Friday, May 23, 2008

A Tiny Terahertz Laser

A new kind of semiconductor terahertz laser works at room temperature.

By Kate Greene

Laser lab: Federico Capasso, a professor of applied physics at Harvard, and his research associate Mikhail Belkin. Capasso and his team have built the world’s first semiconductor terahertz laser that operates at room temperature. Credit: Eliza Crinell, Harvard School of Engineering and Applied Sciences

Terahertz radiation has captivated researchers, engineers, and security experts with its promise of sensitive chemical detection, ultrafast data transmission, and the ability to "see through" walls and clothing. Today, however, terahertz devices are in limited use. Part of the problem is that engineers are still developing cost-effective and portable ways to emit and control the radiation.

But now, researchers at Harvard University have developed a semiconductor-based terahertz laser that is smaller than a fingernail. And importantly, it works at room temperature, unlike other semiconductor terahertz lasers that need to be cooled with tanks of liquid nitrogen. When you shrink a laser to the size of a chip and make it work at high temperatures, it suddenly becomes much more useful, says Federico Capasso, a professor of physics at Harvard and lead researcher on the work. A room-temperature semiconductor terahertz laser is portable, and since it can be fabricated in bulk on a wafer, it would be more economical than other types of terahertz lasers. Removing the need for cryogenic cooling also makes it easier and less expensive to use.

Capasso is a leader in developing semiconductor lasers. In the 1990s, he was on the team of researchers at Bell Laboratories who developed a specific type of semiconductor laser called the quantum-cascade laser. In 2004, the first quantum-cascade lasers were commercialized, and now they're used in medical diagnostic tests and for detecting certain types of chemicals and pollutants. "The next frontier," Capasso says, "is finding a way to capture terahertz in a quantum-cascade laser."

Quantum-cascade lasers use a series of differing energy gaps within an optical chip to produce mid-infrared light. Researchers believe that this design is the most practical one for a terahertz semiconductor laser, but it's been difficult to make them work well. The size of the energy gaps, or quantum wells, within quantum-cascade lasers determines the frequency of light emitted; electrons are injected into the upper energy levels, and when they fall to the lower energy levels, they produce photons. But in order to produce terahertz frequencies, the energy gaps need to be extremely close together, and it's difficult to selectively inject electrons in the right level. When the laser is cooled, the energy gaps spread out, and it's not as difficult, but as the laser warms, the output power drops off dramatically.

While researchers at MIT and at universities in Italy and Switzerland are also developing terahertz quantum-cascade lasers, Capasso's team is the first to show that a powerful beam of radiation, with a frequency of about five terahertz, is possible at room temperature.