X-Rays Thrown For a Loop

High energy x-rays are a great way to probe matter, but it's hard to control a beam of them, since they shoot straight through lenses. Now, inspired by the famous "whispering gallery" of St. Paul's Cathedral in London, physicists have developed the most efficient way yet to bend a powerful x-ray beam. The device, described in this week's issue of Physical Review Letters, might someday become a crucial component of an x-ray laser.

Scientists have previously tried bending an x-ray beam by bouncing it off a metal plate at a very shallow angle, like skipping a stone on water, or by manufacturing layers and layers of artificial crystals. But these techniques only deflect the beam by a few degrees. They have now had better luck imitating the acoustics of St. Paul's, where a beam of sound waves--a whisper, for instance--passes around one side of the circular gallery, bounces along the curved walls, and ends at the other side.

The same principle can work for light. To make a whispering gallery for x-rays, Jene Golovchenko and Chien Liu polished an 18-millimeter-long silicon wafer, which they bent into an arc. When they fired a beam of high-energy x-rays (0.7 angstroms) at a shallow angle, the photons grazed the wafer's surface. Almost all were reflected and then hit the wafer again, further down the curve. The process repeated down the length of the wafer, with the beam bouncing nearly 100 times against the wall. At the end of the line, the ray had been deflected 13 degrees.

Though whispering galleries had been tried before, nobody had succeeded with such energetic x-rays. "By far, this is the shortest wavelength example of a whispering gallery," says Malcolm Howells, a physicist at the Lawrence Berkeley National Laboratory. And it's just possible, he says, that x-rays trapped in a racetrack-shaped whispering gallery could stimulate the production of more x-rays--a type of amplification needed for an x-ray laser.