Growing and carving micro-laser forests.

Growing and carving micro-laser forests

On a dime-sized chip, researchers have created a high-tech forest of 2 million cylindrical lasers, each about a twentieth the girth of a human hair and a tenth of a hair-width in height. Likely the world's smallest lasers, the devices represent a stretching step toward harnessing light for speeding up computing and communications and for designing otherwise impossible light-based technologies, the scientists say.

As electronic chips get denser and faster, the wires that carry bits of data to and from them seem increasely sluggish, especially for the ever-more-complicated problems and decisions now being relegated to computers. To ease the bottleneck, scientists have been looking toward materials such as gallium arsenide, which can transform electrical currents into beams of light zooming through optical fibers.

"The rate at which you can transfer information along an optical fiber is much higher than the [transfer] rate along an electrical wire," notes James P. Harbison, one of three researchers from Bell Communications Research in Red Bank, N.J., who are working on the project with four colleagues from AT&T Bell Laboratories in Murray Hill, N.J. Jack L. Jewell of AT&T initiated the project and unveiled the results July 18 at a conference in Kobe, Japan.

To create the laser forest, the scientists start with a technique called molecular beam epitaxy to grow semiconductor chips with a composition they can regulate at each molecular layer. Using a relatively thick layer of gallium arsenide as a crystal template, they stack alternating layers of gallium arsenide and aluminum arsenide molecules to form two mirror-like regions. These mirrors will sandwich the lasers' light-emitting "gain medium," made of indium gallium arsenide.

The next job is to chisel individual lasers from the multilayered chip. After a thin coat of gold, which will serve as an electrical contact for pumping the lasers, the chip gets an icing of a photoresist material that toughens when illuminated. Shining light onto the coated chip through a polka-dotted mask, then washing away the unexposed photoresist icing, yields a polka dot photoresist pattern. A beam of xenon ions then cuts through the chip's photoresist-free parts like a cookie cutter, producing the 2 million multilayered microcylinders.

A gentle current (a thousandth of an amp) injected into the gold layer with an electrical probe pumps the lasers into action. Excited electrons travel through the top mirror into the gain medium, where they emit light as they combine with nearby sites of positive charge originating in the bottom mirror. The flanking mirrors return some of the light to the gain medium to stimulate emission of more light of exactly the same wavelength and phase.

Most existing semiconductor lasers are at least 50 times larger, emit light from their edges rather than their surfaces and require either higher electrical currents or other lasers to run them, Harbison notes. The smaller, more them, Harbison notes. The smaller, more readily pumped, surface-emitting lasers should integrate more smoothly with electronic circuitry into hybrid "opto-electronic chips," he says.

"It's certainly a major step toward realizing these devices," comments laser-making physicist Paul L. Gourley of Sandia National Laboratories in Albuquerque, N.M.