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Showing posts with label Francisco Guinea. Show all posts
Showing posts with label Francisco Guinea. Show all posts
In recent years, enhanced light-matter interactions through a plethora of dipole-type polaritonic excitations have been observed in two-dimensional (2D) layered materials. In graphene, electrically tunable and highly confined plasmon-polaritons were predicted and observed, opening up opportunities for optoelectronics, bio-sensing and other mid-infrared applications. In hexagonal boron nitride (hBN), low-loss infrared-active phonon-polaritons exhibit hyperbolic behavior for some frequencies, allowing for ray-like propagation exhibiting high quality factors and hyperlensing effects. In transition metal dichalcogenides (TMDs), reduced screening in the 2D limit leads to optically prominent excitons with large binding energy, with these polaritonic modes having been recently observed with scanning near field optical microscopy (SNOM). Here, we review recent progress in state-of-the-art experiments, survey the vast library of polaritonic modes in 2D materials, their optical spectral properties, figures-of-merit and application space. Taken together, the emerging field of 2D material polaritonics and their hybrids provide enticing avenues for manipulating light-matter interactions across the visible, infrared to terahertz spectral ranges, with new optical control beyond what can be achieved using traditional bulk materials.
(Nanowerk News) A new study by an international team of researchers led by the University of Minnesota highlights how manipulation of 2D materials could make our modern day devices faster, smaller, and better. The findings are now online and will be published in Nature Materials ("Polaritons in layered two-dimensional materials").
Two-dimensional materials allow strong light-matter interactions through polaritons. Two-dimensional materials are a class of nanomaterials that are only a few atoms in thickness. Electrons in these materials are free to move in the two-dimensional plane, but their restricted motion in the third direction is governed by quantum mechanics. Research on these nanomaterials is still in its infancy, but 2D materials such as graphene, transition metal dichalcogenides and black phosphorus have garnered tremendous attention from scientists and engineers for their amazing properties and potential to improve electronic and photonic devices. In this study, researchers from the University of Minnesota, MIT, Stanford, U.S. Naval Research Laboratory, IBM, and universities in Brazil, UK and Spain, teamed up to examine the optical properties of several dozens of 2D materials. The goal of the paper is to unify understanding of light-matter interactions in these materials among researchers and explore new possibilities for future research. They discuss how polaritons, a class of quasiparticles formed through the coupling of photons with electric charge dipoles in solid, allow researchers to marry the speed of photon light particles and the small size of electrons. “With our devices, we want speed, efficiency, and we want small. Polaritons could offer the answer,” said Tony Low, a University of Minnesota electrical and computer engineering assistant professor and lead author of the study. By exciting the polaritons in 2D materials, electromagnetic energy can be focused down to a volume a million times smaller compared to when its propagating in free space. “Layered two-dimensional materials have emerged as a fantastic toolbox for nano-photonics and nano-optoelectronics, providing tailored design and tunability for properties that are not possible to realize with conventional materials,” said Frank Koppens, group leader at the Institute of Photonic Sciences at Barcelona, Spain, and co-author of the study. “This will offer tremendous opportunities for applications.”Others on the team from private industry also recognize the potential in practical applications.“ The study of the plasmon-polaritons in two-dimensions is not only a fascinating research subject, but also offers possibilities for important technological applications,” said Phaedon Avoruris, IBM Fellow at the IBM T. J. Watson Research Center and co-author of the study. “For example, an atomic layer material like graphene extends the field of plasmonics to the infrared and terahertz regions of the electromagnetic spectrum allowing unique applications ranging from sensing and fingerprinting minute amounts of biomolecules, to applications in optical communications, energy harvesting and security imaging. ”The new study also examined the possibilities of combining 2D materials. Researchers point out that every 2D material has advantages and disadvantages. Combining these materials create new materials that may have the best qualities of both. “Every time we look at a new material, we find something new,” Low said. “Graphene is often considered a ‘wonder’ material, but combining it with another material may make it even better for a wide variety of applications.” In addition to Low, Avoruris and Koppens, other researchers involved in the study include Andrey Chaves, Universidade Federal do Cearán (Brazil) and Columbia University; Joshua D. Caldwell, U.S. Naval Research Laboratory; Anshuman Kumar, University of Minnesota and Massachusetts Institute of Technology; Nicholas X.Fang, MIT; Tony Heinz, Stanford University; Francisco Guinea, IMDEA Nanociencia and University of Manchester; and Luis Martin-Moreno, University of Zaragoza (Spain).
In the image, plasmon dispersion in graphene on silicon dioxide substrate, reveals coupling with long-lived substrate phonons. They can be excited by patterning graphene into nanoribbons.
The IBM researchers demonstrated that graphene can either be positive or negative depending on its gate bias. The positive is due to a photovoltaic effect and the negative is due to a bolometric effect.
The bolometric effect involves photo-generated carriers that, while propagating across graphene, emit quanta of lattice vibrations called phonons and thereby transfer their energy into the lattice. Heating up the lattice implies enhancing the electron-phonon scattering process and reducing the carrier’s mobility. The IBM researchers discovered this effect was dominant in the photo response of graphene and is what leads to the photocurrent flowing in the opposite direction of the source-drain current.
The research team, which includes Hugen Yan, Tony Low, Wenjuan Zhu, YanqingWu, Marcus Freitag, Xuesong Li, Francisco Guinea, Phaedon Avouris, and Fengnian Xia, began by first studying the fundamental property of plasmons in graphene metamaterials by purely optical methods, revealing important information about its dispersion and damping mechanisms. This knowledge guided them in their design of graphene photodetectors, leading to the first demonstration of a graphene infrared detector driven by intrinsic plasmons.
Graphene’s high mobility and zero gap nature gives it fast optoelectronic response and detection in an extremely broad spectral range from the visible over the infrared and into the terahertz range.
In the visible and near-IR, semiconductors are more efficient in detecting light than graphene because they can have matched bandgaps to a particular spectral window, and because a single layer of graphene absorbs only a small fraction of the incoming light. So it is very unlikely that we will some day be able to buy a cell-phone or camera with a graphene photodetector in it.
However, at lower energies, for example in the mid-IR or terahertz regime, graphene could be much more competitive and provide a unique technology solution. Currently, superconducting transition-edge detectors and bolometers are state of the art in these regimes, and these detectors are very expensive. The absorption in a single layer of graphene can be as high as 40 percent in the terahertz, and the window of high absorption can be moved into the mid-IR by patterning the graphene and harvesting graphene plasmons.
The graphene-based photodetectors, which utilize their intrinsic plasmons, have been demonstrated to yield an order of magnitude improvement in the device’s photo-responsivity in comparison to its non-plasmonic counterpart.
The graphene used in the photodetectors were first grown by CVD on copper foil. Copper was then dissolved in etchant, and finally graphene was transferred to a silicon/silicon oxide chip. The researchers built the graphene photodetector itself by patterning graphene into superlattices of graphene nanoribbons using e-beam lithography. The ribbons widths range from 80 to 200 nm and lateral confinement in ribbons provides the necessary momentum to couple with the graphene plasmons. It is then illuminated with a chopped CO2 infrared laser beam.
The researchers believe that graphene plasmonics could potentially provide a natural platform for a range of technologies in the infrared regime such as light detection and modulation, optical communications, photovoltaics, and spectroscopy.
With this basic understanding of how graphene plasmon disperses, damps, and generates photocurrent, the IBM team is now more confident about this line of research. The merging of graphene plasmonics with optoelectronics is a field that has essentially just began so there remain fundamental and technological issues to resolve.
