Electron imaging is an important tool of modern science, but producing the electrons has typically required a big, expensive, power-hungry setup. Now a team of researchers from MIT and Germany have built a device the size of a matchbox that could use electron imaging to make X-ray movies. The device uses a UV laser to excite electrons out of a 30nm-thick copper film contained between two plates, and then capitalizes on the right-hand rule by using a perpendicular burst of polarized terahertz radiation to fling those electrons into a focused beam.
The key to the system is the way the containing plates are bent and shaped. Their physical shape focuses the beam and confines the terahertz radiation to an area — a 75-micrometer field — that is narrower than its own wavelength. "That’s something special," remarks Ronny Huang, an MIT PhD student in electrical engineering and first author on the new paper. "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."THz gun concept. (a) Photograph of the THz gun; (b) a single-cycle THz pulse, generated via optical rectification in lithium niobate (LN), is coupled into the THz gun, which takes the form of a parallel-plate waveguide for field confinement. A UV-backilluminated photocathode emits an electron bunch, which is accelerated by the THz field. The bunch exits through a slit on the top plate, and a retarding field analyzer (RFA) measures its energy spectrum. (c) Cross section of the gun, showing the UV-photoemitted electrons accelerated by the THz field and exiting through the slit.
That's not the only place the team has reduced overhead. Because of the increased accelerating power, the device can use terahertz beams instead of higher-energy RF radiation. And the same laser can generate both UV and terahertz wavelengths, which makes for a smaller device.
Shrinking down the electron gun in this way could have some shiny implications for academic and industrial research. Last year, this same group of people established prior art by making a prototype "linear accelerator" that could reduce the size of the electron gun, bringing x-ray imaging of reactions, bonds and phase changes right down to benchtop scale. Now with the higher-energy terahertz beam, the whole shebang is the size of a shoebox.
Because they operate at such high energies, too, the beam can perform its imaging with extremely short durations of exposure. While the electron bursts reported in this paper(Opens in a new window) (paywalled) have a duration of hundreds of femtoseconds, which is in line with what the best existing electron guns can do, the MIT News notes(Opens in a new window) that the researchers' technique could bring the burst duration down to a single femtosecond. Such a short burst would create attosecond-long x-ray pulses. Taking lots of short exposures would enable "real-time imaging of cellular machinery in action": in other words, x-ray movies."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 Huang. "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."