Powerful lab microscopes may soon get a screwy upgrade.
By twisting a stream of electrons into a tornado-like vortex, physicists have created a new type of beam to take snapshots of atoms, biological tissue and tiny computer parts, a team reports in the Jan. 14 Science.
Though they haven’t put one in a microscope yet, such a vortex beam could be used in the near future to take sharper pictures, says Ben McMorran, a physicist who led the research at the National Institute of Standards and Technology in Gaithersburg, Md. What’s more, the beams could possibly grab onto atoms to manipulate them, other researchers say.
Electrons, which can behave like waves as well as particles, have a wavelength much smaller than that of visible light. So compared with light, or optical, microscopes, the electron microscope excels at probing tiny things like atoms.
Electron microscopes snap images of minuscule objects by shooting a beam of electrons at a target and recording where the electrons scatter. A microscope using a vortex electron beam would work similarly, except the electron beam must first pass through a hologram that twists the beam into a helix, says McMorran. A beam that spirals offers even better resolution than a straight beam because its energy isn’t concentrated in the beam’s center, which is difficult to focus.
Holograms, like the colorful ones on credit cards, are formed when light bounces off or through an etched surface. In this case, the “hologram” is a thin layer of silicon nitride that McMorran’s team etched with lines a few nanometers apart.
The electron beam diffracted as it passed through the hologram, much as light separates into colors when it goes through a prism. But instead of seeing colors fan out at an angle, the team saw corkscrew beams of electrons fan from the hologram. Those beams can then strike a target, scattering the beam in a way that carries information about the object.
Transforming an electron microscope into a vortex microscope is as simple as slipping a thin hologram into a preexisting slot, according to mechanical engineer Rodney Herring of the University of Victoria in Canada.
One intriguing application would be to use the beams to grab onto individual atoms, Herring says. Electrons in a twisted beam couple with electrons in the atoms of a material, and scientists controlling the electron microscope could use its lenses like a joystick to move the beam (and a captive atom) around.
“Now we have hands that can manipulate atoms and electrons,” says Herring.
In September, a research team in Belgium led by materials scientist Jo Verbeeck of the University of Antwerp announced that it had created an electron-vortex beam using holographic techniques. But McMorran says his team’s beam had 25 times more twists, meaning it could potentially yield 25 times better resolution.
McMorran is working to create beams with even more twists. The more twists, the more angular momentum (the same force that underlies an ice skater’s spin) the beam carries. But so far, these vortex beams don’t carry enough angular momentum to strip particles from a surface, says Verbeeck.
“Whether this will lead to a useful application to manipulate particles or atoms remains to be demonstrated,” he says.
