'Quasicrystals' made from 'optical matter.' (using lasers to manipulate particles)

'Quasicrystals' made from 'optical matter'

On atomic and molecular scales, orderly exchanges and interactions of electrons underlie matter's compulsion to organize into specific chemical structures. Chemistry, some say, is the science of this electron behavior.

Last September, three physicists from the Rowland Institute for Science in Cambridge, Mass., reported they had discovered a previously unrecognized laser-induced binding force. They suggested it might provide a new way to manipulate micron-scale particles -- thousands of times larger than atoms -- into precise and regular patterns (SN:9/30/89,p.212). Since these interactions depend on photons, not electrons, the researchers provocatively referred to the simple structures as "optical matter."

Now, in a more extensive report, the same group reports using the laser technique to organize hundreds of tiny plastic spheres into two-dimensional, crystal-like structures -- some of the first examples of optical matter. "We suggest that such organized structures can be considered a new form of matter," write Michael M. Burns, Jean-Marc Fournier and Jene A. Golovchenko in the Aug. 17 SCIENCE.

To make optical matter, the researchers shine a laser through a set of optical devices and mirrors, splitting the single beam into several individual ones. Then they steer the beams through a chamber containing micron-scale polystyrene beads suspended in water. By choosing how many beams to use, and by controlling both the angles at which the beams enter the cell and the angles the beams form with respect to each other, the researchers can sculpt specific optical interference patterns. These patterns of low and high light intensity serve as templates that mimic the molecular patterns of different crystal lattices. Like theatergoers filling an auditorium, the particles file into the sites of maximum light intensity.

With five beams entering the chamber like "a pyramid of light," the scientists have even configured particles into a flat, single-layer quasicrystal structure -- a bizarre molecular arrangement that violates rules of crystallographic symmetry yet characterizes a class of real crystals.

The researchers have yet to reliablyt induce spheres into regulat three-dimensional arrangements, nor have they found a way to preserve the organization after the laser is turned off, Burns notes.

In addition to this patterning effect, intense optical fields can induce binding between particles, the researchers find. Unlike electron bonding, which can link only adjacent atoms, optical binding links particles separated by any of several discrete distances. And this could give materials scientists a new way of assembling particles, or even biological cells, for potential applications ranging from light filters to cellular grids for growing artificial skin, Burns says.

For the moment, however, optical matter remains in the realm of basic research. "We're discovering subtleties and complexities in the interaction of light and matter," Burns says.