Page 10
www.us-
tech.com
TechWaTch
X-ray Movies: Imaging with Ultrashort Electron Beams
By Larry Hardesty U
ltrashort bursts of electrons have several important appli- cations in scientific and indus-
trial imaging, but producing them has typically required a costly, pow- er-hungry apparatus about the size of an automobile. In the journal Optica, re-
searchers at MIT, the German Syn- chrotron, and the University of Ham- burg 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 of the power of its predecessors. Ultrashort electron beams are
used to directly gather information about materials that are undergoing chemical reactions or changes of physical state. After being fired down a particle accelerator a half a mile long, they’re also used to produce ul- trashort X-rays. Last year, in Nature Communi-
cations, the same group of MIT and Hamburg researchers reported the prototype of a small “linear accelera- tor” that could serve the same pur-
pose as the much larger and more ex- pensive 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 academ- ic and industrial labs. Indeed, while the electron bursts
reported in the new paper have a du- ration measured in hundreds of fem- toseconds, or quadrillionths of a sec- ond (which is about what the best ex- isting electron guns can manage), the researchers’ approach has the poten- tial to lower their duration to a single femtosecond. An electron burst of a single femtosecond could generate at- tosecond 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 engi- neering 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 under- going a chemical reaction. A femtosec- ond X-ray light source requires more hardware, but it utilizes electron guns.” In particular, Huang explains,
with a technique called electron dif- fraction imaging, physicists and chemists use ultrashort bursts of elec- trons to investigate phase changes in
microwaves and visible light. The researchers’ device, which
is about the size of a matchbox, con- sists of two copper plates that, at their centers, are only 75 µm apart. Each plate has two bends in it, so that it looks rather like a trifold let- ter that’s been opened and set on its side. The plates bend in opposite di- rections, so that they’re farthest
February, 2017
A miniature electron gun driven by terahertz radiation.
IPTE Depaneling Stress-free Production
EXPERTS IN FACTORY AUTOMATION
materials, such as the transition from an electrically conductive to a noncon- ductive state, and the creation and dissolution of bonds between mole- cules in chemical reactions. Ultrashort X-ray pulses have
Low stress High quality output
Routing and/or sawing Reduced investment
EasyRouter off-line, low volume
TopRouter off-line, medium mix
Automatic tool change Long autonomy
Fast cutting speed High output
Clear cut Outstanding quality
High performance Best return on investment
the same advantages that ordinary X-rays do: They penetrate more deeply into thicker materials. The current method for producing ultra- short X-rays involves sending elec- tron bursts from an automobile-sized electron gun through a billion-dollar, kilometer-long particle accelerator that increases their velocity. Then they pass between two rows of mag- nets — known as an “undulator” — that converts them to X-rays. In the paper published last
year, the MIT-Hamburg group, to- gether with colleagues from the Max Planck Institute for the Structure and Dynamics of Matter in Hamburg and the University of Toronto, de- scribed a new approach to accelerat- ing electrons that could shrink parti- cle accelerators to tabletop size. “This is supposed to complement that,” Huang says of the new study. Franz Kartner, who was a pro-
FlexRouter II in-line, high volume, high mix
SpeedRouter in-line, high volume
IPTE America LLC 5935 Shiloh Road East - suite 100 Alpharetta GA 30005 USA T: +1 (0) 678 807 0067 x101 F: +1 (0) 678 807 0072 E:
sales.usa@
ipte.com
WWW.IPTE.COM
fessor of electrical engineering at MIT for 10 years before moving to the German Synchrotron and the University of Hamburg in 2011, led the project. Kartner remains a prin- cipal investigator at MIT’s Research Laboratory of Electronics.
Sub-wavelength Confinement The researchers’ new electron
gun is a variation on a device called an RF gun. But where the RF gun us- es radio frequency (RF) radiation to accelerate electrons, the new device uses terahertz radiation, the band of electromagnetic radiation between
apart 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 nm 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 op- posite side of the film. At the same time, a burst of ter-
ahertz 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 oth- er. The terahertz radiation is polar- ized so that its electric component ac- celerates the electrons directly to- ward the second plate. The key to the system is that
the tapering of the plates confines the terahertz radiation to an area — the 75 µm gap — that is narrower than its own wavelength. “That’s something special,” Huang says. “Typically, in optics, you can’t con- fine something to below a wave- length. Using this structure we were able to. Confining it increases the en- ergy density, which increases the ac- celerating power.” Because of that increased accel-
erating power, the device can make do with terahertz beams whose pow- er is much lower than that of the ra- dio-frequency beams used in a typi- cal RF gun. Moreover, the same laser can
generate both the ultraviolet beam and, with a few additional optical com- ponents, the terahertz beam. The work was funded by the U.S. Air Force Office of Scientific Research and by the European Research Council. Web:
www.news.mit.edu r
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112 |
Page 113 |
Page 114 |
Page 115 |
Page 116 |
Page 117 |
Page 118 |
Page 119 |
Page 120 |
Page 121 |
Page 122 |
Page 123 |
Page 124