'Optical matter' emerges under laser.

'Optical Matter' Emerges Under Laser

There definitely is something new under the sun, or at least under a laser. For several years, researchers have known that a laser beam can push bacterium-sized particles in the direction the light is traveling. But the beam can also induce a previously undetected attraction between the laser-soaked particles, three physicists report.

The group describes the first, simple examples of "optical matter" -- tiny spheres stuck together under laser light--in the Sept. 18 PHYSICAL REVIEW LETTERS.

"A consequence is that light waves can serve to bind matter in new organized forms," say Michael M. Burns and Jean-Marc Fournier of the Rowland Institute for Science in Cambridge, Mass., and Harvard physicist Jene A. Golovchenko, who also works at the Rowland Institute. With further development, "optical binding" could join the small club of chemical and physical interactions that govern how molecules and larger material building blocks organize into increasingly larger-scale structures, Golovchenko says.

"The atoms that come together to make up the various forms of organized matter around us are bound together by forces that can be viewed as originating from the exchange of electrons between atoms," he notes. "Our group has been studying the possibility that new forms of matter might exist, which we think of as optical matter, in which that [electronic] binding is replaced by the exchange of photons."

"I'm struck by its novelty," comments Arthur Ashkin, a physicist at AT&T Bell Laboratories in Holmdel, N.J., who uses lasers to manipulate cells and other microscopic objects. Physicist Noel A. Clark of the University of Colorado at Boulder suggests the newly described interaction might prove useful for getting bacteria to stick together or for aligning particles in preferred configurations before chemically bonding them. "It's important work," Ashkin says, but he thinks talking about applications is premature.

In their experiments, Golovchenko and his colleagues inject a solution containing polystyrene mini-spheres (1.43 microns in diameter) between two closely spaced glass plates, then shine an intense laser beam through the plates. Radiation pressure from the beam traps a few spheres against the top plate. The researchers also observe the effects of an optical binding force among the trapped spheres.

In the simplest example of the force's effects, pairs of spheres that start out roughly 5 microns apart take discrete, wavelength-sized steps (0.387 microns in this example) toward one another until they touch. At room temperature, switching off the laser enables Brownian, or thermally induced, motion to separate the spheres, but the scientists note that freezing the sample solution preserves the optical matter.

Golovchenko told SCIENCE NEWS his group already has seen the spheres form into much more complex optical matter. He declined to discuss these observations until the work is published in a journal article.

How does optical binding work? Like sunlight, radio signals and other forms of electromagnetic energy, laser light consists of oscillating electric and magnetic fields. The light induces oscillating electrical currents within the spheres, turning them into minuscule antennas that respond by also emitting radiation -- a process called light scattering.

The electric and magnetic fields from the laser interfere with those from the light scattering off the spheres, creating a busy electromagnetic landscape pocked with energy wells. The spheres "hop" from well to well until they settle into a pair of deeper wells separated by a distance equal to a sphere's diameter. That's when the spheres make physical contact and stick together.