Tiny silicon grains for lasers on a chip

Jan. 11, 2002 at 2:01 PM

CHAMPAIGN, Ill., Jan. 11 (UPI) -- Newly developed silicon grains that emit laser light may in the future serve as the backbone of an optical computer network light years faster than today's Internet.

The microscopic particles are only 3 nanometers -- or billionths of a meter -- in diameter, making them more than 30,000 times thinner than a human hair. They glow red laser light when green light from a mercury lamp shines on them.

"The particles are ultra-bright -- you can detect single ones with the naked eye," said lead researcher Munir Nayfeh, a physicist at the University of Illinois at Champaign-Urbana. "This type of laser could possibly replace the wires used to communicate between components in a circuit."

Today's microchips are miniature networks of metal wires data travels down in electric pulses. Scientists are working on future computers that depend instead on light pulses that zip much faster and more efficiently through optical fibers than electricity moves through metal circuitry.

Key to this optical technology are lasers on a chip. While miniature lasers can be made from semiconductor alloys, integrating them into silicon circuitry often is too costly and difficult to be economical.

The new nanoparticles, on the other hand, are lasers made from silicon itself, and in theory could easily be incorporated into silicon chips.

"There are a number of new effects left to be discovered in these systems -- they haven't explored even 10 percent of the possibilities which are in there, and they're coming up with new results fairly often," commented physicist Lubos Mitas at North Carolina State University in Raleigh. "That's very exciting -- to have a group break new ground in what can be done and find new effects we didn't even think about previously. The number of applications could be enormous."

To create the nanoparticles, the researchers begin with a silicon wafer, which they pulverize using a combination of chemistry and electricity in a etching solution of acid and bleach.

"We use an electrochemical treatment that involves gradually immersing the wafer into an etchant bath while applying an electrical current," Nayfeh said. "This process erodes the surface layer of the material, leaving behind a delicate network of weakly interconnected nanostructures. The silicon wafer is then removed from the etchant and immersed briefly in an ultrasound bath."

Under the ultrasound treatment, the fragile nanostructure network crumbles into individual particles of different size groups. The slightly larger, heavier particles precipitate out, while the ultra-small particles remain in suspension where they can be recovered.

"They can produce the nanoparticles in fairly large quantities by really simple experimental processes, so that's important for practical applications," Mitas said. "You want something you can reproducibly produce in sufficiently large quantities."

The researchers also have developed nanoparticles of various smaller sizes that glow blue, green and yellow-orange. They are searching for larger nanoparticles they believe may emit infrared light.

"We're at the cusp of hard-core applications -- I think we're talking months. We're certainly very close," Nayfeh said in an interview with United Press International. "But for lasers we still need some control. The phenomenon is in place, but how then do you integrate such things in a chip, and add control through reflectors and such? We have some collaborators who may be more inclined to work in that regard. Hopefully we'll get somewhere soon."

Since the particles have no known toxic effects in living tissues, the research team is focusing on biological applications for the nanoparticles.

"You can use is them as a tag, attach them selectively to cells -- for instance, ones with defects like cancer cells -- and then study the action of the cells," Nayfeh said.

The researchers reported their findings in Applied Physics Letters.

(Reported by Charles Choi in New York.)

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