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Smallest cavity for light realized by graphene plasmons

Posted By Graphene Council, Friday, June 12, 2020
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".

Tags:  Duke University. Sensors  Frank Koppens  Graphene  ICFO  Itai Epstein 

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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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New health monitors are flexible, transparent and graphene enabled

Posted By 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.”

Tags:  Antonios Oikonomou  Emre Ozan Polat  Frank Koppens  Graphene  Healthcare  ICFO  nanomaterials  quantum dots  Sensors  Stijn Goossens 

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Graphene gives a tremendous boost to future terahertz cameras

Posted By 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.

Tags:  Bernat Terres  CIC NanoGUNE  Donostia International Physics Center  Frank Koppens  Graphene  ICERA  ICFO  Klaas-Jan Tielrooij  Nanjing University  National Institute for Material Sciences  photodetectors  Sebastian Castilla  University of Ioannina 

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