Dark Pulses from Quantum-Dot Laser

An experimental type of laser could lead to faster communications.

When you think of a laser, you probably imagine a continuous beam of light. But many lasers emit incredibly intense and short pulses of light–these “pulsed lasers” are used in medical and laboratory devices, and in industrial equipment. Now, researchers have developed a new type of pulsed laser that uses quantum dots to emit bursts not of light, but of darkness–a trick that could prove useful for optical communication and rapid chemical analysis.

On again, off again: The dark pulse laser, which uses quantum dots, is seen here as the thin strip attached to wires.

The new “dark pulse laser” was developed by scientists at the National Institute of Standards and Technology (NIST) and the research institute JILA in Boulder, CO. The NIST laser emits light punctuated with extremely short bursts of darkness. “Think of it as a continuous wave laser, except with a really fast shutter,” says Richard Mirin, a scientist at NIST.

This shutter creates dark pulses that last 90 picoseconds. This speed of operation could help scientists probe ultrafast chemical and biological reactions. A dark pulse laser could also be used in a fiber-optic telecommunication scheme where information would be encoded as dark pulses, which tend to be able to travel long distances without degrading in quality.

The pulses are generated by quantum dots inside a chip made of ultrathin layers of semiconducting materials. A periodic drop in intensity of about 70 percent is caused by a mismatch in the speed with which the quantum dots and the surrounding materials interact with the electrical current and internally produced photons. Semiconductor lasers are already found in telecommunications systems, DVD players, and laser pointers. But this laser design is different in that it uses quantum dots–atom-sized structures that emit light when excited–to produce dark pulses.

NIST’s Mirin says that the group initially wanted to make a bright pulse laser using quantum dots. Quantum dots can be used to make lasers that have a broad range of colors. “It turns out that the process of discovery led us to something interesting with this particular [quantum dot] configuration,” he says.

Dirk Englund, professor of electrical engineering and applied physics at Columbia University, says that the dark pulses created by the scientists at NIST and JILA are similar to well-known quasi-particles called “dark solitons.” Regular solitons are pulses of light that are passed through a special optical material that keeps them from dispersing, or losing energy over a distance. Dark solitons are the “absence of energy in a continuous beam background,” Englund says.

But it is difficult to generate dark solitons, which is why the technique hasn’t been used in telecommunications, says Mirin. The setup is cumbersome, and sometimes only a single dark soliton is produced. The new dark pulse laser makes it easier to produce a soliton-like effect, says Mirin.

“While it does not appear that these dark pulses are actually solitons,” says Englund, “they are similar and could prove useful in communications and optical measurements applications.”

It is too early to promise that dark pulses will revolutionize telecommunications, Mirin says. Since today’s communications systems use bright pulses of light, optical fibers have been engineered to reduce the amount of energy lost due to dispersion, which means dark pulses couldn’t travel effectively along existing fiber. Dark pulse lasers would need their own specially engineered type of fiber. Still, he’s encouraged that the discovery of a compact and reliable source of dark pulses could open up new areas of research.

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