Scientists had already theoretically predicted that specifically structured surfaces can turn the wavefronts of light upside down when it propagates along them. "On such surfaces, called hyberbolic metasurfaces, the waves emitted from a point source propagate only in certain directions and with open (concave) wavefronts", explains Javier Alfaro, PhD student at nanoGUNE and co-author of the paper. These unusual waves are called hyperbolic surface polaritons. Because they propagate only in certain directions, and with wavelengths that are much smaller than that of light in free space or standard waveguides, they could help to miniaturize optical devices for sensing and signal processing.
Now, the researchers developed such a metasurface for infrared light. It is based on boron nitride, a graphene-like 2D material, and was selected because of its capability to manipulate infrared light on extremely small length scales, which could be applied for the development of miniaturized chemical sensors or for heat management in nanoscale optoelectronic devices. On the other hand, the researchers succeeded to directly observe the concave wavefronts with a special optical microscope, which have been elusive so far.
Hyperbolic metasurfaces are challenging to fabricate because an extremely precise structuring on the nanometer scale is required. Irene Dolado, PhD student at nanoGUNE, and Saül Vélez, former postdoctoral researcher at nanoGUNE (now at ETH Zürich) mastered this challenge by electron beam lithography and etching of thin flakes of high-quality boron nitride provided by Kansas State University. "After several optimization steps, we achieved the required precision and obtained grating structures with gap sizes as small as 25 nm”, Dolado says. “The same fabrication methods can also be applied to other materials, which could pave the way to realize artificial metasurface structures with custom-made optical properties”, adds Saül Vélez.
To see how the waves propagate along the metasurface, the researchers used a state-of the-art infrared nanoimaging technique that was pioneered by the nanoptics group at nanoGUNE. They first placed an infrared gold nanorod onto the metasurface. “It plays the role of a stone dropped into water”, says Peining Li. The nanorod concentrates incident infrared light into a tiny spot, which launches waves that then propagate along the metasurface. With the help of a so-called scattering-type scanning near-field microscope (s-SNOM) the researchers imaged the waves. “It was amazing to see the images. They indeed showed the concave curvature of the wavefronts that were propagating away form the gold nanorod, exactly as predicted by theory“, says Rainer Hillenbrand, Ikerbasque Professor at nanoGUNE, who led the work.
The results promise nanostructured 2D materials to become a novel platform for hyberbolic metasurface devices and circuits, and further demonstrate how near-field microscopy can be applied to unveil exotic optical phenomena in anisotropic materials and for verifying new metasurface design principles.
The research has been mainly funded by individual fellowship grants of the European Union Marie Sklodowsca-Curie Actions and the pre-doctoral research grants program of the Basque and Spanish Governments, as well as by the National Science Foundation (USA), and has been carried out in line with nanoGUNEs projects within the EU's Graphene Flagship.
This work package deals with exploiting plasmon polaritons in graphene nanostructures for enhanced gas and biosensing. Nanogune specifically does not work on graphene but on hexagonal boron nitride (hBN, a van der Waals material) where instead of plasmons there are phonon polaritons. Plasmon and phonon polaritons in nanostrucutres enhanced the incoming infrared light. When molecules are in the vicinity of the nanostrucutres they absorb more light than without a nanostructure, and thus the transmitted light should have strong absorption signature.
Nanogune is involved in developing infrared and terahertz photodetectors based on the photothermal effect in graphene. Compared to current infrared and terahertz photodetectors, graphene photodetectors could combine the following features: operate at room temperature, at high speed, and lower cost.
FET Flagships Graphene Flagship project has the ambition to take graphene and related materials from the research laboratories to industrial exploitation in a huge range of application areas. In this case, GrapheneCore3 project aims to secure a major role for Europe in the ongoing technological revolution, helping to bring graphene innovation out of the lab and into commercial applications by 2023. In its third core project, the Graphene Flagship gathers over 160 academic and industrial partners from 23 countries, all exploring different aspects of graphene and related materials. Bringing diverse competencies together, the Graphene Flagship facilitates cooperation between its partners, accelerating the timeline for industry acceptance of graphene technologies.
In this three-year phase of the project, the Graphene Flagship expects to advance much further toward the commercialization of graphene and layered materials. While keeping an eye on fundamental research, the Graphene Flagship Core 3 will have a special focus on innovative research to boost graphene-enabled technologies to higher technology readiness levels.
Optical spectroscopy with infrared light, such as Fourier transform infrared (FTIR) spectroscopy, allows for chemical identification of organic and inorganic materials. The smallest objects which can be distinguished with conventional FTIR microscopes have sizes on the micrometre-scale. Scientists at nanoGUNE, however, employed nano-FTIR to resolve objects, which can be as small as a few nanometres.
In nano-FTIR (which is based on near-field optical microscopy), infrared light is scattered at a sharp metallized tip of a scanning-probe microscope. The tip is scanned across the surface of a sample of interest and the spectra of scattered light are recorded using Fourier transform detection principles. Recording of the tip-scattered light yields the sample’s infrared spectral properties and thus the chemical composition of an area located directly below the tip apex. Because the tip is scanned across the sample surface, nano-FTIR is typically considered to be a surface-characterization technique.
Importantly though, the infrared light that is nano-focussed by the tip does not only probe a nanometric area below the tip, but in fact probes a nanometric volume below the tip. Now the researchers at nanoGUNE showed that spectral signatures of materials located below the sample surface can be detected and chemically identified up to a depth of 100 nm. Furthermore, the researchers showed that nano-FTIR signals from thin surface layers differ from that of subsurface layers of the same material, which can be exploited for determination of the materials distribution within the sample. Remarkably, surface layers and subsurface layers can be distinguished directly from experimental data without involving time-consuming modelling.
Light plays an essential role in modern science and technology, with applications ranging from fast optical communication to medical diagnosis and laser surgery. In many of these applications, the interaction of light with matter is of fundamental importance.
At infrared frequencies, light can interact with molecules via their vibrations that occur at molecule-specific frequencies. For that reason, molecular materials can be identified by measuring their infrared reflection or transmission spectra. This technique, often called infrared fingerprint spectroscopy, is widely used for the analysis of chemical, biological and medical substances.
Recently, it was found that the interaction between infrared light and molecular vibrations can be so strong that eventually the material properties are modified, such as conductivity and chemical reactivity. This effect – called vibrational strong coupling – can occur when a material is placed into a microcavity (typically formed by mirrors that are separated by micrometer-size distances) in which the light is concentrated.
The strength of the interaction between light and matter strongly depends on the amount of matter. Consequently, the interaction weakens when the number of molecules is reduced, challenging infrared spectroscopy applications and eventually preventing strong vibrational coupling to be achieved. This problem can be overcome by concentrating light in nanocavities or by compressing its wavelength, which leads to light confinement.
“A particularly strong compression of infrared light can be achieved by coupling it to lattice vibrations (phonons) of thin layers of high-quality polar crystals. This coupling leads to the formation of infrared waves – so-called phonon polaritons – that propagate along the crystal layer with a wavelength that can be more than ten times smaller than that of the corresponding light wave in free space”, says Andrei Bylinkin, first author of the work.
Now, the researchers have studied the coupling between molecule vibrations and propagating phonon polaritons. First, they placed a thin layer of hexagonal boron nitride (less than 100 nm thick) on top of organic molecules. Hexagonal boron nitride is a van der Waals crystal from which thin high-quality layers can be easily obtained by exfoliation. Next, it was necessary to generate phonon polaritons in the thin boron nitride layer. “This cannot be achieved by just shining infrared light onto the boron nitride layer, because the momentum of light is much smaller than the momentum of the phonon polaritons”, says Andrei Bylinkin.
The problem of the momentum mismatch was solved with the help of the sharp metal tip of a scanning near-field microscope, which acts as an antenna for infrared light and concentrates it to a nanoscale infrared spot at the tip apex that provides the necessary momentum to generate phonon polaritons. The microscope also plays a second important role. “It allowed us for imaging the phonon polaritons that propagate along the boron nitride while interacting with the nearby organic molecules”, says Rainer Hillenbrand who led the study. “That way we could observe in real space how the phonon polaritons couple with the molecular vibrations, thereby forming hybrid polaritons”, he added.
The set of images that were recorded at various infrared frequencies around the resonance of the molecular vibrations revealed various fundamental aspects. The hybrid polaritons are strongly attenuated at the frequency of the molecular vibration, which could be interesting for future on-chip sensing applications. The spectrally resolved images also showed that the waves propagate with negative group velocity, and most important, that the coupling between the phonon polaritons and the molecular vibrations is so strong that it falls into the regime of vibrational strong coupling.
“With the help of electromagnetic calculations we could confirm our experimental results, and further predict that strong coupling should be possible even between few atom thick layers of boron nitride and molecules”, says Alexey Nikitin.
The possibility of strong vibrational coupling on the extreme nanometer scale could be used in the future for development of ultrasensitive spectroscopy devices or to study quantum aspects of strong vibrational coupling that have been not accessible so far.
Rainer Hillenbrand received the award from the hands of Prof. Dressel, chairman of the selection committee, during the LEES 2014 conference in Loire Valley (France) for his pioneering and world-leading developments in the field of optical near-field microscopy. Hillenbrand studied at the University of Augsburg and developed his PhD research at the Max-Planck Institute of Biochemistry in Martinsried, both in Germany. During his PhD, Hillenbrand and his colleagues developed a novel scanning near-field optical microscope for background-free amplitude and phase resolved optical imaging with nanoscale spatial resolution, which they named scattering-type scanning near-field microscopy (s-SNOM). This development was a breakthrough in the field and inspired many groups worlwide to start working in this direction. He continued working at the same German institute and built up his own independent junior research group. Further developments made possible to found the start-up company Neaspec in 2007, which has been the first company offering commercial s-SNOM systems.
In 2008, Hillenbrand joined nanoGUNE as the group leader of the Nanooptics group and Ikerbasque Research Professor. He has applied his technique to perform cutting-edge research in different fields, including fundamental solid-state physics, materials science, life science, and nanophotonics. Recent developments include nanoscale infrared Fourier transform spectroscopy (nano-FTIR), which enables infrared spectroscopy with a more than 100 times improved spatial resolution compared to conventional infrared spectroscopy systems. Hillenbrand currently applies the techniques for chemical identification of nanomaterials, protein studies and the development of graphene-based nanophotonics.
According to the price committee, the revolutionary surface-imaging technique allows to perform spectrally resolved measurements from the visible to the microwave regime with an unprecedented spatial resolution. Hillenbrand work “reveals a unique combination of engineering skills and deep insights into the scientific problems of the various topics, addressing both fundamental questions and applications reaching all the way to industrial maturity”.
The Ludwig-Genzel-Prize is awarded to a young scientist for exceptional contributions to the field of condensed-matter spectroscopy. Related to Ludwig Genzel’s scientific oeuvre the focus is on the far-infrared spectral range. The award contains a diploma and prize money of 4000 €. Bruker Optics (Ettlingen) is the sponsor of this prize. The prize is awarded every two years during the International Conference on Low Energy Electrodynamics in Solids (LEES). The selection is made by a committee. The current members of the selection committee are: Martin Dressel —chairman— (Univ. Stuttgart, Germany), Leonardo Degiorgi (EHT Zürich, Switzerland), Jan Petzelt (Academy of Sciences, Praha, Czech Republic), Karl Renk (Univ. Regensburg, Germany) and Hartmut Roskos (Univ. Frankfurt, Germany).
Proteins are basic building blocks of life. The chemistry and structure of proteins are essential for their biological function. Indeed, the structure of proteins determines their mechanical and catalytic properties (e.g. enzymes). Such functions literally shape all living beings. Furthermore, the protein structure also plays a major role in many diseases. For example, the secondary structure of a protein (whether it has helical (alpha-) or sheet-like (beta-) internal substructures) is highly relevant in the pathogenous mechanism leading to Alzheimer, Parkinson, and other neuro-degenerative diseases. Although a variety of methods have been developed to study the protein chemistry and structure, recognizing and mapping the secondary structure on the nanometer scale, or even with single protein sensitivity, is still a major challenge. A new infrared spectroscopy technique, called nano-FTIR, has now enabled nanoscale chemical imaging and probing of protein’s secondary structure with enormous sensitivity.
nano-FTIR is an optical technique that combines scattering-type scanning near-field optical microscopy (s-SNOM) and Fourier transform infrared (FTIR) spectroscopy. The latter is a tool often used for studying secondary structure of proteins that, however, does not allow for nanoscale mapping of proteins by itself. In nano-FTIR, a sharp metalized tip is illuminated with a broadband infrared laser beam, and the backscattered light is analyzed with a specially designed Fourier transform spectrometer. With this technique, the researchers could now demonstrate local infrared spectroscopy of proteins with a spatial resolution of less than 30 nm.
“The tip acts as an antenna for infrared light and concentrates it at the very tip apex. The nanofocus at the tip apex can be thus considered as an ultra-small infrared light source. It is so small that it only illuminates an area of about 30×30 nm, which is the scale of large protein complexes”, says project leader Rainer Hillenbrand.
In order to demonstrate the versatility of nano-FTIR for nanoscale-resolved protein spectroscopy, the researchers measured infrared spectra of single viruses, ferritin complexes, purple membranes and insulin fibrils. “They all exhibit variations of their secondary structure – describes Iban Amenabar, who performed the nanospectroscopy experiments-; viruses and ferritin are mainly made of alpha-helical structures, while insulin fibrils are mainly made of beta-sheet structures”. Simon Poly, the biologist in the team, explains that “in a mixture of insulin fibrils and few viruses, standard FTIR spectroscopy did not reveal the presence of the alpha-helical viruses. By probing the protein nanostructures one by one with nano-FTIR we could clearly identify the virus, that is the alpha-helical structures within the beta-sheet ones”.
An important aspect of enormous practical relevance is that the nano-FTIR spectra of proteins match extremely well with conventional FTIR spectra, while the spatial resolution is increased by more than 100. “We could measure infrared spectra of even single ferritin particles. These are protein complexes of only 24 proteins. The mass of one ferritin complex is extremely small, only 1 attogram, but we could clearly recognize its alpha-helical structure”, says Amenabar.
The researchers also studied single insulin fibrils, which are a model system for neurodegenerative diseases. It is known that insulin fibrils have a core of beta-sheet structure but their complete structure is still not fully clarified. “In nano-FTIR spectra of individual fibrils we recognized not only beta-sheet structure, but also alpha-helical structures, which might be of relevance for fibril association,” says Alexander Bittner, leader of the Self-Assembly Group at nanoGUNE.
“We are excited about the novel possibilities that nano-FTIR offers. With sharper tips and improved antenna function, we also hope to obtain infrared spectra of single proteins in the future. We see manifold applications, such as studies of conformational changes in amyloid structures on the molecular level, the mapping of nanoscale protein modifications in biomedical tissue or the label-free mapping of membrane proteins. This could lead to a new era in infrared nano-bio-spectroscopy”, concludes Rainer Hillenbrand, head of the Nanooptics Group at nanoGUNE.
I. Amenabar, S. Poly, W. Nuansing, E. H. Hubrich, A. A. Govyadinov, F. Huth, R. Krutokhvostov, L. Zhang, M. Knez, J. Heberle, A.M. Bittner and R. Hillenbrand. “Structural analysis and mapping of individual protein complexes by infrared nanospectroscopy” Nature Communications, 2013, DOI: 10.1038/ncomms3890
Figure: Illustration of infrared protein nano-spectroscopy. A metal tip (yellow) is illuminated with infrared light. Due the antenna function of the tip, the light is concentrated at the tip apex and creates a nanofocus, which illuminates the proteins. Copyright: CIC nanoGUNE
Optical circuits and devices could make signal processing and computing much faster. “However, although light is very fast it needs too much space”, explains Rainer Hillenbrand, Ikerbasque Professor at nanoGUNE and the UPV/EHU. In fact, propagating light needs at least the space of half its wavelength, which is much larger than state-of-the-art electronic building blocks in our computers. For that reason, a quest for squeezing light to propagate it through nanoscale materials arises.
The wonder material graphene, a single layer of carbon atoms with extraordinary properties, has been proposed as one solution. The wavelength of light captured by a graphene layer can be strongly shortened by a factor of 10 to 100 compared to light propagating in free space. As a consequence, this light propagating along the graphene layer – called graphene plasmon – requires much less space.
However, transforming light efficiently into graphene plasmons and manipulating them with a compact device has been a major challenge. A team of researchers from nanoGUNE, ICFO and Graphenea – members of the EU Graphene Flagship – now demonstrates that the antenna concept of radio wave technology could be a promising solution. The team shows that a nanoscale metal rod on graphene (acting as an antenna for light) can capture infrared light and transform it into graphene plasmons, analogous to a radio antenna converting radio waves into electromagnetic waves in a metal cable.
“We introduce a versatile platform technology based on resonant optical antennas for launching and controlling of propagating graphene plasmons, which represents an essential step for the development of graphene plasmonic circuits”, says team leader Rainer Hillenbrand. Pablo Alonso-González, who performed the experiments at nanoGUNE, highlights some of the advantages offered by the antenna device: “the excitation of graphene plasmons is purely optical, the device is compact and the phase and wavefronts of the graphene plasmons can be directly controlled by geometrically tailoring the antennas. This is essential to develop applications based on focusing and guiding of light”.
The research team also performed theoretical studies. Alexey Nikitin, Ikerbasque Research Fellow at nanoGUNE, performed the calculations and explains that “according to theory, the operation of our device is very efficient, and all the future technological applications will essentially depend upon fabrication limitations and quality of graphene”.
Based on Nikitin´s calculations, nanoGUNE’s Nanodevices group fabricated gold nanoantennas on graphene provided by Graphenea. The Nanooptics group then used the Neaspec near-field microscope to image how infrared graphene plasmons are launched and propagate along the graphene layer. In the images, the researchers saw that, indeed, waves on graphene propagate away from the antenna, like waves on a water surface when a stone is thrown in.
Graphic representation of the refraction of graphene plasmons – launched by a tiny gold antenna – when passing through a one-atom-thick prism
In order to test whether the two-dimensional propagation of light waves along a one-atom-thick carbon layer follow the laws of conventional optics, the researchers tried to focus and refract the waves. For the focusing experiment, they curved the antenna. The images then showed that the graphene plasmons focus away from the antenna, similar to the light beam that is concentrated with a lens or concave mirror.
The team also observed that graphene plasmons refract (bend) when they pass through a prism-shaped graphene bilayer, analogous to the bending of a light beam passing through a glass prism. “The big difference is that the graphene prism is only two atoms thick. It is the thinnest refracting optical prism ever”, says Rainer Hillenbrand. Intriguingly, the graphene plasmons are bent because the conductivity in the two-atom-thick prism is larger than in the surrounding one-atom-thick layer. In the future, such conductivity changes in graphene could be also generated by simple electronic means, allowing for highly efficient electric control of refraction, among others for steering applications.
Altogether, the experiments show that the fundamental and most important principles of conventional optics also apply for graphene plasmons, in other words, squeezed light propagating along a one-atom-thick layer of carbon atoms. Future developments based on these results could lead to extremely miniaturized optical circuits and devices that could be useful for sensing and computing, among other applications.
P. Alonso-González1, A.Y. Nikitin1,5, F. Golmar1,2, A. Centeno3, A. Pesquera3, S. Vélez1, J. Chen1, G. Navickaite4, F. Koppens4<, A. Zurutuza3, F. Casanova 1,5, L.E. Hueso 1,5 and R. Hillenbrand 1,5. “Controlling grapheme plasmons with resonant metal antennas and spatial conductivity patterns” Science (2014), DOI: 10.1126/science.1253202
- CIC nanoGUNE, 20018 Donostia-San Sebastián, Spain.
- I.N.T.I-CONICET and ECyT-UNSAM, San Martín, Bs. As., Argentina.
- Graphenea SA, 20018 Donostia-San Sebastián, Spain.
- ICFO-Institut de Ciéncies Fotoniques, Mediterranean Technology Park, 08860 Casteldefells, Barcelona, Spain.
- IKERBASQUE, Basque Foundation for Science, 48011 Bilbao, Spain.
The nanoGUNE Cooperative Research Center, located in Donostia-San Sebastian, Basque Country, is a research centre set up with the mission to conduct excellence research into nanoscience and nanotechnology with the aim of increasing the Basque Country’s business competitiveness and economic and social development.
Graphenea is a pioneer graphene production start-up company founded in 2010 by private investors and CIC nanoGUNE. The company produces and commercializes graphene films by Chemical Vapor Deposition technology and graphene powders by Chemical Exfoliation techniques.
ICFO is a young research institution located in Barcelona that aims to advance the very limits of knowledge in Photonics, namely the science and technology of harnessing Light. Its research programs target the global forefront of photonics, and aim to tackle important challenges faced by society at large. ICFO is focused on current and future problems in Health, Energy, Information, Safety, Security and caring for the Environment.
The Scholarships will be of a total amount of 3,000€. This amount is for the whole period and will not be compatible with any other grant or funding awarded for the same purpose. Candidates have to be pre-registered and accepted at the above mentioned Master degrees in order to be eligible for these grants. Interested candidates can find all the information about the offered master projects and the application process following this link.
Besides the grants, nanoGUNE offers Master students coming from any official Master degree the possibility to develop their Master Thesis within one of its research groups.
Hyperbolic materials are very special because they behave like a metal in one direction, but like an insulator in the other. Until now, these materials have been used to fabricate complex nanostructures that permit subwavelength-scale imaging, as well as the focusing and controlling of light at the nanoscale. However, in order to fully exploit their potential, it is necessary to study and understand how light behaves inside them.
The work lays the foundations for studying the precise manner in which light travels through complex optical systems at the subwavelength scale in extremely high levels of detail. Such a capability will be vital for verifying that future nanophotonic devices, perhaps with biosensing or optical computing applications, are functioning as expected.
"The difficulty in performing the reported experiments is the extremely short wavelength of light when it is inside a hyperbolic material” explains Ikerbasque Professor Rainer Hillenbrand, leader of the nanooptics group at nanoGUNE. When light moves inside the material – in our case mid-infrared light in a 135 nm boron nitride slab - it travels in the form of what we call a polariton, where the light is actually coupled to the vibrations of the matter itself".
These polaritons can be considered a double-edged sword to the scientists trying to study them. On the one hand, they squeeze light into much smaller volumes than is normally possible. This is helpful for a wide range of applications that require the manipulation of light in tiny spaces, such as detecting and identifying individual molecules. On the other hand, this ultra-high confinement means that special techniques have to be developed to look at their behavior.
Edward Yoxall, who performed the experiments at nanoGUNE along with Martin Schnell, elaborates: "Because the wavelength of a polariton is so small, we cannot use 'conventional' optical equipment, such as lenses and cameras, to image it. Instead, we have to use a special type of microscope." This microscope - a scattering-type scanning near-field infrared microscope - is capable of seeing details 1000 times smaller than a standard infrared microscope, visualizing "objects" of just 10 nanometers.
"But it's not just the spatial resolution that makes tracking polaritons tricky", continues Yoxall. "If we want to see how a polariton moves, we need to detect and track it in both space and time. This can be accomplished by using extremely short flashes of light - or pulses- that are just 100 femtoseconds long." That is an extremely small number; less than one millionth of a millionth of a second. By using these very short flashes in combination with their near-field microscope, the researchers are able to watch the polaritons passing different locations along the boron nitride slab, allowing for measuring their speed.
By using both the space and time information that is gathered during the experiment, the scientists have been able to exactly determine how the polariton was travelling. The time- and space-resolved maps revealed a range of intriguing behaviors of the polaritons, including a dramatic slowing down of the pulse velocity - below 1 percent of the light velocity in vacuum- and a reversal of the direction in which the polariton waves were propagating in relation to the direction of the energy flow.
“An exciting result is the speed at which the polariton moves”, says Yoxall. “There’s a lot of interest in slow light, and what we’ve shown here is a novel way of achieving this.” Slow light in conventional photonic structures has great potential for manifold applications in sensing and communication technologies, owing to enhance light-matter interactions. The deep subwavelength-scale confinement of slow polaritons in hyperbolic materials could help to miniaturize these devices.
The work has been published by Nature Photonics (see https://www.nanogune.eu/newsroom/nanolight-edge) and now appeared on the cover of the april 2016 issue.
The CENTINELA project is seeking to develop a system of remote detection that can be used as a basis for the detection and early warning of substances responsible for unpleasant odours in the environs of the refinery. The project has been specifically designed to address the basic needs raised by Petronor.
The detection systems will be developed at the facilities of the CFM/MPC and nanoGUNE and will later be transferred to the University of Burgos where its laboratories are equipped to work on samples of chemical substances in safe conditions. During the final phase, the equipment will be moved to the Petronor facilities for field testing.
Tasks of each principal member
The participant and the collaborators will be carrying out the following functions:
- CFM/MPC (Project and Innovation Management Unit): This is responsible for the work to manage and coordinate the project. It will be participating in the work to define the specifications, design the software and measuring systems, and develop the measuring and pre-industrial-scale production systems.
- CIC nanoGUNE (Nanooptics Group): It is set to participate in developing the measuring systems (Fourier transform spectrometer, optical design, signal processing) and the characterising of them.
- University of Burgos (Instrumental Analysis Group): It will be gathering the spectroscopic data on the substances responsible for the unpleasant odours, characterising the measuring instrument, and developing the multivariate analysis software for automatic detection and early warning.
- Petronor: It will be responsible for the general monitoring of progress. Specification of requirements. Mapping of substance types and potential emission zones.
Radiation in the terahertz (THz) frequency range is attracting large interest because of its manifold application potential for non-destructive imaging, next-generation wireless communication or sensing. But still, the generating, detecting and controlling of THz radiation faces numerous technological challenges. Particularly, the relatively long wavelengths (from 30 to 300 μm) of THz radiation require solutions for nanoscale integration of THz devices or for nanoscale sensing and imaging applications.
In recent years, graphene plasmonics has become a highly promising platform for shrinking THz waves. It is based on the interaction of light with collective electron oscillations in graphene, giving rise to electromagnetic waves that are called plasmons. The graphene plasmons propagate with strongly reduced wavelength and can concentrate THz fields to subwavelength-scale dimensions, while the plasmons themselves can be controlled electrically.
Now, researchers at CIC nanoGUNE (San Sebastian, Spain) in collaboration with ICFO (Barcelona, Spain), IIT (Genova, Italy) - members of the EU Graphene Flagship - Columbia University (New York, USA), Radboud University (Nijmegen, Netherlands), NIM (Tsukuba, Japan) and Neaspec (Martinsried, Germany) could visualize strongly compressed and confined THz plasmons in a room-temperature THz detector based on graphene. To see the plasmons, they recorded a nanoscale map of the photocurrent that the detector produced while a sharp metal tip was scanned across it. The tip had the function to focus the THz illumination to a spot size of about 50 nm, which is about 2000 times smaller than the illumination wavelength. This new imaging technique, named THz photocurrent nanoscopy, provides unprecedented possibilities for characterizing optoelectronic properties at THz frequencies.
The team recorded photocurrent images of the graphene detector, while it was illuminated with THz radiation of around 100 μm wavelength. The images showed photocurrent oscillations revealing that THz plasmons with a more than 50 times reduced wavelength were propagating in the device while producing a photocurrent.
“In the beginning we were quite surprised about the extremely short plasmon wavelength, as THz graphene plasmons are typically much less compressed”, says former nanoGUNE researcher Pablo Alonso, now at the University of Oviedo, and first author of the work. “We managed to solve the puzzle by theoretical studies, which showed that the plasmons couple with the metal gate below the graphene”, he continues. “This coupling leads to an additional compression of the plasmons and an extreme field confinement, which could open the door towards various detector and sensor applications”, adds Rainer Hillenbrand, Ikerbasque Research Professor and Nanooptics Group Leader at nanoGUNE who led the research. The plasmons also show a linear dispersion – that means that their energy is proportional to their momentum - which could be beneficial for information and communication technologies. The team also analysed the lifetime of the THz plasmons, which showed that the damping of THz plasmons is determined by the impurities in the graphene.
THz photocurrent nanoscopy relies on the strong photothermoelectric effect in graphene, which transforms heat generated by THz fields, including that of THz plasmons, into a current. In the future, the strong thermoelectric effect could be also applied for on-chip THz plasmon detection in graphene plasmonic circuits. The technique for THz photocurrent nanoimaging could find further application potential beyond plasmon imaging, for example, for studying the local THz optoelectronic properties of other 2D materials, classical 2D electron gases or semiconductor nanostructures.
After lunch, a lab tour combined with a scavenger hunt took place. The main goal of this activity was to show the equipment and the labs such that participants can get an idea of the work that is performed in each lab. The activity included an introduction into the work that is conducted in the lab and within the research group. Furthermore, in each lab a riddle, puzzle or experiment had to be solved to get a clue. After all the labs were visited, the sum of the clues led to a final destination.
The event concluded with a poster session where the PhD researchers showed off their work. This part of the workshop was open to the general public as well. In this activity, the PhD researchers had the chance to defend and discuss their work with scientific colleagues from the different research centers in San Sebastian.
On Tuesday, 19 June, we welcomed a group of internship students that will carry out a research project at NanoGUNE during the summer. The director of the center, Jose M. Pitarke, received the students with a presentation talk about nanoGUNE, that was also attended by the researchers that will conduct the students’ projects.
The 11 students come from different universities, among which are, Unibersity of the Basque Country (UPV /EHU), Tecnun, Univeristy of Barcelona (UB), Autonomous University of Barcelona (UAB). This program offers to the students a real experience of work in a research laboratory in order to make it easier for them to take decision about their future professional life.
The students will collaborate and learn with the different research groups at nanoGUNE, such as nanooptics, nanodevices or nanomagnetism. They will carry out a research project for two months following the instructions of a researcher of their group.
Some of them started the internships at the beginning of the month and they have been very involved in the group’s work. "The truth is that we started very suddenly; the very first day they took me to the laboratory," says Amaia Ochandorena, a student of Biochemistry and Molecular Biology at the UPV/EHU.
All the students knew CIC nanoGUNE and stressed that "it is an important research center" and "offers and works with topics of much interest".
For these students, and also for undergraduate students of general, nanoGUNE offers the possibility of collaborating with the center for the completion of final graduate or master thesis projects, for which also opens a call for grants every year.
Through this programme the Basque nanoscience research centre will this summer be receiving about ten new students in their 3rd and 4th years of Physics, Chemistry, Biology and Engineering. For a period of six weeks or two months the young students will be collaborating with nanoGUNE researchers in their research projects on subjects such as electron/spin phenomena and magnetism, nanoscale optics, nanoscale materials and nanobioengineering, among others.
To participate in the summer internship programme any students who are interested will need to submit their applications online via the nanoGUNE website, the deadline being 16 February. Full information relating to the call is available via the nanoGUNE website (www.nanogune.eu)
Research with nanolight based on phonon polaritons has developed considerably in recent years thanks to the use of sheet-structured nanomaterials such as graphene, boron nitride or molybdenum trioxide: the so-called van der Waals materials. Nanolight based on phonon polaritons is very promising because it can live longer than other forms of nanolight, but one of the main drawbacks to the technological applications of this nanolight based on phonon polaritons is the limited frequency ranges characteristic of each material, it exists only in narrow frequency region.
But now, an international team has proposed a novel method that allows to widely extend this range of working frequencies of phonon polaritons in van der Waals materials. This consists in the intercalation of alkaline and alkaline earth atoms, such as sodium, calcium or lithium, in the laminar structure of the van der Waals vanadium pentaoxide material, thus allowing to modify its atomic bonds and consequently its optical properties.
Considering that a large variety of ions and ion contents can be intercalated in layered materials, on-demand spectral response of phonon polaritons in van der Waals materials can be expected, eventually covering the whole mid-infrared range, something critical for the emerging field of phonon polariton photonics.
The finding, published in the journal Nature Materials, will allow progress in the development of compact photonic technologies, such as high-sensitivity biological sensors or information and communication technologies at the nanoscale.
Due to their size, the physical properties of nanoparticles are very different from those of macroscopic materials, even when they have the same chemical composition. Therefore, the use of nanoparticles to administer drugs has revolutionised the fields of both nanotechnology and medicine.
'When administering a drug, in order to guarantee efficient performance, it is vital to ensure it is effectively delivered to the right place. Nanoparticles are able to store and transport drugs to the required site in an effective manner. It is best to load them with the maximum possible quantity of the drug in question, providing the particle remains stable,' explains Iban Amenabar, a researcher with nanoGUNE's Nanoóptica team. 'To do so, we obviously need techniques to enable us to measure the real quantity of the drug present in the nanoparticles. However, establishing said techniques is no easy matter,' he continues. 'Conventional measurement techniques (such as FTIR–Fourier transform infrared spectroscopy) offer limited spatial resolution and sensitivity, and only enable drug load measurements in samples containing thousands of nanoparticles. Unfortunately, this amount is not representative, since the drug may not be stored or loaded in the nanoparticles, which means it will not be transported and delivered effectively.' Thus, 'in order to determine the real, effective load, it is necessary to measure the amount of the drug in individual nanoparticles. To do this, we use the nanoscale Fourier transform infrared spectroscopy (nano-FTIR) technique, which was developed at nanoGUNE. This novel technique enables us to measure both optical images and absorption spectra with nanometric resolution and sensitivity. Specifically, the collaboration has enabled us to measure and identify drugs in individual nanoparticles for the very first time,' he adds.
The nano-FTIR technique combines the analytical capability (or chemical information) of Fournier transform infrared spectroscopy (FTIR) with the nanoscale spatial resolution of atomic force microscopy (AFM).
'As a result of the collaboration project, we were able to demonstrate the potential of the novel spectroscopy technique for analysing nanoparticles. Thanks to this technique, we can now measure and identify the drugs present in individual nanoparticles, based on the formulations developed by Kusudama Therapeutics S.L for treating various different types of lung disease,' explains Iban Amenabar.
It is important to highlight the fact that this collaboration has opened up new avenues of research aimed at ensuring greater control over nanoparticles with a wider variety of drugs.
In this particular case, the aid provided by Fomento de San Sebastián was channelled through the Technological Bonds programme, set up in collaboration with CIC nanoGUNE and other entities to promote and develop projects with a strong technological component among local companies and entrepreneurs. The programme seeks to foster the transfer of the technology and knowledge generated in research centres based in San Sebastián.
Dr. Hillenbrand has a successful track record in the field of Scanning Near Field Optical Microscopy (SNOM) having developed the so-called scattering-type scanning near-field optical microscopy (s-SNOM). He was the prizewinner of the “Young Scientist Competition in Nanotechnology 2002”, awarded by the Bundesministerium für Bildung und Forschung (BMBF). Dr. Rainer Hillenbrand is an Ikerbasque Senior Researcher.
Illuminating the sample with THz radiation of 118 μm (2,5 THz) they have been able to resolve details as small as 40 nm (0,04 μm). Therefore, this THz microscope breaks the diffraction barrier by a factor of 1500. The THz near-field microscopy does not only allow for nanoscale imaging of materials, but it can be also used for recognition of mobile carriers in semiconductor nanodevices. This opens the door to quantitative studies of local carrier concentration and mobility at the nanometer scale. Hitherto, no powerful metrology tools are available allowing for simultaneous and quantitative mapping of both materials and carrier concentrations with nanoscale resolution. The added values of seeing and even quantifying conducting carriers opens an enormous industrial application potential for the THz near-field microscope. Future improvements could allow for THz characterization of even single electrons or biomolecules. The results have been published today in Nanoletters.
Visualizing strain at length scales below 100 nm is a key requirement in modern metrology because strain determines the mechanical and electrical properties of high-performance ceramics or modern electronic devices, respectively. The non-invasive mapping of strain with nanoscale spatial resolution, however, is still a challenge.
A promising route for highly sensitive and non-invasive mapping of nanoscale material properties is scattering-type Scanning Near-field Optical Microscopy (s-SNOM). Part of the team had pioneered this technique over the last decade, enabling nanoscale resolved chemical recognition of nanostructured materials and local conductivity in industrial semiconductor devices. The technique makes use of extreme light concentration at the sharp tip of Atomic Force Microscope (AFM), yielding infrared and terahertz images with a spatial independent of the wavelength. The s-SNOM thus breaks the diffraction barrier throughout the electromagnetic spectrum and with its 20 nm resolving power matches the needs of modern nanoscience and technology.
Now, the research team has provided first experimental evidence that the microscopy technique is capable of mapping local strain and nanocracks in crystals using infrared light. “Compared to other methods such as electron microscopy, our technique offers the advantage of non-invasive and contact-free imaging without the need of special sample preparation” says Andy Huber who performed the experiments. Specific applications of technological interest could be the non-invasive detection of nanocracks before they reach critical dimensions, e.g. in Micro-Electro-Mechanical Systems (MEMS) and ceramics, and the study of crack tip propagation.
By controlled straining of semiconductors, the properties of the free carriers can be designed, which is essential to further shrink and speed-up future computer chips. For both development and quality control, the quantitative and reliable mapping of the carrier mobility is strongly demanded but hitherto no tool has been available. The researchers demonsrate that s-SNOM also offers the intriguing possibility of mapping free-carrier properties such as density and mobility in strained silicon. “Our results thus promise interesting applications of s-SNOM in semiconductor science and technology such as the quantitative analysis of the local carrier properties in strain-engineered electronic nanodevices” says Rainer Hillenbrand, leader of the Nanooptics Laboratory at nanoGUNE.
A. J. Huber, A. Ziegler, T. Köck, and R. Hillenbrand, Infrared nanoscopy of strained semiconductors, Nat. Nanotech., advanced online publication, 11. January 2009. See abstract here.
This problem has now been solved, as shown by researchers from ICFO (Barcelona), in a collaboration with nanoGUNE, CNR/Scuola Normale Superiore (Pisa, Italy) – members of the EU Graphene Flagship – and Columbia University (New York, USA).
Since the discovery of graphene, many other two-dimensional materials have been isolated in the laboratory. One example is boron nitride, a very good insulator. A combination of these two unique two-dimensional materials has provided the solution to the quest for controlling light in tiny circuits and suppression of losses. When graphene is encapsulated in boron nitride, electrons can move ballistically for long distances without scattering, even at room temperature. This new research shows that the graphene/boron nitride material system is also an excellent host for extremely strongly confined light and suppression of plasmon losses.
The research, carried out by ICFO PhD students Achim Woessner and Yuando Gao and postdoctoral fellow Mark Lundeberg, is just the beginning of a series of discoveries on nano-optoelectronic properties of new heterostructures based on combining different kinds of two-dimensional materials. The material heterostructure was first discovered by the researchers at Columbia University.
Ikerbasque Professor Rainer Hillenbrand, nanoGUNE’s Nanooptics group leader, comments: “Now we can squeeze light and at the same time make it propagate over significant distances through nanoscale materials. In the future, low-loss graphene plasmons could make signal processing and computing much faster, and optical sensing more efficient.”
The research team also performed theoretical studies. Marco Polini, from CNR/Scuola Normale Superiore (Pisa) and the IIT Graphene Labs (Genova, Italy), laid down a theory and performed calculations together with their collaborators.
These findings pave the way for extremely miniaturized optical circuits and devices that could be useful for optical and/or biological sensing, information processing or data communications.
A. Woessner,M. B. Lundeberg, Y. Gao, A. Principi, P. Alonso-González, M. Carrega, K. Watanabe, T. Taniguchi, G. Vignale, M. Polini, J. Hone, R. Hillenbrand, and F. H. L. Koppens
Nature Materials (2014) Highly confined low-loss plasmons in graphene–boron nitride heterostructures
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nanoGUNE researchers Pablo Alonso González and Ikerbasque Prof. Rainer Hillenbrand, together with ICFO researchers Achim Woessner, Mark B. Lundeberg, Gabriele Navickaite, Davide Janner, ICREA Prof. Valerio Pruneri and ICREA Prof. Frank Koppens (leader oft the international collaboration), developed a noninvasive technique for efficient characterization of the optoelectronic properties of graphene devices at the nanoscale.
Large-scale integration of electronic and optoelectronic graphene devices is becoming a reality. However, to achieve a high device performance, nanoscale and atomic scale imperfections such as grain boundaries, charge density inhomogeneties or additional doping near metal contacts need to be minimized or even eliminated.
The current techniques used for nanoscale graphene device characterization are a major challenge, as most of them are invasive, require specifically designed device structures, can only image small areas, or lack nanometer-scale resolution.
Now, the team of researchers developed infrared photocurrent nanoscopy. This technique combines scanning near-field infrared nanoscopy with electrical read-out, allowing infrared photocurrent mapping at length scales of tens of nanometres. It enables non-invasive and nanoscale probing of the electrical and optical properties of graphene devices at room temperature.
The new technique opens a path for easy, non-invasive and efficient optoelectronic characterization of graphene devices to be integrated in electronic and optoelectronic technologies.
The absorption of infrared radiation is characteristic for the chemical composition and structure of materials. For this reason, an infrared spectrum can be considered as a material’s “fingerprint”. Infrared spectroscopy has thus become an important tool for characterizing and identifying materials and is widely applied in different sciences and technologies including materials sciences and biomedical diagnostics. However, with conventional optical instruments, such as Fourier-transform (FTIR) infrared spectrometers, the light cannot be focused to spot sizes below several micrometers. This fundamental limitation prevents infrared-spectroscopic mapping of single nanoparticles, molecules or modern semiconductor devices.
Figure: Infrared nanospectroscopy with a thermal source. The sketch shows an atomic force microscope tip probing a sample. The tip is illuminated with the broadband infrared radiation from of a thermal source and the backscattered light is analyzed with a Fourier-transform spectrometer, yielding local infrared spectra with a spatial resolution better than 100 nm. The displayed graph shows local infrared spectra of differently processed oxides in an industrial semiconductor device. Copyright F. Huth, CIC nanoGUNE.
Researchers at nanoGUNE and Neaspec have now developed an infrared spectrometer that allows for nanoscale imaging with thermal radiation. The setup – in short nano-FTIR (see Figure) – is based on a scattering-type near-field microscope (NeaSNOM, www.neaspec.com) that uses a sharp metallic tip to scan the topography of a sample surface. While scanning the surface, the tip is illuminated with the infrared light from a thermal source. Acting like an antenna, the tip converts the incident light into a nanoscale infrared spot (nanofocus) at the tip apex. By analyzing the scattered infrared light with a specially designed FTIR spectrometer, the researchers were able to record infrared spectra from ultra-small sample volumes.
In their experiments, the researchers managed to record infrared images of a semiconductor device from Infineon Technologies AG (Munich). “We achieved a spatial resolution better than 100 nm. This directly shows that thermal radiation can be focused to a spot size that is hundred times smaller than in conventional infrared spectroscopy”, says Florian Huth, who performed the experiments. The researcher demonstrated that nano-FTIR can be applied for recognizing differently processed silicon oxides or to measure the local electron density within complex industrial electronic devices. “Our technique allows for recording spectra in the near- to far-infrared spectral range. This is an essential feature for analyzing the chemical composition of unknown nanomaterials“, explains Rainer Hillenbrand, leader of the Nanooptics group at nanoGUNE.
Nano-FTIR has interesting application potential in widely different sciences and technologies, ranging from semiconductor industry to nanogeochemistry and astrophysics. “Based on vibrational fingerprint spectroscopy, it could be applied for nanoscale mapping of chemical composition and structural properties of organic and inorganic nano-systems, including organic semiconductors, solar cells, nanowires, ceramics and minerals”, adds Florian Huth.
The CIC nanoGUNE Consolider, nanoGUNE in short, is the Basque nanoscience and nanotechnology research center, inaugurated in 2009 in Donostia – San Sebastián, Spain. Neaspec GmbH has been established in 2007 as a spin-off from the Max Planck Institute of Biochemistry (Martinsried, Germany) and is the first supplier of scattering-type near-field optical microscopes. “The fruitful collaboration of research and industry has been the key for the development of nano-FTIR”, concludes Nenad Ocelic, CTO of Neaspec GmbH.
As explained in the abstract, the authors have studied the fundamental optical properties of pure nickel nanostructures by far-field extinction spectroscopy and optical near-field microscopy, providing direct experimental evidence of the existence of particle plasmon resonances predicted by theory. Experimental and calculated near-field maps allow for unambiguous identification of dipolar plasmon modes. By comparing calculated near-field and far-field spectra, they find dramatic shifts between the near-field and far-field plasmon resonance, which are much stronger than in gold nanoantennas. Based on a simple damped harmonic oscillator model to describe plasmonic resonances, it is possible to explain these shifts as due to plasmon damping.
The cover shows the near-field amplitude image of dipolar plasmon modes in nickel nanodisks. Each disk exhibits two bright spots oscillating along the polarization direction of the incident light, revealing the enhanced near-field at the rims of the nickel disks. The image was recorded by a scattering-type scanning near-field microscope (s-SNOM) within a study of the optical and magnetic properties of nickel nanostructures. For more information, please read the Full Paper Plasmonic Nickel Nanoantennas
Original publication: J. Chen, P. Albella, Z. Pirzadeh, P. Alonso-González, F. Huth, S. Bonetti, V. Bonanni, J. Åkerman, J. Nogués, P. Vavassori, A. Dmitriev, J. Aizpurua, and R. Hillenbrand, Plasmonic Nickel Nanoantennas, Small, doi: 10.1002/smll.201100640
The article reports the development of an instrument that allows for recording infrared spectra with a thermal source at a resolution that is 100 times better than in conventional infrared spectroscopy. In future, the technique could be applied for analyzing the local chemical composition and structure of nanoscale materials in polymer composites, semiconductor devices, minerals or biological tissue (F. Huth et al., Nature Materials 10, 352). Read a previous highlight on this article .
The Center for NanoScience was founded in 1998 at the Ludwig-Maximilians-Universität (LMU) in Munich as one of the first nanoscience networks worldwide. Today, CeNS crosslinks the work of more than 100 senior and junior scientists and 200 PhD and Diploma students. The CeNS members share their extended interdisciplinary knowledge of physics, chemistry, biology, medicine and pharmacy to promote progress in nanosciences and nano-bio-sciences.
Each year, CeNS awards prizes for excellent publications of CeNS members which have been published during the past 12 months. This year, the candidates submitted numerous articles which appeared in high-impact journals between October 2010 and October 2011. With this award, remarkably successful cooperation projects within CeNS as well as outstanding research of an individual research group of CeNS are distinguished.
Nature Communications: Resolving the electromagnetic mechanism of surface-enhanced light scattering at single hot spots
(a) Inelastic scattering process (ω1≠ω2) from an object (O) in the presence of a metal nanostructure that acts as an optical antenna (A). (b) Elastic scattering process (ω1=ω2=ω). Einc(ω) denotes the incident field, EA(ω) is the field directly radiated by the antenna, and EAOA is the field radiated by the object via the antenna. To select EAOA, the antenna–object distance d is modulated at frequency Ω, and the detector signal is demodulated at the higher harmonic frequency nΩ.
The results provide a rigorous verification of the widely accepted electromagnetic enhancement mechanism in surface- and antenna-enhanced spectroscopy, more than 30 years after it was proposed to explain the electromagnetic contribution in SERS. It is also established that the enhancement mechanism for elastic and inelastic (Raman) scattering is the same, and thus generalize the electromagnetic mechanism of SERS. Furthermore, the verification of the phase shift involved in the elastic scattering process introduces a new paradigm to characterize and manipulate surface-enhanced coherent scattering processes, which might be of fundamental and technological importance in optical information and sensing applications.
The findings might have particular implications in surface-enhanced infrared absorption (SEIRA) spectroscopy, where objects adsorbed on rough metal films or antennas exhibit an enhanced infrared fingerprint spectrum. On the one hand, the scattering from the object, which is usually neglected, might be a significant contribution to the spectrum. On the other hand, the strongly enhanced scattering cross-section might be applied for the development of surface-enhanced infrared scattering spectroscopy.
Original publication: P. Alonso-González, P. Albella, M. Schnell, J. Chen, F. Huth, A. García-Etxarri, F. Casanova, F. Golmar, L. Arzubiaga, L.E. Hueso, J. Aizpurua, and R. Hillenbrand, Nat. Commun. 3 : 684; doi: 10.1038/ncomms1674
“Infrared Nanophotonics based on Metal Antennas and Transmission Lines” NanoGUNE's first PhD Thesis defended by Martin Schnell
The thesis presents a near-field microscope that allows to verify the merging of radiofrequency and plasmonic concepts at infrared frequencies. This near-field microscope is applied to study infrared antennas and transmission lines for the nanofocusing of light and the control of near fields at the nanoscale. Thus, the thesis provides both a valuable tool and new design ideas for the development of novel near-field probes and spectroscopy lab-on-a-chip applications for (bio)medical and chemical sensing.
An international committee including leading researchers in the field was selected by the UPV/EHU to assess the research project:
- Niek van Hulst (ICFO, Spain)
- Paolo Vavassori (nanoGUNE, Spain)
- P. Scott Carney (U. Illinois at Urbana-Champaign, USA)
- Fritz Keilmann (Max Planck Institute für Quantenoptik, Germany)
- Javier Aizpurua (DIPC, Spain)
Martin Schnell joined the Nanooptics Group in 2008 and was involved from the very beginning in the set-up of the nanooptics laboratories of the center, even before the official opening of the facilities in January 2009.
The article reports first ever visualizations of light guided with nanometric precision on graphene (a one-atom-thick sheet of carbon atoms). This visualization proves what theoretical physicists have long predicted; that it is possible to trap and manipulate light in a highly efficient way, using graphene as a novel platform for optical information processing and sensing. Synergies between theoretical proposals from IQFR-CSIC, specializations in graphene nano-photonics and nano-optoelectonics at ICFO, and the experimental expertise in optical nano-imaging at nanoGUNE give rise to these noteworthy results reported in Nature this week in a back-to-back publication alongside a similar study by the group of Dmitry Basov in UCSD in California. Along with nanoGUNE, other Basque research centers, the Center of Material Physics, the Donostia International Physics Center, as well as the Ikerbasque Foundation and nanoGUNE’s start-up company Graphenea, have contributed to this work as well.
Figure: Optical nanoimaging of graphene plasmons. Upper panel: Sketch of the imaging method. A laser illuminated scanning tip launches plasmons on graphene. Detection is by recording the light backscattered from the tip. Lower panel: Optical image of graphene, where the fringes visualize the interference of the graphene plasmons
Graphene is a material that, among many other fascinating properties, has an extraordinary optical behavior. Particularly interesting optical properties had been predicted for the case that light couples to so-called plasmons, wave-like excitations that were predicted to exist in the “sea” of conduction electrons of graphene. However, no direct experimental evidence of plasmons in graphene had been shown up to this work. This is because the wavelength of graphene plasmons is 10 to 100 times smaller than what can be seen with conventional light microscopes. Now, the researchers show the first experimental images of graphene plasmons. They used a near-field optical microscope that uses a sharp tip to convert the illumination light into a nanoscale light spot that provides the extra push needed for the plasmons to be created. At the same time the tip probes the presence of plasmons (see figure). Rainer Hillenbrand, leader of the nanoGUNE group comments: “Seeing is believing! Our near-field optical images definitely proof the existence of propagating and localized graphene plasmons and allow for a direct measurement of their dramatically reduced wavelength.”
Jianing Chen, Michela Badioli, Pablo Alonso-González, Susokin Thongrattanasiri, Florian Huth, Johann Osmond, Marko Spasenović, Alba Centeno, Amaia Pesquera, Philippe Godignon, Amaia Zurutuza, Nicolas Camara, Javier García de Abajo, Rainer Hillenbrand & Frank Koppens
Contributions and institutes:
- Optical nano-imaging: CIC nanoGUNE Consolider (San Sebastian, Spain), CFM-CSIC-UPV/EHU (San Sebastian, Spain), DIPC (San Sebastian, Spain), Neaspec GmbH (Martinsried, Germany), Ikerbasque (Bilbao, Spain)
- Graphene nano-photonics and optoelectronics: ICFO (Barcelona, Spain)
- Theory: IQFR-CSIC (Madrid, Spain)
- Graphene synthesis: Graphenea (San Sebastian, Spain) University of Tours (Tours, France), and CNM-IMB-CSIC (Barcelona, Spain)
Correlative infrared-electron nanoscopy technique allows performing s-SNOM and TEM of one and the same sample.
A joint Basque research team (CIC nanoGUNE and IK4-CIDETEC) presents the method in Nature Communications.
Researchers from the Nanoscience Cooperative Research Center CIC nanoGUNE and IK4-CIDETEC, both located in San Sebastian (Basque Country, Spain), present correlative infrared-electron nanoscopy, a novel nanoimaging technique that allows for a deeper understanding of the interplay between structure, conductivity and chemical composition (J.M. Stiegler et al., Nat. Commun., 2012, DOI: 10.1038/ncomms2118).
The joint Basque research team has developed a new method named correlative infrared-electron nanoscopy for taking s-SNOM – a relatively new technique that allows for nanoscale infrared imaging and spectroscopy – and TEM images of one and the same nanostructure. “We have developed a special sample preparation that allows combining these two techniques using a unique sample”, explains Andrey Chuvilin, the nanoGUNE’s TEM expert. “Thanks to our method we can better understand the properties of materials at the nanometer scale and this opens new avenues for studying widely different material properties and their mutual relationship”, says nanoGUNE’s researcher Johannes Stiegler.
This work is in line with cutting edge research in modern materials science and especially in nanotechnology, where the understanding of the properties of materials at the nanometer scale is an ultimate goal. A variety of high-resolution imaging techniques providing different information about the different material properties exist. It is, however, the combined use of the different characterization techniques that will further the understanding of functional nano materials.
Correlative infrared-electron nanoscopy is a good example of combining different imaging methods. While TEM is a well established imaging technique in materials science, providing structural information with atomic resolution, s-SNOM is a imaging method yielding maps of the chemical composition and conductivity with a spatial resolution of less than 20 nm. “As we demonstrate in our study, the correlation of the TEM and SNOM images will help to obtain a deeper and more comprehensive understanding of the material properties at the nanometer level”, Stiegler states.
In order to demonstrate the potential of correlative infrared-electron nanoscopy the researchers have studied cross-sections of ZnO nanowires using a NeaSNOM from Neaspec GmbH (www.neaspec.com) and the Titan TEM from FEI Company (www.fei.com). These nanowires are of high technological relevance due to their potential to be used as universal electron transport building blocks in different technological applications such as solar cells, light emitting sensors and piezoelectric nanogenerators. Although being of great importance, very little is known about the local conductivity within these wires.
From the infrared s-SNOM images the researchers find a radial conductivity profile, whose origin cannot be explained without further information. The origin of this profile, however, can be explained by studying the local structural properties with TEM, where in the regions of low conductivity the ZnO wire exhibit significant crystal defects. From the two different images an inverse correlation between defect density and free-carrier (conductivity) concentration can be concluded.Ramón Tena-Zaera, Head of Photovoltaics Unit at IK4-CIDETEC, explains that the novel technique “allows us, for the first time, to obtain all this information about one unique sample”. “Our results open new avenues in the growth and device integration of ZnO nanowires. Until now, for example, the scientific community has focused on obtaining nanowires with diameters as thin as possible because it was considered they had more potentiality. However, our results suggest that lateral growth – a bigger diameter – is advantageous to obtain defect-free and high electronic conductivity materials”, says Tena-Zaera.
Although shown for a special sample, “the method of correlated infrared-electron nanoscopy has a much broader applicability – concludes Rainer Hillenbrand, Nanooptics Group Leader at nanoGUNE and coordinator of the study – and has great potential to study novel materials such as graphene, topological insulators, phase change materials, or biological and organic nanostructures”.
Stiegler J.M., Tena-Zaera R., Idigoras O., Chuvilin A. and Hillenbrand R. Correlative infrared–electron nanoscopy reveals the local structure–conductivity relationship in zinc oxide nanowires. Nat. Commun. 3:1131 doi: 10.1038/ncomms2118 (2012).
The results presented in the publication are an overview of the activity currently being conducted in the field. As it is mentioned in the editorial of the Special Issue, “apart from being the thinnest existing material, graphene is very attractive for photonics due to its extreme flexibility, high mobility and the possibility of controlling its carrier concentration (and hence its electromagnetic response) via external gate voltage”. The editorial underlines graphene’s potential for innovative technological applications, such as its already shown capabilities in areas like photodetection, photovoltaics, lasing, etc., as well as other challenges such as complementing (or replacing) the existing semiconductor/metallic photonic platforms.
One of the articles of the Special Issue, entitled “Analytical solution for the diffraction of an electromagnetic wave by a graphene grating”, is a collaboration between nanoGUNE’s researcher Alexey Nikitin, member of the nanooptics group, and researchers from the Materials Science Institute of Aragón and the Department of Condensed Matter Physics at the University of Zaragoza (Spain). A feature about the paper has been published at the “Lab Talk” section of Journal of Optics. In short, the research conducts a comprehensive theoretical analysis of the diffraction of an electromagnetic wave at a periodically structured graphene sheet. An interesting result is the fact that reducing the electron losses inside the graphene sheet increases their ability to be involved in plasmonic oscillations, which results in more photons being absorbed. As expressed in the Lab Talk, “the authors believe that this demonstration can help to fully exploit the potential of graphene in a variety of applications like tunable ultrathin subwavelength antennas, oscillators, amplifiers, photodetectors, etc.”.
T. M. Slipchenko, M. L. Nesterov, L. Martin-Moreno, and A. Yu Nikitin. Analytical solution for the diffraction of an electromagnetic wave by a graphene grating. Journal of Optics 15, 114008 (doi: 10.1088/2040-8978/15/11/114008)
After getting his PhD, he began to work as a post-doctoral researcher in the Nanooptics group of nanoGUNE led by the Ikerbasque researcher Rainer Hillenbrand, where he is now doing his research work. Some of his pieces of work on the optical properties of nanoscale graphene have been published in journals of major international prestige like Nature and Science. In an article recently published in Science, he showed experimentally that confined nanoscale light in graphene obeys the laws of conventional optics, which constitutes the first step towards the development of two-dimensional nanooptics.
RSEF-BBVA Foundation Awards for Physics [www.resef.es]
The Royal Spanish Society for Physics (RSEF), founded in 1903, began to award these prizes in 1958 to recognize creativity and effort and to encourage researchers, young ones in particular. The awards are divided into the following sections: The RSEF Medal, totalling 15,000 euros; New Researcher, in two categories, Theoretical Physics and Experimental Physics, each worth 4,000 euros; Physics Education and Popularisation in two categories, University Education and Secondary Education (8,000 euros each); Physics, Innovation and Technology (8,000 euros); Best Papers in RSEF publications (two prizes, one for education and the other for research, each worth 1,500 euros). The BBVA Foundation has been collaborating with the RSEF since 2007 in these prizes which are awarded every year.
An international committee including leading researchers in the field was selected to assess the research project:
- Dr. Thomas Taubner (Institute of Organic Chemistry RWTH)
- Dr. Alexander Bittner (CIC nanoGUNE)
- Dr. P. Scott Carney (University of Illinois at Urbana-Champaign)
- Dr. Andres Arnau Pino (UPV/EHU)
- Dr. F. Javier Aizpurua Iriazabal (UPV/EHU)
After the defense, we had a little interview with Dr. Huth and we asked him to explain us a bit more about his project:
Which was the subject of your thesis?
My thesis is focused on Nano-FTIR, an innovating technology to increasing the spatial resolution of infrared spectroscopy to enable spectroscopic analysis of nanoscale objects.
Why did you choose this subject?
Because nanotechnology experiences a huge interest in the recent years, creating a demand for novel ultrahigh-resolution techniques such as nano-FTIR.
Which metodology or techniques did you use?
We improved existing technologies such as IR-spectroscopy and Near-field spectroscopy to develop the nano-FTIR system.
Which have been the main conclusions?
The developed Nano-FTIR system achieves >100 times better spatial resolution compared to standard IR-spectroscopy and could be proven to work with a large variety of materials including semiconductors, polymers and organic samples.
What could be the contribution of your research for present or future nanotechnologies?
The new high-resolution analytical capabilities of Nano-FTIR could help understanding and improving a manifold of different material-systems, from nanoscale electronics to molecular biology, or even the nanostructure of stardust.
How do you feel now that you have finished the thesis? Which are your plans for the future?
It has been a great experience and honor to work here in this amazing scientific environment. In the future I will continue to work for Neaspec GmbH, a company that was co-founded by Rainer Hillenbrand, my thesis supervisor. My duties there will be mostly related to the development and improvement of the nano-FTIR system, which is already commercially available since very recently.
In conventional optical instruments, light cannot be focused to spot sizes smaller than half the wavelength because of diffraction effects. An important approach to beat this diffraction limit is based on optical antennas, their name being an allusion to their radiofrequency counterparts. They have the ability to concentrate (focus) light to tiny spots of nanometer-scale dimensions, which are orders of magnitude smaller than what conventional lenses can achieve. Tiny objects such as molecules or semiconductor nanoparticles that are placed into these so-called “hot spots” of the antenna can efficiently interact with light. Thus, optical antennas boost single molecule spectroscopy or the sensitivity of optical detectors. However, the hot spot is bound to the antenna structure, which limits flexibility in designing nanooptical circuits.
The experiments conducted at nanoGUNE now show that infrared light can be transported and nanofocused with miniature transmission lines, consisting of two closely spaced metal nanowires. While lenses and mirrors manipulate light in its form of a free-space propagating wave, transmission lines guide the infrared light in form of a tightly bound surface wave.
The researchers at nanoGUNE adapted the concept of classic transmission lines to the infrared frequency range. Transmission lines are specialized cables for carrying for example radio frequency signals. A simple form consists of two metal wires running closely in parallel, also called ladder line. This structure was widely used in former times for connecting the radio receiver or television set to the rooftop antenna. Applied at MHz frequencies, where typical wavelengths are in the range of centimeters to several meters, it is a prime example for transport of energy in waveguides of strongly subwavelength-scale diameter.
Figure 1:Concept and design of the device. (c) Martin Schnell, nanoGUNE
Figure 2: Near-field microscopy image of the tapered transmission line structure, taken at 9.3 µm wavelength (30 THz). It shows the infrared field intensity along the transmission line, revealing the tiny infrared hot spot at the taper apex. (c) Martin Schnell, nanoGUNE
In their experiments, the researchers demonstrated that infrared light can be transported in the same way, by scaling down the size of the transmission lines to below 1 micrometer (Figure 1). To that end, they fabricated two metal nanowires connected to an infrared antenna. The antenna captures infrared light and converts it into a propagating surface wave traveling along the transmission line. By gradually reducing the width of the transmission line (“tapering”), the researchers demonstrate that the infrared surface wave is compressed to a tiny spot at the taper apex with a diameter of only 60 nm (Figure 2). This tiny spot is 150 times smaller than the free-space wavelength, emphasizing the extreme subwavelength-scale focus achieved in the experiments. The researchers applied their recently introduced near-field microscopy technique (Schnell et al., Nano Lett. 10 3524 (2010)) to map the different electrical field components of the infrared focus with nanoscale resolution.
Nanofocusing of infrared light with transmission lines has important implications in spectroscopy and sensing applications. Connecting a transmission line to the antenna, the infrared light captured by the nanoantenna can be transported over significant distances and nanofocused in a remote place. “This opens new pathways for the development of infrared nanocircuits” says Rainer Hillenbrand leader of the Nanooptics Group at the nanoscience institute nanoGUNE. “It is amazing that the classical radiofrequency concepts still work at infrared frequencies. That is 30 THz!” adds Martin Schnell who performed the experiments.
“Near-field optical microscopy techniques urgently seek for new ways to confine light down to the nanometer scale” explains Rainer Hillenbrand. “The concept of tapered transmission lines is a promising way to do achieve this. Acting as an ultra-small torch, it conducts infrared light exactly to the spot under analysis” says Martin Schnell.
The collaborative environment at nanoGUNE has been one of the keys to the success of the idea. The fabrication of the transmission lines was carried out by members of the Nanodevices Group and the TEM Laboratory, while the infrared transport and focusing functionality was designed and verified in the Nanooptics Group. “It’s great having the tools and expertise you need in the labs right next to yours” says Hillenbrand.
Original publication: M. Schnell, P. Alonso-González, F. Casanova, L. Arzubiaga, L. E. Hueso,
A. Chuvilin, R. Hillenbrand
Nanofocusing of Mid-Infrared Energy with Tapered Transmission Lines
Nature Photonics 5, 283–287,
Graphene-based technologies enable extremely small optical nanodevices. The wavelength of light captured by a graphene sheet – a monolayer sheet of carbon atoms can be shortened by a factor of 100 compared to light propagating in free space. As a consequence, this light propagating along the graphene sheet - called graphene plasmon - requires much less space. For that reason, photonic devices can be made much smaller. The plasmonic field concentration can be further enhanced by fabricating graphene nanostructures acting as nanoresonators for the plasmons. The enhanced field have been already applied for enhanced infrared and terahertz photodetection or infrared vibrational sensing of molecules, among others.
“The development of efficient devices based on plasmonic graphene nanoresonators will critically depend on precise understanding and control of the plasmonic modes inside them” says Dr. Pablo Alonso-Gonzalez, (now at Oviedo University) who performed the real-space imaging of the graphene nanoresonators (disks and rectangles) with a near-field microscope. “We have been strongly impressed by the diversity of plasmonic contrasts observed in the near-field images” continues Dr. Alexey Nikitin, Ikerbasque Research Fellow at nanoGUNE, who developed the theory to identify the individual plasmon modes.
The research team has disentangled the individual plasmonic modes and separated them into two different classes. The first class of plasmons – “sheet plasmons” - can exist ”inside” graphene nanostructures, extending over the whole area of graphene. Conversely, the second class of plasmons – “edge plasmons” –can exclusively propagate along the edges of graphene nanostructures, leading to whispering gallery modes in disk-shaped nanoresonators or Fabry-Perot resonances in graphene nanorectangles due to reflection at their corners. The edge plasmons are much better confined than the sheet plasmons and, most importantly, transfer the energy only in one dimension. The real-space images reveal dipolar edge modes with a mode volume that is 100 million times smaller that a cube of the free-space wavelength. The researchers also measured the dispersion (energy as a function of momentum) of the edge plasmons based on their near-field images, highlighting the shortened wavelength of edge plasmons compared to sheet plasmons. Thanks to their unique properties, edge plasmons could be a promising platform for coupling quantum dots or single molecules in future quantum opto-electronic devices.
“Our results also provide novel insights into the physics of near-field microscopy of graphene plasmons, which could be very useful for interpreting near-field images of other light-matter interactions in two-dimensional materials”, adds Ikerbasque Research Professor Rainer Hillenbrand who led the project.
Plasmons are key players when trying to squeeze light into tiny circuits and control its flow electrically. Last year, the same team showed that high-quality graphene could guide plasmons and confine them to length scales of nanometers, one hundred to two hundred times below the wavelength of light, yet while maintaining a long plasmon lifetime. However, one missing link in this picture was the lack of an on-chip detector of plasmons.
Now, their investigation has gone a huge step forward. In a recent study, published in Nature Materials, researchers from ICFO, nanoGUNE, Columbia University and the National Institute for Materials Science in Japan have been able to fabricate an all-graphene mid-infrared plasmon detector operating at room temperature, where a single graphene sheet serves simultaneously as the plasmonic generation medium and detector.
In their study, the team of scientists used a new experimental design in which they employed two local gates—a split metal sheet located underneath the graphene—to fully tune the thermoelectric and plasmonic behaviour of the graphene. In contrast to the conventional back gating through a thick silicon oxide layer, the separate local gates not only induce free carriers in the graphene to act as a plasmonic channel, but also create a thermoelectric detector for the plasmons. The resulting device converts the conveniently available natural decay product of the plasmon, electronic heat, directly into a voltage through the thermoelectric effect.
The results of this paper open a new pathway to graphene 'plasmo-electronics', which would allow to perform mid-infrared opto-electronics at very small length scales. The team looks forward to creating a fully integrated system in which all three parts, source, channel, and receiver, are made of graphene, enabling a new ´all-carbon optics´ technology.
An ultimate goal in materials science, biomedicine or nanotechnology is the non-invasive compositional mapping of materials with nanometer-scale spatial resolution. A variety of high-resolution imaging techniques exist (for example, electron or scanning probe microscopies), however, they cannot meet the increasing demand in research, development and industry of being noninvasive while offering highest chemical sensitivity.
Nanoscale chemical analysis has recently become possible with nano-FTIR spectroscopy, an optical technique that combines scattering-type scanning near-field optical microscopy (s-SNOM) and Fourier transform infrared (FTIR) spectroscopy. By illuminating the metalized tip of an atomic force microscope (AFM) with a broadband infrared laser or a synchrotron, and analyzing the backscattered light with a specially designed Fourier Transform spectrometer, local infrared spectroscopy with a spatial resolution of less than 20 nm has been demonstrated. However, only point spectra or spectroscopic line scans comprising not more than a few tens of nano-FTIR spectra could be achieved on organic samples, owing to the long acquisition times.
Now, researchers from CIC nanoGUNE (San Sebastian, Spain), Ikerbasque (Bilbao, Spain), Cidetec (San Sebastian, Spain) and the Robert Koch-Institut (Berlin, Germany) developed hyperspectral infrared nanoimaging. The technique allows for recording two-dimensional arrays of several thousand of nano-FTIR spectra - usually referred as to hyperspectral data cubes - in a few hours and with a spatial resolution and precision better than 30 nm.
“The excellent data quality allows for extracting nanoscale-resolved chemical and structural information with the help of statistical techniques (multivariate data analysis) that use the complete spectroscopic information available at each pixel”, says Iban Amenabar, first author of the work. Even without any previous information about the sample and its components, pixels with similar infrared spectra can be grouped automatically with the help of hierarchical cluster analysis. By imaging and analysis of a three-component polymer blend and (Figure 1), the researchers obtained nanoscale chemical maps that do not only reveal the spatial distribution of the individual components but also spectral anomalies that were explained by local chemical interaction. The researcher also demonstrated in situ hyperspectral infrared nanoimaging of native melanin in human hair.
For their experiments, the researchers used the commercial nano-FTIR system from Neaspec GmbH including a mid-infrared laser continuum that covers the spectral range from 1000 to 1900 cm-1. Multivariate analysis of the hyperspectral data was done with the software tool CytoSpec, which was developed by coauthor Peter Lasch.
“With the rapid development of high-performance mid-infrared lasers and by applying advanced noise reduction strategies, we envision high-quality hyperspectral infrared nanoimaging in few minutes”, concludes Rainer Hillenbrand who led the work. “We see a large application potential in various fields of science and technology, including the chemical mapping of polymer composites, pharmaceutical products, organic and inorganic nanocomposite materials or biomedical tissue imaging ”, he adds.
Researchers have studied how light can be used to “see” the quantum nature of an electronic material. They managed to do that by capturing light in a net of carbon atoms and slowing down light that it moves almost as slow as the electrons in the graphene. Then, something special happens: electrons and light start to move in concert, unveiling their quantum nature at such large scale that it could be observed with a special type of microscope.
The experiments were performed with ultra-high quality graphene. To excite and image the ultra-slow ripples of light in the graphene (also called plasmons), the researchers used a special antenna for light that scans over the surface at a distance of a few nanometers. With this near-field nanoscope they saw that the light ripples on the graphene moved more than 300 times slower than light, and dramatically different from what is expected from classical physics laws.
The work, authored by researchers from nanoGUNE, ICFO, IIT, and Columbia University, has been published in Science. In reference to the accomplished experiments, ICREA Prof. at ICFO Frank Koppens comments: “Usually it is very difficult to probe the quantum world, and to do so it requires ultra-low temperatures; here we could just “see” it with light and even at room temperature”.
This technique paves now the way for exploring many new types of quantum materials, including superconductors where electricity can flow without energy consumption, or topological materials that allow for quantum information processing with topological qubits. In addition, Ikerbasque Prof. at nanoGUNE Rainer Hillenbrand states that “this could just be the beginning of a new era of near field nanoscopy”.
Prof. Polini, from IIT, adds that “this discovery may eventually lead to understanding in a truly microscopic fashion complex quantum phenomena that occur when matter is subject to ultra-low temperatures and very high magnetic fields, like the fractional quantum Hall effect”.
This research has been partially supported by the European Research Council, the European Graphene Flagship, the Government of Catalonia, Fundació Cellex and the Severo Ochoa Excellence program of the Government of Spain.
Modulating the amplitude and phase of light is a key ingredient for many of applications such as wavefront shaping, transformation optics, phased arrays, modulators, and sensors. Performing this task with high efficiency and small footprint is a major challenge for the development of optoelectronic devices.
In a recent paper published in Nature Photonics, ICFO researchers Dr. Achim Woessner and Dr. Mark Lundeberg, led by ICREA Prof. at ICFO Frank Koppens, in collaboration with Ikerbasque Prof. Rainer Hillenbrand from nanoGUNE, Iacopo Torre and Prof. Marco Polini from IIT and Dr. Yuanda Gao and Prof. James Hone from Columbia University, have developed a phase modulator based on graphene capable of tuning the light phase between 0 and 2π in situ.
To achieve this, they exploited the unique wavelength tunability of graphene plasmons, light coupled to electrons in graphene. In their experiment, they used ultra-high quality graphene and build a fully functional phase modulator with a device footprint of only 350 nm, which is 30 times than the wavelength of the infrared light used for this experiment. A near-field microscope was used to excite and image the plasmons, allowing an unprecedented insight into the plasmon properties such as their wavelength and phase.
This new type of phase modulator enables graphene plasmons to be used for ultra-compact light modulators and phase arrays with the possibility to control, steer, and focus light in situ. This has potential applications for on-chip biosensing and two dimensional transformation optics.
This research has been partially supported by the European Research Council, the European Graphene Flagship, the Government of Catalonia, Fundació Cellex and the Severo Ochoa Excellence program of the Government of Spain.
NanoGUNE was recently recognized with the Maria de Maeztu distinction of excellence and, therefore, encourages interested candidates to contact nanoGUNE's Group Leaders to explore the possibilities of working in a research proposal for the JunioLeader and INPhINIT programs.
Boron nitride nanoresonators for phonon-enhanced molecular vibrational spectroscopy at the strong coupling limit
M. Autore, I. Dolado, F.J. Alfaro-Mozaz, R. Esteban, A. Atxabal, F. Casanova, L. Hueso, P. Alonso-Gonzalez, J. Aizpurua, A. Nikitin, S. Velez and R. Hillenbrand
Light-Science & Applications, (2018)
Enhanced light-matter interactions are the basis of surface enhanced infrared absorption (SEIRA) spectroscopy, and conventionally rely on plasmonic materials and their capability to focus light to nanoscale spot sizes. Phonon polariton nanoresonators made of polar crystals could represent an interesting alternative, since they exhibit large quality factors, which go far beyond those of their plasmonic counterparts. The recent emergence of van der Waals crystals enables the fabrication of high-quality nanophotonic resonators based on phonon polaritons, as reported for the prototypical infrared-phononic material hexagonal boron nitride (h-BN). In this work we use, for the first time, phononpolariton-resonant h-BN ribbons for SEIRA spectroscopy of small amounts of organic molecules in Fourier transform infrared spectroscopy. Strikingly, the interaction between phonon polaritons and molecular vibrations reaches experimentally the onset of the strong coupling regime, while numerical simulations predict that vibrational strong coupling can be fully achieved. Phonon polariton nanoresonators thus could become a viable platform for sensing, local control of chemical reactivity and infrared quantum cavity optics experiments.
The interaction of light and matter at the nanoscale is a key element for many fundamental studies and technological applications, ranging from light harvesting to the detection of small amounts of molecules.
An interesting but much less explored approach for enhancing nanoscale light-matter interaction is based on infrared-phononic materials, in which light couples to crystal lattice vibrations to form so-called phonon polaritons. “Phonon-polariton resonators offer much lower losses and field confinement than their mid-infrared plasmonic counterparts. For that reason, we decided to develop and apply infrared-phononic resonators to enhance the coupling of infrared light to molecular vibrations” says Marta Autore, first author of the paper.
In order to develop a method that one could call “phononic SEIRA”, the researchers fabricated a set of ribbon arrays made of hexagonal-boron nitride (h-BN) flakes. By infrared transmission spectroscopy they indeed observed narrow phonon polariton resonances. Then, they deposited thin layers of an organic molecule onto the ribbons. It led to a strong modification of the phonon polariton resonance, which could be used to detect ultra-small amounts of molecules (N<10-15 mol) that were not detectable when deposited on conventional substrates.
“Interestingly, when we deposited thicker layers of molecules onto the ribbons, we observed a splitting of the phonon polariton resonance. This is a typical signature of a phenomenon that is known as strong coupling. In this regime, the interaction of light and matter is so strong that exciting phenomena such as modification of chemical reactions, polariton condensation or long-range and ultrafast energy transfer can occur” says Rainer Hillenbrand, group leader at nanoGUNE who led the work. “In the future we want to have a closer look into phonon-enhanced strong coupling and what we could do with it.”
The findings show the potential of phonon polariton resonators to become a new platform for mid-infrared sensing of ultra-small quantities of materials and for exploring strong coupling at the nanoscale, opening the way for future fundamental studies of quantum phenomena or applications such as local modification of chemical bond strength and selective catalysis at the nanoscale.
Eligible candidates must fulfill all the requirements necessary to apply for the PREDOC BERRI Fellowship program of the Department of Education of the Basque Government. Note that candidates must be currently resident in the Basque Country since at least 1 January 2018.
The PhD projects will be related to the current interests of the research groups at nanoGUNE:
- Nanomagnetism - Andreas Berger and Paolo Vavassori
- Nanooptics - Rainer Hillenbrand
- Self-Assembly - Alexander Bittner
- Nanobiomechanics - Raul Perez-Jimenez
- Nanodevices - Luis Hueso and Felix Casanova
- Electron Microscopy - Andrey Chuvilin
- Theory - Emilio Artacho
- Nanomaterials - Mato Knez
- Nanoimaging - Jose Ignacio Pascual
- Nanoengineering - Andreas Seifert
The PREDOC BERRI call of the Basque Government for 2018-2019 is now open and interested candidates are welcomed to submit their application together with their CV and Academic Record to the Group Leader of their interest no later than 15 June 2018.
All the information about the fellowships is available at these links:
Future information and communication technologies will rely on the manipulation of not only electrons but also of light at the nanometer-scale. Squeezing (confining) light to such a small size has been a major goal in nanophotonics for many years. A successful strategy is the use of polaritons, which are electromagnetic waves resulting from the coupling of light and matter. Particularly strong light squeezing can be achieved with polaritons at infrared frequencies in 2D materials, such as graphene and hexagonal boron nitride. However, although extraordinary polaritonic properties - such as electrical tuning of graphene polaritons - have been recently achieved with these materials, the polaritons have always been found to propagate along all directions of the material surface, thereby losing energy quite fast, which limits their application potential.
Now, an international team led by Qiaoliang Bao (Monash Engineering’s Associate Professor, Melbourne, Australia), Pablo Alonso-González (Distinguished researcher at University of Oviedo, Spain) and Rainer Hillenbrand (Ikerbasque Research Professor at CIC nanoGUNE, San Sebastián, Spain) have discovered ultra-confined infrared polaritons that propagate only in specific directions along thin slabs of the natural 2D material molybdenum trioxide (a-MoO3).
“Our findings promise a-MoO3 to become a unique platform for infrared nanophotonics”, says Qiaoliang Bao. “It was amazing to discover polaritons on our a-MoO3 thin flakes travelling only along certain directions”, says Weiliang Ma, postgraduate-student and co-first-author. “Until now, the directional propagation of polaritons has been observed experimentally only in artificially structured materials, where the ultimate polariton confinement is much more difficult to achieve than in natural materials”, adds co-first author Shaojuan Li.
Apart of directional propagation, the study also revealed that the polaritons on a-MoO3 can have an extraordinarily long lifetime. “Light seems to take a nanoscale highway on a-MoO3; it travels along certain directions with almost no obstacles”, says Pablo Alonso-González, co-first author of the paper. He adds: “Our measurements show that polaritons on a-MoO3 live up to 20 picoseconds, which is 40 times larger than the best-possible polariton lifetime in high-quality graphene at room temperature”.
Because the wavelength of the polaritons is much smaller than that of light, the researchers had to use a special microscope, a so-called near-field optical microscope, to image them. “The establishment of this technique coincided perfectly with the emergence of novel van der Waals materials, enabling the imaging of a variety of unique and even unexpected polaritons during the past years”, adds Rainer Hillenbrand.
For a better understanding of the experimental results, the researchers developed a theory that allowed them to extract the relation between the momentum of polaritons in a-MoO3 with their energy. "We have realized that light squeezed in a-MoO3 can become “hyperbolic” making the energy and wave-fronts to propagate in different directions along the surface, which can lead to interesting exotic effects in optics (such as e.g. negative refraction or “superlensing”)", says Alexey Nikitin, Ikerbasque Research Associate at Donostia International Physics Center (DIPC), who developed the theory in collaboration with Javier Taboada-Gutiérrez, and Javier Martín-Sánchez, PhD and postdoctoral researchers, respectively at Alonso-Gonzalez´s group.
The current work is just the beginning of a series of studies focused on directional control and manipulation of light with the help of ultra-low-loss polaritons at the nanoscale, which could benefit the development of more efficient nanophotonic devices for optical sensing and signal processing or heat management.