No Phasers or Light Shows: Tiny Lasers Hone Efficiency

In science fiction, it's the biggest laser that gets the most respect. In real science, it may well be the opposite. A number of scientists around the world are trying to build the smallest possible lasers.

While their devices are laboratory curiosities for the moment, they could one day power fiber-optic communications and computers that run on light instead of electricity.

"A tiny laser can do the job of a big laser and do it better," says Eli Yablonovitch, professor of electrical engineering at the University of California at Los Angeles. "It can work at lower power, and you can get big arrays of them. I'm sure that this structure is only one in a series of clever designs, which are ultimately going to become a new type of nanostructure."

Seng-Tiong Ho and his research team here at Northwestern University's engineering school are building miniature lasers shaped in a ring only 5 microns in diameter. How small is that? It would take 10 of them side-by-side to equal the width of a single strand of hair. The devices are so tiny, in fact, that they squeeze photons into a space that's only about one-tenth the diameter of their natural wavelength. Photons are the particles that make up a beam of light. The result is a remarkably efficient device that Dr. Ho calls a photonic-wire laser.

Lasers focus beams of light very narrowly. Unlike, say, a flashlight, whose beam spreads out the farther its light travels, laser beams hardly spread at all. This property makes them extremely powerful and accurate. Lasers cut tiny holes through diamonds. They weld together very fine wires. One of their most popular uses is in eye surgery; their accuracy also helps scientists keep particle accelerators in line.

The trouble is that most lasers are inefficient. To get photons into their highly unusual narrow-beam "lasing" mode, the typical semiconductor laser wastes all but one out of every 10,000 available photons when it powers up. Sometimes they can use only one out of every 100,000 photons. And these lasers, used in compact-disc players, are considered among the most efficient, far more than the much larger gas lasers used in laser-light shows. It takes relatively large amounts of energy to turn these devices on.

Ho's photonic-wire lasers, by contrast, are very efficient. They typically use 1 out of every 5 to 10 photons on power up. One of his models uses 7 out of every 10. This efficiency allows them to start up very quickly. If miniature lasers ever become a commercial reality, they should be able to speed up the processing of optical signals (for, say, antimissile defenses) and fiber-optic communications (used in long-distance telephone cables). The devices might also one day power optical computers, which make calculations using light-sensitive switches.

Using a device known as a quantum well, Ho's laser generates photons that move inside its inner loop. Squeezed into a tunnel-like space that's only 0.4 microns wide and 0.2 microns high, most of the photons have only one way to travel - along the loop's circular path. (A micron is 1/1,000th of a millimeter.) This is a quantum effect and the key to making Ho's laser so efficient.

As the photons circle, they slowly "leak" to the outer horseshoe structure. This "leaking" effect works a lot like two tuning forks placed side-by-side. When the first one resonates, the second one begins to sound too. Eventually, the first one stops while the second one continues. In similar fashion, the photons resonate their way from the inner loop to the outer horseshoe and then shoot out the ends, creating a visible laser beam.

Ho's prototypes can't do the precision diamond-drilling and micro-welding of today's lasers - they don't have the power. For this to be viable, "it should be [used in] a low-power application," Ho says. Initial versions of his device produced only a few thousandths of a watt. To be practical, commercial versions of the device will have to meld the technology of larger, more traditional lasers.

"In theory, we know there will be improvements," Ho says. But they'll have to wait for laboratory breakthroughs. Commercialization is at least five years away, he adds.

Meanwhile, the race for the world's tiniest laser continues. At a conference in Baltimore last month, researchers unveiled their micro-laser designs.

"I'm trying to make them even smaller than these guys," Dr. Yablonovitch says.