Helium Beam's Gentle, Sensitive Touch

A microscope with unprecedented sensitivity, based on a beam of atoms rather than a standard setup using light or electrons, is one step closer to reality. Researchers have coaxed helium atoms into an intense, needle-fine beam that could one day probe micrometer-scale detail without damaging an object's surface.

Atoms can play the role of light in a microscope because, according to quantum mechanics, they too exist as waves--albeit thousands of times shorter than light waves. Shorter waves make for crisper microscopic images. And in contrast to energetic probes such as x-rays or electrons, helium atoms bounce lightly off the target surface without damaging it, explains Peter Toennies of the Max Planck Institute for Fluid Dynamics in Göttingen, Germany. "With helium atoms you see the flesh, whereas with all other probe particles you see the bones," he says.

Toennies and his colleagues start with a jet of helium atoms spurting out of a fine nozzle at 1 kilometer per second. To trim this jet, the atoms pass through a funnel-like "skimmer," a drawn-out glass micropipette with a tip just 1 micrometer across. The beam then passes through a "lens" half a millimeter wide that contains a series of concentric rings. As the atom waves emerge from the rings, they interfere with each other. Because the wave crests match up along the axis of the beam, it gets concentrated into a central spot just 2 micrometers wide--10 times narrower and 100 million times brighter than in earlier atom-focusing efforts, they report in the 22 November issue of Physical Review Letters.

What's more, the Göttingen group's helium atoms are in their lowest energy state, unlike the atoms in earlier efforts, says Jabez McClelland of the National Institute of Standards and Technology in Gaithersburg, Maryland. That's crucial for any future atom microscope, because sedate helium atoms scatter from a surface more predictably than do energetic ones.