Illuminating tiniest microlaser on a chip.

The notion of a laser beam shooting out of a disk small enough to fit easily inside a red blood cell seems more the stuff of science fiction than the laboratory. But advances in technology have put just such a device well within reach.

Investigators at AT&T Bell Laboratories in Murray Hill, N.J., have now fabricated and successfully operated disk-shaped semiconductor lasers measuring 2 to 10 microns in diameter. Commercial semiconductor lasers, such as those used in compact-disk players, typically measure 250 microns across.

In terms of volume, the new devices are the smallest semiconductor lasers yet produced, says Sam McCall, who designed the disk lasers. McCall and his collaborators described these devices earlier this month at a meeting of the Optical Society of America, held in San Jose, Calif. Further details will appear in APPLIED PHYSICS LETTERS in January.

Resembling a miniature thumbtack, the novel microlaser consists of a layered disk only 400 or so atoms thick mounted on a slender pedestal. The disk -- a thin layer of indium gallium arsenide sandwiched between layers of indium gallium arsenide phosphide -- absorbs light from a helium-neon laser to generate coherent infrared radiation at wavelengths ranging from 1.3 to 1.5 microns.

"We've been working at low temperatures where we cool the substrate with liquid nitrogen," McCall says. "But we've also warmed them up, and they've worked at a few degrees above freezing."

These microdisk lasers operate in what researchers call a "whispering gallery" mode, named for the way words whispered at one location near the interior wall of a circular, domed chamber can be readily overheard anywhere else along the wall. Like these echoing whispers, photons travel along a disk's edge for long periods with little loss.

"In the perfect geometry, the beam would come out along the edge and would sort of spray out in the plane of the disk," McCall says. "We can change the ideal geometry a little bit to get the beam to go where we want it to. For example, by putting grooves at just the right places on the top surface, we can get the beam to come out vertically."