Vienna Group Generates Coherent X-Ray Source

Michael D. Wheeler

VIENNA, Austria -- A group of researchers at the Vienna University of Technology has generated a coherent x-ray source that could pave the way for high-contrast biological imaging.
The collimated x-ray beam, long-sought by legions of researchers, is particularly important because it falls within what is commonly called the "water window." The water window refers to the range between 2.33 nm (the absorption edge of oxygen) and 4.37 nm (the absorption edge of carbon). Within this range, carbon-containing biological objects absorb radiation efficiently while water remains comparatively transparent. The result: Cells and macromolecules appear three-dimensionally with superior clarity.
The research group, headed by Ferenc Krausz in conjunction with two researchers from the University of Alberta at Edmonton, Canada, directed a 5-fs, 780-nm Ti:sapphire laser at a jet of helium gas. When exposed to the laser beam, the helium gas atoms responded by radiating x-ray/UV light. The resulting collimated beam extended into the water window down to <2.5 nm, exhibiting superior brightness of about 5 3 10⁸ photons per millimeter.

New options
These experimental results could translate into some interesting possibilities. Researchers interested in high-contrast biological imaging have had limited options: They could take their samples to very large facilities that use either magnetic undulators in electron storage rings or multikilojoule lasers to produce a plasma-column x-ray amplifier.
Neil Burnett, one of Canadian researchers who worked on the project, is optimistic that the Vienna team's discovery eventually could lead to some real-world applications. "This is a spectacular new capability in terms of light, [but] I think it will be a decade before this really shows up in practice," he said.
Adding credence to the group's findings were similar results reported in the Oct. 20 issue of Physical Review Letters from a research group at the Center for Ultrafast Optical Science at the University of Michigan. Rather than a 5-fs source, the Michigan group used a 26-fs, 800-nm Ti:sapphire laser. This yielded a lower-power x-ray between 2.7 and 5.2 nm, also well within the water window.