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News Article | April 17, 2017
Site: www.nanotech-now.com

Home > Press > Shedding light on the absorption of light by titanium dioxide Abstract: Titanium dioxide (TiO2) is one of the most promising materials for photovoltaics and photocatalysis nowadays. This material appears in different crystalline forms, but the most attractive one for applications is called "anatase". Despite decades of studies on the conversion of the absorbed light into electrical charges in anatase TiO2, the very nature of its fundamental electronic and optical properties was still unknown. EPFL scientists, with national and international partners, have now shed light onto the problem by a combination of cutting-edge steady-state and ultrafast spectroscopic techniques, as well as theoretical calculations. The work is published in Nature Communications. Anatase TiO2 is involved in a wide range of applications, ranging from photovoltaics and photocatalysis to self-cleaning glasses, and water and air purification. All of these are based on the absorption of light and its subsequent conversion into electrical charges. Given its widespread use in various applications, TiO2 has been one of the most studied materials in the twentieth century, both experimentally and theoretically. When light shines on a semiconducting material such as TiO2, it generates either free negative (electrons) and positive (holes) charges or a bound neutral electron-hole pair, called an exciton. Excitons are of great interest because they can transport both energy and charges on a nanoscale level, and form the basis of an entire field of next-generation electronics, called "excitonics". The problem with TiO2 so far is that we have not been able to clearly identify the nature and properties of the physical object that absorbs light and characterize its properties. The group of Majed Chergui at EPFL, along with national and international colleagues, have shed light on this long-standing question by using a combination of cutting-edge experimental methods: steady-state angle-resolved photoemission spectroscopy (ARPES), which maps the energetics of the electrons along the different axis in the solid; spectroscopic ellipsometry, which determines the optical properties of the solid with high accuracy; and ultrafast two-dimensional deep-ultraviolet spectroscopy, used for the first time in the study of materials, along with state-of-the-art first-principles theoretical tools. They discovered that the threshold of the optical absorption spectrum is due to a strongly bound exciton, which exhibits two remarkable novel properties: First, it is confined on a two-dimensional (2D) plane of the three-dimensional lattice of the material. This is the first such case ever reported in condensed matter. And secondly, this 2D exciton is stable at room temperature and robust against defects, as it is present in any type of TiO2 -- single crystals, thin films, and even nanoparticles used in devices. This "immunity" of the exciton to long-range structural disorder and defects implies that it can store the incoming energy in the form of light and guide it at the nanoscale in a selective way. This promises a huge improvement compared to current technology, in which the absorbed light energy is dissipated as heat to the crystal lattice, making the conventional excitation schemes extremely inefficient. Furthermore, the newly discovered exciton is very sensitive to a variety of external and internal stimuli in the material (temperature, pressure, excess electron density), paving the way to a powerful, accurate and cheap detection scheme for sensors with an optical read-out. "Given that it is cheap and easy to fabricate anatase TiO2 materials, these findings are crucial for many applications and beyond", says Majed Chergui. "To know how electrical charges are generated after light is absorbed is a key ingredient for efficient photocatalysts." ### This work was carried out in a collaboration of the EPFL's Laboratoire de Spectroscopie Ultrarapide (LSU) and the Institute of Physics (IPHYS) within the Lausanne Centre for Ultrafast Science (LACUS), with the Max Planck Institute for the Structure and Dynamics of Matter, the University of Fribourg, the Università Campus Bio-Medico di Roma, the Università Roma "Tor Vergata", and the Universidad del Pais Vasco. It was funded by the Swiss National Science foundation (SNSF; NCCR:MUST), the European Research Council Advanced Grants "DYNAMOX" and "Qspec-Newmat", the Grupos Consolidados del Gobierno Vasco and COST Actions, EUSpec. Reference E. Baldini, L. Chiodo, A. Dominguez, M. Palummo, S. Moser, M. Yazdi-Rizi, G. Auböck, B.P.P. Mallett, H. Berger, A. Magrez, C. Bernhard, M. Grioni, A. Rubio, M. Chergui. Strongly bound excitons in anatase TiO2 single crystals and nanoparticles. Nature Communications 13 April 2017. DOI: s41467-017-00016-6. For more information, please click If you have a comment, please us. Issuers of news releases, not 7th Wave, Inc. or Nanotechnology Now, are solely responsible for the accuracy of the content.


Subedi A.,Max Planck Institute for the Structure and Dynamics of Matter
Physical Review B - Condensed Matter and Materials Physics | Year: 2015

I study the lattice dynamics and electron-phonon coupling in noncentrosymmetric quasi-one-dimensional K2Cr3As3 using density functional theory based first principles calculations. The phonon dispersions show stable phonons without any soft-mode behavior. They also exhibit features that point to a strong interaction of K atoms with the lattice. I find that the calculated Eliashberg spectral function shows a large enhancement around 50 cm-1. The phonon modes that show large coupling involve in-plane motions of all three species of atoms. The q dependent electron-phonon coupling decreases strongly away from the qz=0 plane. The total electron-phonon coupling is large, with a value of λep=3.0, which readily explains the experimentally observed large mass enhancement. © 2015 American Physical Society.


Miller R.J.D.,Max Planck Institute for the Structure and Dynamics of Matter | Miller R.J.D.,University of Toronto
Science | Year: 2014

With the recent advances in ultrabright electron and x-ray sources, it is now possible to extend crystallography to the femtosecond time domain to literally light up atomic motions involved in the primary processes governing structural transitions. This review chronicles the development of brighter and brighter electron and x-ray sources that have enabled atomic resolution to structural dynamics for increasingly complex systems. The primary focus is on achieving sufficient brightness using pump-probe protocols to resolve the far-from-equilibrium motions directing chemical processes that in general lead to irreversible changes in samples. Given the central importance of structural transitions to conceptualizing chemistry, this emerging field has the potential to significantly improve our understanding of chemistry and its connection to driving biological processes.


Home > Press > Ultra-long, one-dimensional carbon chains are synthesised for the first time: Researchers involved in an international study, in which the UPV/EHU-University of the Basque Country has participated, have stabilised chains of more than 6,400 carbon atoms using double-walled nanotub Abstract: Elemental carbon appears in many different forms, some of which are very well-known and have been thoroughly studied: diamond, graphite, graphene, fullerenes, nanotubes and carbyne. Within this "carbon family", carbyne (a truly one-dimensional carbon structure) is the only one that has not been synthesised until now, despite having been studied for more than 50 years. Organic chemists across the world had been trying to synthesise increasingly longer carbyne chains by using stabilizing agents; the longest chain obtained so far (achieved in 2010) was 44 carbon atoms. A research group at the University of Vienna, led by Prof Thomas Pichler, has presented a new, simple means for stabilising carbon chains with a record-breaking length of over 6,400 carbon atoms. They have thus broken the previous record by more than two orders of magnitude. To do this, they used the confined space inside a double-walled carbon nanotube as a nano-reactor to make the ultra-long carbon chains grow and also to provide the chains with great stability. This stability is tremendously important for future applications. The existence has been confirmed The work carried out in collaboration with various highly prominent research groups worldwide, including the UPV/EHU's Nano-Bio Spectroscopy research Group led by Prof Ángel Rubio, has unambiguously confirmed the existence of these chains by means of structural and optical probes. The researchers have presented their study in the latest edition of the prestigious Nature Materials journal. According to the researchers, the direct experimental proof of the confined, ultra-long carbon chains, which are two orders of magnitude longer than the previously proven ones, can be seen as a promising step towards the final objective to obtain perfectly linear carbon chains. Theoretical studies have shown that after having made these linear chains grow inside the carbon nanotube, the hybrid system could have a metallic nature due to the load transfer from the carbon nanotubes towards the chain, although both the nanotube and the chain are vacuum semi-conductors. So it is possible to control the electronic properties of this hybrid system. Therefore, this new system is not only interesting from the chemical point of view, it could also be very important in the field of nano devices. According to theoretical models, carbyne has mechanical properties that are unmatched by any known material, as it even outperforms the mechanical resistance and flexibility properties of graphene and diamond. Furthermore, its electronic properties are pointing towards new nano-electronic applications, such as in the development of new magnetic semiconductors, high power density batteries, or in quantum spin transport electronics (spintronics). However, the researchers point out that to do this it would be necessary to extract these ultra-long, linear carbon chains from the double-walled nanotube containing them and stabilise them in some liquid environment. ### Additional information The research was carried out in collaboration with various research groups at different organisations: University of Vienna, AIST (Japan), ETH Zürich, Nano-bio Spectroscopy Group (UPV/EHU) and the Max Planck Institute for the Structure and Dynamics of Matter (Hamburg). The Nano-bio Spectroscopy research Group is led by Ángel Rubio, a UPV/EHU professor, a member of the Department of Materials Sciences, and director of the Theory Department of the Max Planck Institute for the Structure and Dynamics of Matter. The group's activity focusses on the theoretical research and modelling of electronic and structural properties of condensed matter as well as the development of new theoretical tools and computer codes to explore the electronic response of solids and nanostructures when handling external electromagnetic fields. For more information, please click If you have a comment, please us. Issuers of news releases, not 7th Wave, Inc. or Nanotechnology Now, are solely responsible for the accuracy of the content.


News Article | December 5, 2016
Site: www.scientificcomputing.com

Ultrashort bursts of electrons have several important applications in scientific imaging, but producing them has typically required a costly, power-hungry apparatus about the size of a car. In the journal Optica, researchers at MIT, the German Synchrotron, and the University of Hamburg in Germany describe a new technique for generating electron bursts, which could be the basis of a shoebox-sized device that consumes only a fraction as much power as its predecessors. Ultrashort electron beams are used to directly gather information about materials that are undergoing chemical reactions or changes of physical state. But after being fired down a particle accelerator a half a mile long, they’re also used to produce ultrashort X-rays. Last year, in Nature Communications, the same group of MIT and Hamburg researchers reported the prototype of a small “linear accelerator” that could serve the same purpose as the much larger and more expensive particle accelerator. That technology, together with a higher-energy version of the new “electron gun,” could bring the imaging power of ultrashort X-ray pulses to academic and industry labs. Indeed, while the electron bursts reported in the new paper have a duration measured in hundreds of femtoseconds, or quadrillionths of a second (which is about what the best existing electron guns can manage), the researchers’ approach has the potential to lower their duration to a single femtosecond. An electron burst of a single femtosecond could generate attosecond X-ray pulses, which would enable real-time imaging of cellular machinery in action. “We’re building a tool for the chemists, physicists, and biologists who use X-ray light sources or the electron beams directly to do their research,” says Ronny Huang, an MIT PhD student in electrical engineering and first author on the new paper. “Because these electron beams are so short, they allow you to kind of freeze the motion of electrons inside molecules as the molecules are undergoing a chemical reaction. A femtosecond X-ray light source requires more hardware, but it utilizes electron guns.” In particular, Huang explains, with a technique called electron diffraction imaging, physicists and chemists use ultrashort bursts of electrons to investigate phase changes in materials, such as the transition from an electrically conductive to a nonconductive state, and the creation and dissolution of bonds between molecules in chemical reactions. Ultrashort X-ray pulses have the same advantages that ordinary X-rays do: They penetrate more deeply into thicker materials. The current method for producing ultrashort X-rays involves sending electron bursts from a car-sized electron gun through a billion-dollar, kilometer-long particle accelerator that increases their velocity. Then they pass between two rows of magnets — known as an “undulator” — that converts them to X-rays. In the paper published last year — on which Huang was a coauthor — the MIT-Hamburg group, together with colleagues from the Max Planck Institute for the Structure and Dynamics of Matter in Hamburg and the University of Toronto, described a new approach to accelerating electrons that could shrink particle accelerators to tabletop size. “This is supposed to complement that,” Huang says, about the new study. Franz Kärtner, who was a professor of electrical engineering at MIT for 10 years before moving to the German Synchrotron and the University of Hamburg in 2011, led the project. Kärtner remains a principal investigator at MIT’s Research Laboratory of Electronics and is Huang’s thesis advisor. He and Huang are joined on the new paper by eight colleagues from both MIT and Hamburg. The researchers’ new electron gun is a variation on a device called an RF gun. But where the RF gun uses radio frequency (RF) radiation to accelerate electrons, the new device uses terahertz radiation, the band of electromagnetic radiation between microwaves and visible light. The researchers’ device, which is about the size of a matchbox, consists of two copper plates that, at their centers, are only 75 micrometers apart. Each plate has two bends in it, so that it looks rather like a trifold letter that’s been opened and set on its side. The plates bend in opposite directions, so that they’re farthest apart — 6 millimeters — at their edges. At the center of one of the plates is a quartz slide on which is deposited a film of copper that, at its thinnest, is only 30 nanometers thick. A short burst of light from an ultraviolet laser strikes the film at its thinnest point, jarring loose electrons, which are emitted on the opposite side of the film. At the same time, a burst of terahertz radiation passes between the plates in a direction perpendicular to that of the laser. All electromagnetic radiation can be thought of as having electrical and magnetic components, which are perpendicular to each other. The terahertz radiation is polarized so that its electric component accelerates the electrons directly toward the second plate. The key to the system is that the tapering of the plates confines the terahertz radiation to an area — the 75-micrometer gap — that is narrower than its own wavelength. “That’s something special,” Huang says. “Typically, in optics, you can’t confine something to below a wavelength. But using this structure we were able to. Confining it increases the energy density, which increases the accelerating power.” Because of that increased accelerating power, the device can make do with terahertz beams whose power is much lower than that of the radio-frequency beams used in a typical RF gun. Moreover, the same laser can generate both the ultraviolet beam and, with a few additional optical components, the terahertz beam. According to James Rosenzweig, a professor of physics at the University of California at Los Angeles, that’s one of the most attractive aspects of the researchers’ system. “One of the main problems you have with ultrafast sources like this is timing jitter between, say, the laser and accelerating field, which produces all sorts of systematic effects that make it harder to do time-resolved electron diffraction,” Rosezweig says. “In the case of Kärtner’s device, the laser produces the terahertz and also produces the photoelectrons, so the jitter is highly suppressed. You could do pump-probe experiments where the laser is the driver and the electrons would be the probe, and they would be more successful than what you have right now. And of course it would be a very small-sized and modest-cost device. So it might turn out to be very important as far as that scenario goes.” The researchers’ work was funded by the U.S. Air Force Office of Scientific Research and by the European Research Council. Ronny Huang was supported by a National Defense Science and Engineering Graduate fellowship.


News Article | November 22, 2016
Site: www.eurekalert.org

Ultrashort bursts of electrons have several important applications in scientific imaging, but producing them has typically required a costly, power-hungry apparatus about the size of a car. In the journal Optica, researchers at MIT, the German Synchrotron, and the University of Hamburg in Germany describe a new technique for generating electron bursts, which could be the basis of a shoebox-sized device that consumes only a fraction as much power as its predecessors. Ultrashort electron beams are used to directly gather information about materials that are undergoing chemical reactions or changes of physical state. But after being fired down a particle accelerator a half a mile long, they're also used to produce ultrashort X-rays. Last year, in Nature Communications, the same group of MIT and Hamburg researchers reported the prototype of a small "linear accelerator" that could serve the same purpose as the much larger and more expensive particle accelerator. That technology, together with a higher-energy version of the new "electron gun," could bring the imaging power of ultrashort X-ray pulses to academic and industry labs. Indeed, while the electron bursts reported in the new paper have a duration measured in hundreds of femtoseconds, or quadrillionths of a second (which is about what the best existing electron guns can manage), the researchers' approach has the potential to lower their duration to a single femtosecond. An electron burst of a single femtosecond could generate attosecond X-ray pulses, which would enable real-time imaging of cellular machinery in action. "We're building a tool for the chemists, physicists, and biologists who use X-ray light sources or the electron beams directly to do their research," says Ronny Huang, an MIT PhD student in electrical engineering and first author on the new paper. "Because these electron beams are so short, they allow you to kind of freeze the motion of electrons inside molecules as the molecules are undergoing a chemical reaction. A femtosecond X-ray light source requires more hardware, but it utilizes electron guns." In particular, Huang explains, with a technique called electron diffraction imaging, physicists and chemists use ultrashort bursts of electrons to investigate phase changes in materials, such as the transition from an electrically conductive to a nonconductive state, and the creation and dissolution of bonds between molecules in chemical reactions. Ultrashort X-ray pulses have the same advantages that ordinary X-rays do: They penetrate more deeply into thicker materials. The current method for producing ultrashort X-rays involves sending electron bursts from a car-sized electron gun through a billion-dollar, kilometer-long particle accelerator that increases their velocity. Then they pass between two rows of magnets -- known as an "undulator" -- that converts them to X-rays. In the paper published last year -- on which Huang was a coauthor -- the MIT-Hamburg group, together with colleagues from the Max Planck Institute for the Structure and Dynamics of Matter in Hamburg and the University of Toronto, described a new approach to accelerating electrons that could shrink particle accelerators to tabletop size. "This is supposed to complement that," Huang says, about the new study. Franz Kärtner, who was a professor of electrical engineering at MIT for 10 years before moving to the German Synchrotron and the University of Hamburg in 2011, led the project. Kärtner remains a principal investigator at MIT's Research Laboratory of Electronics and is Huang's thesis advisor. He and Huang are joined on the new paper by eight colleagues from both MIT and Hamburg. The researchers' new electron gun is a variation on a device called an RF gun. But where the RF gun uses radio frequency (RF) radiation to accelerate electrons, the new device uses terahertz radiation, the band of electromagnetic radiation between microwaves and visible light. The researchers' device, which is about the size of a matchbox, consists of two copper plates that, at their centers, are only 75 micrometers apart. Each plate has two bends in it, so that it looks rather like a trifold letter that's been opened and set on its side. The plates bend in opposite directions, so that they're farthest apart -- 6 millimeters -- at their edges. At the center of one of the plates is a quartz slide on which is deposited a film of copper that, at its thinnest, is only 30 nanometers thick. A short burst of light from an ultraviolet laser strikes the film at its thinnest point, jarring loose electrons, which are emitted on the opposite side of the film. At the same time, a burst of terahertz radiation passes between the plates in a direction perpendicular to that of the laser. All electromagnetic radiation can be thought of as having electrical and magnetic components, which are perpendicular to each other. The terahertz radiation is polarized so that its electric component accelerates the electrons directly toward the second plate. The key to the system is that the tapering of the plates confines the terahertz radiation to an area -- the 75-micrometer gap -- that is narrower than its own wavelength. "That's something special," Huang says. "Typically, in optics, you can't confine something to below a wavelength. But using this structure we were able to. Confining it increases the energy density, which increases the accelerating power." Because of that increased accelerating power, the device can make do with terahertz beams whose power is much lower than that of the radio-frequency beams used in a typical RF gun. Moreover, the same laser can generate both the ultraviolet beam and, with a few additional optical components, the terahertz beam. The researchers' work was funded by the U.S. Air Force Office of Scientific Research and by the European Research Council. Ronny Huang was supported by a National Defense Science and Engineering Graduate fellowship.


Home > Press > Scientists shrink electron gun to matchbox size: Terahertz technology has the potential to enable new applications Abstract: In a multi-national effort, an interdisciplinary team of researchers from DESY and the Massachusetts Institute of Technology (MIT) has built a new kind of electron gun that is just about the size of a matchbox. Electron guns are used in science to generate high-quality beams of electrons for the investigation of various materials, from biomolecules to superconductors. They are also the electron source for linear particle accelerators driving X-ray free-electron lasers. The team of DESY scientist Franz Kärtner, who is also a professor at University of Hamburg and continues to run a research group at MIT, where he taught till 2010 before coming to Hamburg, presents its new electron gun in the scientific journal Optica. The new device uses laser generated terahertz radiation instead of the usual radio-frequency fields to accelerate electrons from rest. As the wavelength of the terahertz radiation is much shorter than radio-frequency radiation, the device can shrink substantially. While state-of-the-art electron guns can have the size of a car, the new device measures just 34 by 24.5 by 16.8 millimetres. "Electron guns driven by terahertz radiation are miniature and efficient," explains main author Dr. W. Ronny Huang from MIT, who carried out this work at the Center for Free-Electron Laser Science CFEL in Hamburg, a cooperation of DESY, the University of Hamburg and the German Max Planck Society. "Also, the materials used to guide the radiation are susceptible to much higher fields at terahertz wavelengths as compared to radio frequency wavelengths, allowing terahertz radiation to give a much stronger 'kick' to the electrons. This has the effect of making the electron beams much brighter and shorter." Ultrashort electron beams with narrow energy spread, high charge and low jitter are essential for ultrafast electron diffraction experiments to resolve phase transitions in metals, semiconductors and molecular crystals, for example. "Our device has a nanometer thin film of copper which, when illuminated with ultraviolett light from the back, produces short bursts of electrons," describes Huang. "Laser radiation with Terahertz frequency is fed into the device which has a microstructure specifically tailored to channel the radiation to maximize its impact on the electrons." This way the device reached an accelerating power of 350 Megavolts per metre. "The accelerating field was almost twice that of current state-of-the-art guns," says Huang. "We achieved an acceleration of a dense packet of 250,000 electrons from rest to 0.5 kilo-elecronvolts (keV) with minimal energy spread. Because of this, the electron beams coming out of the device could already be used for low-energy electron diffraction experiments." In their set-up, the researchers used the large CFEL laser lab. The ultraviolet flash used to eject the electrons from the copper film is generated from the same laser as the accelerating terahertz radiation. "This ensures absolute timing synchronisation, substantially reducing jitter," explains Huang. The device worked stably over at least one billion shots, easing every-day operation. "Electron guns are used ubiquitously for making atomic-resolution movies of chemical reactions via ultrafast electron diffraction as pioneered in Prof. Dwayne Miller's group at the Max Planck Institute for the Structure and Dynamics of Matter and CFEL," says Kärtner. "With smaller and better electron guns, biologists can gain better insight to the intricate workings of macromolecular machines, including those responsible for photosynthesis. And physicists can better understand the fundamental interaction processes in complex materials." "Furthermore, electron guns are an important component of X-ray light source facilities," explains Kärtner. Next generation terahertz electron guns producing ultrashort and ultrabright electron bunches up to relativistic energies and of ten femtoseconds (quadrillionth of a second) duration are currently in development at CFEL, according to Kärtner. "These devices will be used as photo injectors for attosecond table-top free-electron lasers to be constructed within the program AXSIS," says Kärtner. An attosecond is a thousandth of a femtosecond. AXSIS (frontiers of Attosecond X-ray Science-Imaging and Spectroscopy) , funded by the European Research Council through an ERC Synergy Grant which also funded this work, also involves DESY scientists Prof. Henry Chapman and Dr. Ralph Aßman and Arizona State University Professor Petra Fromme. About Deutsches Elektronen-Synchrotron DESY Deutsches Elektronen-Synchrotron DESY is the leading German accelerator centre and one of the leading in the world. DESY is a member of the Helmholtz Association and receives its funding from the German Federal Ministry of Education and Research (BMBF) (90 per cent) and the German federal states of Hamburg and Brandenburg (10 per cent). At its locations in Hamburg and Zeuthen near Berlin, DESY develops, builds and operates large particle accelerators, and uses them to investigate the structure of matter. DESY's combination of photon science and particle physics is unique in Europe. For more information, please click If you have a comment, please us. Issuers of news releases, not 7th Wave, Inc. or Nanotechnology Now, are solely responsible for the accuracy of the content.


News Article | November 22, 2016
Site: www.cemag.us

In a multi-national effort, an interdisciplinary team of researchers from DESY and the Massachusetts Institute of Technology (MIT) has built a new kind of electron gun that is just about the size of a matchbox. Electron guns are used in science to generate high-quality beams of electrons for the investigation of various materials, from biomolecules to superconductors. They are also the electron source for linear particle accelerators driving X-ray free-electron lasers. The team of DESY scientist Franz Kärtner, who is also a professor at University of Hamburg and continues to run a research group at MIT, where he taught till 2010 before coming to Hamburg, presents its new electron gun in the scientific journal Optica. The new device uses laser generated terahertz radiation instead of the usual radio-frequency fields to accelerate electrons from rest. As the wavelength of the terahertz radiation is much shorter than radio-frequency radiation, the device can shrink substantially. While state-of-the-art electron guns can have the size of a car, the new device measures just 34 by 24.5 by 16.8 millimeters. “Electron guns driven by terahertz radiation are miniature and efficient,” explains main author Dr. W. Ronny Huang from MIT, who carried out this work at the Center for Free-Electron Laser Science CFEL in Hamburg, a cooperation of DESY, the University of Hamburg and the German Max Planck Society. “Also, the materials used to guide the radiation are susceptible to much higher fields at terahertz wavelengths as compared to radio frequency wavelengths, allowing terahertz radiation to give a much stronger 'kick' to the electrons. This has the effect of making the electron beams much brighter and shorter.” Ultrashort electron beams with narrow energy spread, high charge and low jitter are essential for ultrafast electron diffraction experiments to resolve phase transitions in metals, semiconductors and molecular crystals, for example. “Our device has a nanometer thin film of copper which, when illuminated with ultraviolett light from the back, produces short bursts of electrons,” describes Huang. “Laser radiation with Terahertz frequency is fed into the device which has a microstructure specifically tailored to channel the radiation to maximize its impact on the electrons.” This way the device reached an accelerating power of 350 Megavolts per metre. “The accelerating field was almost twice that of current state-of-the-art guns,” says Huang. “We achieved an acceleration of a dense packet of 250,000 electrons from rest to 0.5 kilo-elecronvolts (keV) with minimal energy spread. Because of this, the electron beams coming out of the device could already be used for low-energy electron diffraction experiments.” In their set-up, the researchers used the large CFEL laser lab. The ultraviolet flash used to eject the electrons from the copper film is generated from the same laser as the accelerating terahertz radiation. “This ensures absolute timing synchronisation, substantially reducing jitter,” explains Huang. The device worked stably over at least one billion shots, easing every-day operation. “Electron guns are used ubiquitously for making atomic-resolution movies of chemical reactions via ultrafast electron diffraction as pioneered in Prof. Dwayne Miller's group at the Max Planck Institute for the Structure and Dynamics of Matter and CFEL,” says Kärtner. “With smaller and better electron guns, biologists can gain better insight to the intricate workings of macromolecular machines, including those responsible for photosynthesis. And physicists can better understand the fundamental interaction processes in complex materials.” “Furthermore, electron guns are an important component of X-ray light source facilities,” explains Kärtner. Next generation terahertz electron guns producing ultrashort and ultrabright electron bunches up to relativistic energies and of ten femtoseconds (quadrillionth of a second) duration are currently in development at CFEL, according to Kärtner. “These devices will be used as photo injectors for attosecond table-top free-electron lasers to be constructed within the program AXSIS,” says Kärtner. An attosecond is a thousandth of a femtosecond. AXSIS (frontiers of Attosecond X-ray Science-Imaging and Spectroscopy) , funded by the European Research Council through an ERC Synergy Grant which also funded this work, also involves DESY scientists Prof. Henry Chapman and Dr. Ralph Aßman and Arizona State University Professor Petra Fromme.


News Article | November 22, 2016
Site: www.eurekalert.org

In a multi-national effort, an interdisciplinary team of researchers from DESY and the Massachusetts Institute of Technology (MIT) has built a new kind of electron gun that is just about the size of a matchbox. Electron guns are used in science to generate high-quality beams of electrons for the investigation of various materials, from biomolecules to superconductors. They are also the electron source for linear particle accelerators driving X-ray free-electron lasers. The team of DESY scientist Franz Kärtner, who is also a professor at University of Hamburg and continues to run a research group at MIT, where he taught till 2010 before coming to Hamburg, presents its new electron gun in the scientific journal Optica. The new device uses laser generated terahertz radiation instead of the usual radio-frequency fields to accelerate electrons from rest. As the wavelength of the terahertz radiation is much shorter than radio-frequency radiation, the device can shrink substantially. While state-of-the-art electron guns can have the size of a car, the new device measures just 34 by 24.5 by 16.8 millimetres. "Electron guns driven by terahertz radiation are miniature and efficient," explains main author Dr. W. Ronny Huang from MIT, who carried out this work at the Center for Free-Electron Laser Science CFEL in Hamburg, a cooperation of DESY, the University of Hamburg and the German Max Planck Society. "Also, the materials used to guide the radiation are susceptible to much higher fields at terahertz wavelengths as compared to radio frequency wavelengths, allowing terahertz radiation to give a much stronger 'kick' to the electrons. This has the effect of making the electron beams much brighter and shorter." Ultrashort electron beams with narrow energy spread, high charge and low jitter are essential for ultrafast electron diffraction experiments to resolve phase transitions in metals, semiconductors and molecular crystals, for example. "Our device has a nanometer thin film of copper which, when illuminated with ultraviolett light from the back, produces short bursts of electrons," describes Huang. "Laser radiation with Terahertz frequency is fed into the device which has a microstructure specifically tailored to channel the radiation to maximize its impact on the electrons." This way the device reached an accelerating power of 350 Megavolts per metre. "The accelerating field was almost twice that of current state-of-the-art guns," says Huang. "We achieved an acceleration of a dense packet of 250,000 electrons from rest to 0.5 kilo-elecronvolts (keV) with minimal energy spread. Because of this, the electron beams coming out of the device could already be used for low-energy electron diffraction experiments." In their set-up, the researchers used the large CFEL laser lab. The ultraviolet flash used to eject the electrons from the copper film is generated from the same laser as the accelerating terahertz radiation. "This ensures absolute timing synchronisation, substantially reducing jitter," explains Huang. The device worked stably over at least one billion shots, easing every-day operation. "Electron guns are used ubiquitously for making atomic-resolution movies of chemical reactions via ultrafast electron diffraction as pioneered in Prof. Dwayne Miller's group at the Max Planck Institute for the Structure and Dynamics of Matter and CFEL," says Kärtner. "With smaller and better electron guns, biologists can gain better insight to the intricate workings of macromolecular machines, including those responsible for photosynthesis. And physicists can better understand the fundamental interaction processes in complex materials." "Furthermore, electron guns are an important component of X-ray light source facilities," explains Kärtner. Next generation terahertz electron guns producing ultrashort and ultrabright electron bunches up to relativistic energies and of ten femtoseconds (quadrillionth of a second) duration are currently in development at CFEL, according to Kärtner. "These devices will be used as photo injectors for attosecond table-top free-electron lasers to be constructed within the program AXSIS," says Kärtner. An attosecond is a thousandth of a femtosecond. AXSIS (frontiers of Attosecond X-ray Science-Imaging and Spectroscopy) , funded by the European Research Council through an ERC Synergy Grant which also funded this work, also involves DESY scientists Prof. Henry Chapman and Dr. Ralph Aßman and Arizona State University Professor Petra Fromme. Deutsches Elektronen-Synchrotron DESY is the leading German accelerator centre and one of the leading in the world. DESY is a member of the Helmholtz Association and receives its funding from the German Federal Ministry of Education and Research (BMBF) (90 per cent) and the German federal states of Hamburg and Brandenburg (10 per cent). At its locations in Hamburg and Zeuthen near Berlin, DESY develops, builds and operates large particle accelerators, and uses them to investigate the structure of matter. DESY's combination of photon science and particle physics is unique in Europe.


News Article | November 22, 2016
Site: www.chromatographytechniques.com

Ultrashort bursts of electrons have several important applications in scientific imaging, but producing them has typically required a costly, power-hungry apparatus about the size of a car. In the journal Optica, researchers at MIT, the German Synchrotron, and the University of Hamburg in Germany describe a new technique for generating electron bursts, which could be the basis of a shoebox-sized device that consumes only a fraction as much power as its predecessors. Ultrashort electron beams are used to directly gather information about materials that are undergoing chemical reactions or changes of physical state. But after being fired down a particle accelerator a half a mile long, they’re also used to produce ultrashort X-rays. Last year, in Nature Communications, the same group of MIT and Hamburg researchers reported the prototype of a small “linear accelerator” that could serve the same purpose as the much larger and more expensive particle accelerator. That technology, together with a higher-energy version of the new “electron gun,” could bring the imaging power of ultrashort X-ray pulses to academic and industry labs. Indeed, while the electron bursts reported in the new paper have a duration measured in hundreds of femtoseconds, or quadrillionths of a second (which is about what the best existing electron guns can manage), the researchers’ approach has the potential to lower their duration to a single femtosecond. An electron burst of a single femtosecond could generate attosecond X-ray pulses, which would enable real-time imaging of cellular machinery in action. “We’re building a tool for the chemists, physicists, and biologists who use X-ray light sources or the electron beams directly to do their research,” says Ronny Huang, an MIT PhD student in electrical engineering and first author on the new paper. “Because these electron beams are so short, they allow you to kind of freeze the motion of electrons inside molecules as the molecules are undergoing a chemical reaction. A femtosecond X-ray light source requires more hardware, but it utilizes electron guns.” In particular, Huang explains, with a technique called electron diffraction imaging, physicists and chemists use ultrashort bursts of electrons to investigate phase changes in materials, such as the transition from an electrically conductive to a nonconductive state, and the creation and dissolution of bonds between molecules in chemical reactions. Ultrashort X-ray pulses have the same advantages that ordinary X-rays do: They penetrate more deeply into thicker materials. The current method for producing ultrashort X-rays involves sending electron bursts from a car-sized electron gun through a billion-dollar, kilometer-long particle accelerator that increases their velocity. Then they pass between two rows of magnets — known as an “undulator” — that converts them to X-rays. In the paper published last year — on which Huang was a coauthor — the MIT-Hamburg group, together with colleagues from the Max Planck Institute for the Structure and Dynamics of Matter in Hamburg and the University of Toronto, described a new approach to accelerating electrons that could shrink particle accelerators to tabletop size. “This is supposed to complement that,” Huang says, about the new study. Franz Kärtner, who was a professor of electrical engineering at MIT for 10 years before moving to the German Synchrotron and the University of Hamburg in 2011, led the project. Kärtner remains a principal investigator at MIT’s Research Laboratory of Electronics and is Huang’s thesis advisor. He and Huang are joined on the new paper by eight colleagues from both MIT and Hamburg. The researchers’ new electron gun is a variation on a device called an RF gun. But where the RF gun uses radio frequency (RF) radiation to accelerate electrons, the new device uses terahertz radiation, the band of electromagnetic radiation between microwaves and visible light. The researchers’ device, which is about the size of a matchbox, consists of two copper plates that, at their centers, are only 75 micrometers apart. Each plate has two bends in it, so that it looks rather like a trifold letter that’s been opened and set on its side. The plates bend in opposite directions, so that they’re farthest apart — 6 millimeters — at their edges. At the center of one of the plates is a quartz slide on which is deposited a film of copper that, at its thinnest, is only 30 nanometers thick. A short burst of light from an ultraviolet laser strikes the film at its thinnest point, jarring loose electrons, which are emitted on the opposite side of the film. At the same time, a burst of terahertz radiation passes between the plates in a direction perpendicular to that of the laser. All electromagnetic radiation can be thought of as having electrical and magnetic components, which are perpendicular to each other. The terahertz radiation is polarized so that its electric component accelerates the electrons directly toward the second plate. The key to the system is that the tapering of the plates confines the terahertz radiation to an area — the 75-micrometer gap — that is narrower than its own wavelength. “That’s something special,” Huang says. “Typically, in optics, you can’t confine something to below a wavelength. But using this structure we were able to. Confining it increases the energy density, which increases the accelerating power.” Because of that increased accelerating power, the device can make do with terahertz beams whose power is much lower than that of the radio-frequency beams used in a typical RF gun. Moreover, the same laser can generate both the ultraviolet beam and, with a few additional optical components, the terahertz beam. According to James Rosenzweig, a professor of physics at the University of California at Los Angeles, that’s one of the most attractive aspects of the researchers’ system. “One of the main problems you have with ultrafast sources like this is timing jitter between, say, the laser and accelerating field, which produces all sorts of systematic effects that make it harder to do time-resolved electron diffraction,” Rosezweig says. “In the case of Kärtner’s device, the laser produces the terahertz and also produces the photoelectrons, so the jitter is highly suppressed. You could do pump-probe experiments where the laser is the driver and the electrons would be the probe, and they would be more successful than what you have right now. And of course it would be a very small-sized and modest-cost device. So it might turn out to be very important as far as that scenario goes.” The researchers’ work was funded by the U.S. Air Force Office of Scientific Research and by the European Research Council. Ronny Huang was supported by a National Defense Science and Engineering Graduate fellowship.

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