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.

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.