanded his research in the field of clean energy. He explored the use of emerging liquid-processed materials such as perovskites, low dimensional perovskites, quantum dots, and their combination, to control energy transfer at the nanoscale. Soon, he turned his attention to energy storage based on hydrogen and CO2 electroreduction. Pelayo, in this area, advanced in the understanding and performance of catalysts for these reactions, offering new insights into their design considering material transformations, and gas, electron and ion management.
At ICFO, García de Arquer will establish a research program focusing on CO2 Mitigation Accelerated by Photons (CO2MAP). His group will explore the conversion of CO2 into renewable fuels and commodities using clean energy. This has the potential to reduce the massive carbon footprint of existing manufacturing and transport processes. Pelayo’s group will use photon-based spectroscopies to shed light on the reaction mechanisms and catalyst reconstruction processes that drive CO2 electroreduction at high conversion rates. Combined with modeling, his group will use these insights to enable the informed design of catalysts and systems that achieve the selectivity, activity, energy efficiency and stability needed for this technology to make a significant impact in the global effort to revert climate change.
Bolometers are devices that measure the power of incident electromagnetic radiation thru the heating of materials, which exhibit a temperature-electric resistance dependence. These instruments are among the most sensitive detectors so far used for infrared radiation detection and are key tools for applications that range from advanced thermal imaging, night vision, infrared spectroscopy to observational astronomy, to name a few.
Even though they have proven to be excellent sensors for this specific range of radiation, the challenge lies in attaining high sensitivity, fast response time and strong light absorption, which not always are accomplished all together. Many studies have been conducted to obtain these higher-sensitivity bolometers by searching to reduce the size of the detector and thus increase the thermal response, and in doing so, they have found that graphene seems to be an excellent candidate for this.
If we focus on the infrared range, several experiments have demonstrated that if you take a sheet of graphene and place it in between two layers of superconducting material to create a Josephson junction, you can obtain a single photon detector device. At low temperatures, and in the absence of photons, a superconducting current flows through the device. When a single infrared photon passes through the detector, the heat it generates is enough to warm up the graphene, which alters the Josephson junction such that no superconducting current can flow. So you can actually detect the photons that are passing through the device by measuring the current. This can be done basically because graphene has an almost negligible electronic heat capacity. This means that, contrary to materials that retain heat like water, in the case of graphene a single low-energy photon can heat the detector enough to block the superconducting current, and then dissipate quickly, allowing the detector to rapidly reset, and thus achieving very fast time responses and high sensitivities.
Trying to take a step further and move to higher wavelengths, in a recent study published in Nature, a team of scientists which includes ICFO researcher Dmitri Efetov, together with colleagues from Harvard University, Raytheon BBN Technologies, MIT, and the National Institute for Material Sciences, has been able to develop a graphene-based bolometer that can detect microwave photons at extremely high sensitivities and with fast time responses.
Just like with the infrared range, the team took a sheet of graphene and placed it in between two layers of superconducting material to create a Josephson junction. This time, they went an entirely new route and attached a microwave resonator to generate the microwave photons and by passing these photons through the device, were able to reach an unprecedented detection levels. In particular, they were able to detect single photons with a much lower energy resolution, equivalent to that of a single 32 Ghz photon, and achieve detection readouts 100.000 times faster than the fastest nanowire bolometers constructed so far.
The results achieved in this study mean a major breakthrough in the field of bolometers. Not only has graphene proven to be an ideal material for infrared sensing and imaging, but it has also proven to span to higher wavelengths, reaching the microwave, where it has also shown to attain extremely high sensitivities and ultra-fast read out times.
As Prof. at ICFO Dmitri Efetov comments "such achievements were thought impossible with traditional materials, and graphene did the trick again. This open entirely new avenues for quantum sensors for quantum computation and quantum communication".
Posted By Graphene Council,
Thursday, July 23, 2020
Surface plasmons in graphene have been widely studied in the past decade due to their very appealing properties, such as the strong tunability of its optical properties through electrical gating and the relatively high plasmon lifetime. However, these exceptional properties are limited to lower frequencies ranging from the mid-infrared (mid-IR) to the terahertz (THz) spectral regions. Additionally, electrical tunability of graphene cannot be achieved in an ultrafast manner, what poses an obstacle for its application in high-speed technological devices that are becoming increasingly important.
In a new paper published in Light Science & Application, a team from ICFO-Institut de Ciencies Fotoniques (Barcelona, Spain) has proposed an all-optical technique to modulate the plasmonic response of graphene- and/or thin-metal-based systems in an ultrafast manner, in a spectrum ranging from mid-infrared to visible (vis-NIR) frequencies. They propose a pump-probe setup where an ultrafast and very intense pump beam is used to heat the electrons of the graphene. Based on the low heat capacity of this 2D material -meaning that a small amount of energy absorbed by this material can induce a large increase in the temperature of its electrons- and on the strong dependence of graphene's conductivity with its electronic temperature, the optical properties of the system will be modulated by the electronic temperature increase, and this can be measured by the probe beam.
Interestingly, this technique can be used to all-optically excite plasmons not only in the graphene sheet, but also in a thin metallic layer placed nearby it. Following a previous work by the same group, they propose to do so by engineering a pump beam such that its wave-front intensity varies spatially in a periodic manner. As such, the electronic temperature in graphene (and subsequently its conductivity) also varies locally in the surface of the sheet, acting as an effective grating that scatters the probe beam and couples it into plasmons. Depending on the wavelength of the probe beam and the presence of a metallic thin film nearby the graphene sheet, this technique can be used to excite either graphene plasmons (mid-IR), metallic plasmons (vis-NIR) or hybrid acoustic plasmons (THz). "In this way, one can excite and manipulate plasmons in a wide spectral range without the need for lateral patterning or using external devices, like SNOM tips, to couple propagating light into plasmons" the authors added.
On a different note, the authors propose to employ nanoscale photothermal effects in order to achieve ultrafast modulation of light. They envision a structure composed of a thin metallic grating on top of a graphene sheet doped to some Fermi level. Then, by increasing the temperature of the graphene electrons via a pump beam, the chemical potential of graphene will decrease, and the interband transitions in graphene will become significant at lower energies, and will quench the plasmonic peak measured by the reflection of a probe beam. "The temperature of graphene electrons can achieve several thousands of Kelvins, resulting in a damping of the reflection peak up to 70%", the authors claim. A similar effect can be observed in graphene acoustic plasmons, but in this case the reason for the quenching is the increasing of the graphene inelastic losses with the electronic temperature. "In both cases, the modulation of the optical response is ultrafast, unlike alternative ways to modulate the response, such as electrically changing the Fermi level of graphene", the authors added.
"Our study opens a promising avenue toward the active photothermal manipulation of the optical response in atomically thin materials with potential applications in ultrafast light modulation", the authors conclude.
Miniaturization has enabled so many unfathomable dreams. Shrinking down electronic circuits has allowed us to access technology like smartphones, health watches, medical probes, nano-satellites, unthinkable a couple decades ago. Just imagine that in the course of 60 years, the transistor has gone from being the size of your hand palm to 14 nanometers in dimension, 1000 times smaller than the diameter of a hair.
Miniaturization has pushed technology to a new era of optical circuitry. But, in parallel, it has also triggered new challenges and obstacles to overcome, for example, on how to deal with controlling and guiding light at the nanometer scale. New techniques have been on the rise searching for ways to confine light into extremely tiny spaces, millions of times smaller than current ones. Researchers had earlier on found that metals can compress light below the wavelength-scale (diffraction limit).
In that aspect, Graphene - a material composed from a single layer of carbon atoms, with exceptional optical and electrical properties, is capable of guiding light in the form of "plasmons", which are oscillations of electrons that are strongly interacting with light. These graphene plasmons have a natural ability to confine light to very small spaces. However, until now it was only possible to confine these plasmons in one direction, while the actual ability of light to interact with small particles, like atoms and molecules, resides in the volume that it can be compressed into. This type of confinement, in all three dimensions, is commonly regarded as an optical cavity.
In a recent study published in Science, ICFO researchers Itai Epstein, David Alcaraz, Varum-Varma Pusapati, Avinash Kumar, Tymofiy Khodkow, led by ICREA Prof. at ICFO Frank Koppens, in collaboration with researchers from MIT, Duke University, Université Paris-Saclay, and Universidad do Minho, have succeeded to build a new type of cavity for graphene plasmons, by integrating metallic cubes of nanometer sizes over a graphene sheet. Their approach enabled to realize the smallest optical cavity ever built for infrared light, which is based on these plasmons.
In their experiment they used silver nanocubes of 50 nanometers in size, which were sprinkled randomly on top of the graphene sheet, with no specific pattern or orientation. This allowed each nanocube, together with graphene, to act as a single cavity. Then they sent infrared light through the device and observed how the plasmons propagated into the space between the metal nanocube and the graphene, being compressed only to that very small volume.
As Itai Epstein, first author of the study, comments, "the main obstacle that we encountered in this experiment resided in the fact that the wavelength of light in the infrared range is very large and the cubes are very small, about 200 times smaller, so it is extremely difficult to make them interact with each other."
In order to overcome this, they used a special phenomenon - when the graphene plasmons interacted with the nanocubes, they were able to generate a special resonance, called a magnetic resonance. As Epstein clarifies, "A unique property of the magnetic resonance is that it can act as a type of antenna that bridges the difference between the small dimensions of the nanocube and the large scale of the light." Thus, the generated resonance maintained the plasmons moving between the cube and graphene in a very small volume, which is ten billion times smaller than the volume of regular infrared light, something never achieved before in optical confinement. Even more so, they were able to see that the single graphene-cube cavity, when interacting with the light, acted as a new type of nano-antenna that is able to scatter the infrared light very efficiently.
The results of the study are extremely promising for the field of molecular and biological sensing, important for medicine, biotechnology, food inspection or even security, since this approach is capable of intensifying the optical field considerably and thus detect molecular materials, which usually respond to infrared light.
As Prof. Koppens states "such achievement is of great importance because it allows us to tune the volume of the plasmon mode to drive their interaction with small particles, like molecules or atoms, and be able to detect and study them. We know that the infrared and Terahertz ranges of the optical spectrum provide valuable information about vibrational resonances of molecules, opening the possibility to interact and detect molecular materials as well as use this as a promising sensing technology".
Posted By Graphene Council, The 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."
Posted By Graphene Council, The Graphene Council,
Thursday, September 26, 2019
European Commission funded research project, the Graphene Flagship, will demonstrate a selection of the project’s most exciting innovations at the Science is Wonderful exhibition in Brussels, Belgium, on 25 and 26 September 2019. The free exhibition, held at Tour & Taxis, a redeveloped industrial space in Brussels, is part of the European Research and Innovation Days and aims to bring a world of science and technology to the general public.
Demonstrations from the Graphene Flagship include technology that has been developed for human health and wellbeing. For example, a graphene-based brain implant that could be used to provide information on the onset of seizures. The new technology, which has been developed by Graphene Flagship partners the Microelectronic Institute of Barcelona (IMB-CNM, CSIC), the Catalan Institute of Nanoscience and Nanotechnology (ICN2) and ICFO, demonstrates a major step in understanding the functions of the human brain.
The exhibition will also showcase examples of graphene dispersions and graphite electrodes manufactured by Graphene Flagship partner Talga. As a high-tech materials company and a leader in bulk graphene and graphite supply, Talga will demonstrate how graphene can easily be exfoliated from graphite, illustrating the journey from material exfoliation, right through to commercialisation.
Other demonstrations at Science is Wonderful include a newly developed virtual reality (VR) system which can be used to construct, manipulate and build graphene and other layered material structures. Developed by Graphene Flagship partner the Technical University of Denmark (DTU), the VR system demonstrates clearly how graphene can be modified and manipulated, with the ability to edit molecules and perform calculations on their electronic properties in real-time.
The VR system gives students and other citizens an unforgettable, low-barrier to entry for the complex machinery of atomic-scale materials and technology. However, it can also provide even experienced researchers with a unique sandbox for scientific problem solving, quantitative analysis, idea generation and discovery.
“The Graphene Flagship’s presence at Science is Wonderful will bolster its efforts to promote the use of graphene in commercial products,” explained Jari Kinaret, director of the Graphene Flagship. “During the first five years of our ambitious Graphene Flagship project, we managed to bring together academic researchers and industrial business leaders to create and commercialise technologies that are already improving European society — the demonstrations at Science is Wonderful will showcase some beautiful examples.”
Posted By Graphene Council, The Graphene Council,
Wednesday, September 18, 2019
New technological devices are prioritizing non-invasive tracking of vital signs not only for fitness monitoring, but also for the prevention of common health problems such as heart failure, hypertension, and stress related complications, among others. Wearables based on optical detection mechanisms are proving an invaluable approach for reporting on our bodies inner workings and have experienced a large penetration into the consumer market in recent years.
Current wearable technologies, based on non-flexible components, do not deliver the desired accuracy and can only monitor a limited number of vital signs. To tackle this problem, conformable non-invasive optical-based sensors that can measure a broader set of vital signs are at the top of the end-users’ wish list.
In a recent study published in Science Advances ("Flexible graphene photodetectors for wearable fitness monitoring"), ICFO researchers have demonstrated a new class of flexible and transparent wearable devices that are conformable to the skin and can provide continuous and accurate measurements of multiple human vital signs. These devices can measure heart rate, respiration rate and blood pulse oxygenation, as well as exposure to UV radiation from the sun.
While the device measures the different parameters, the read-out is visualized and stored on a mobile phone interface connected to the wearable via Bluetooth. In addition, the device can operate battery-free since it is charged wirelessly through the phone.
“It was very important for us to demonstrate the wide range of potential applications for our advanced light sensing technology through the creation of various prototypes, including the flexible and transparent bracelet, the health patch integrated on a mobile phone and the UV monitoring patch for sun exposure. They have shown to be versatile and efficient due to these unique features”, reports Dr. Emre Ozan Polat, first author of this publication.
The bracelet was fabricated in such a way that it adapts to the skin surface and provides continuous measurement during activity (see Figure 1). The bracelet incorporates a flexible light sensor that can optically record the change in volume of blood vessels, due to the cardiac cycle, and then extract different vital signs such as heart rate, respiration rate and blood pulse oxygenation.
Secondly, the researchers report on the integration of a graphene health patch onto a mobile phone screen, which instantly measures and displays vital signs in real time when a user places one finger on the screen (see Figure 2). A unique feature of this prototype is that the device uses ambient light to operate, promoting low-power-consumption in these integrated wearables and thus, allowing a continuous monitoring of health markers over long periods of time.
ICFO’s advanced light sensing technology has implemented two types of nanomaterials: graphene, a highly flexible and transparent material made of one-atom thick layer of carbon atoms, together with a light absorbing layer made of quantum dots. The demonstrated technology brings a new form factor and design freedom to the wearables’ field, making graphene-quantum-dots-based devices a strong platform for product developers.
Dr. Antonios Oikonomou, business developer at ICFO emphasized this by stating that “The booming wearables industry is eagerly looking to increase fidelity and functionality of its offerings. Our graphene-based technology platform answers this challenge with a unique proposition: a scalable, low-power system capable of measuring multiple parameters while allowing the translation of new form factors into products.”
Dr. Stijn Goossens, co-supervisor of the study, also comments that “we have made a breakthrough by showing a flexible, wearable sensing system based on graphene light sensing components. Key was to pick the best of the rigid and flexible worlds. We used the unique benefits of flexible components for vital sign sensing and combined that with the high performance and miniaturization of conventional rigid electronic components.”
Finally, the researchers have been able to demonstrate a broad wavelength detection range with the technology, extending the functionality of the prototypes beyond the visible range. By using the same core technology, they have fabricated a flexible UV patch prototype (see Figure 3) capable of wirelessly transferring both power and data, and operating battery-free to sense the environmental UV-index. continuous monitoring of health markers over long periods of time.
The patch operates with a low power consumption and has a highly efficient UV detection system that can be attached to clothing or skin, and used for monitoring radiation intake from the sun, alerting the wearer of any possible over-exposure.
“We are excited about the prospects for this technology, pointing to a scalable route for the integration of graphene-quantum-dots into fully flexible wearable circuits to enhance form, feel, durability, and performance”, remarks Prof. Frank Koppens, leader of the Quantum Nano-Optoelectronics group at ICFO. “Such results show that this flexible wearable platform is compatible with scalable fabrication processes, proving mass-production of low-cost devices is within reach in the near future.”
Posted By Graphene Council, The Graphene Council,
Tuesday, April 23, 2019
Updated: Saturday, April 20, 2019
Scientists have developed a novel graphene-enabled photodetector that operates at room temperature, is highly sensitive, very fast, has a wide dynamic range and covers a broad range of THz frequencies.
Detecting terahertz (THz) light is extremely useful for two main reasons:
First, 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.
Second, 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 Sebastian Castilla and Dr. Bernat Terres, 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.
The Mobile World Congress (MWC) held annually in Barcelona, Spain is one of the largest technology conferences in the world. For the last three years, the MWC has been hosting the Graphene Pavilion that showcases the research institutes and technologies that they have developed under the EU’s Graphene Flagship.
The Graphene Council visited the Graphene Pavilion last month in Barcelona and we came back with some videos. One of the anchor institutions at the Pavilion is The Institute of Photonics (ICFO) located just outside of Barcelona. The Graphene Council has been speaking to Frank Koppens at ICFO since 2015 about how graphene was impacting photonics and optoelectronics.
In our latest visit with them at MWC this year, we got an update on some of the ways they are applying their technologies to various technologies.
In the one shown in the video below, the researchers have developed ultraviolet (UV) sensors for protecting the wearers from overexposure to the sun.
What the ICFO discovered six years ago was that while graphene generates an electron-hole pair for every single photon the material absorbs generates, it doesn’t really absorb that much light. To overcome this limitation of graphene, they combined it with quantum dots with the hybrid material being capable of absorbing 25 percent of the light falling on it. When you combine this new absorption capability with graphene’s ability to make every photon into an electron-hole pair, the potential for generating current became significant.
The ICFO has been proposing applications like this for this underlying technology for years, and producing working prototypes. At the MWC in 2016, the ICFO was exhibiting a heart rate monitor. In that device, when a finger is placed on the photodetector, the digit acts as an optical modulator, changing the amount of light hitting the photodetector as your heart beats and sends blood through your fingertip. This change in signal is what generates a pulse rate on the screen of the mobile device.
This same basic technology is at the heart of another technology ICFO was exhibiting this year (see video below) in which the graphene-based photodector can determine what kind of milk you are about to drink. This could conceivably be used by someone who has a lactose intolerance that could threaten their lives and by using the detector could determine if it was cow’s milk or soy milk, for instance.
While ICFO goes so far as to discuss prices for the devices, it’s not clear that ICFO is really committed to any of these technologies for its wide-spectrum CMOS graphene image sensor, or not. In the case of the heart monitor, the researchers claimed at the time it was really just intended to demonstrate the capabilities of the technology.
The long-range aim of the technology is to improve the design of these graphene-based image sensors to operate at a higher resolution and in a broader wavelength range. Once the camera is improved, the ICFO expects that will be used inside a smartphone or smart watch. In the meantime, these wearable technologies offer intriguing possibilities and maybe even a real commercial avenue for the technology.
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.
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.