First Ultrabroadband Semiconductor Laser Built

MURRAY HILL, N.J., Feb. 22 -- Scientists at Bell Labs, the research and development arm of Lucent Technologies, have built the world's first semiconductor laser that emits light continuously and reliably over a broad spectrum of infrared wavelengths. The new device, which eliminates the drawbacks of previous attempts at producing broadband laser action, could have potential applications ranging from advanced optical communications to sensitive chemical detectors, the researchers said.

The new laser's properties are described in an article published in the current issue of the journal Nature.

The technique used to produce the new laser could be the basis for future high-performance semiconductor lasers for fiber optics, in addition to other technology applications, according to the authors.

"An ultrabroadband semiconductor laser could be used to make an extremely sensitive and versatile detector that can detect minute traces of pollutants in the atmosphere," said Claire Gmachl, a physicist at Bell Labs and the lead author of the Nature paper. "It could also be used to produce new medical diagnostic tools such as breath analyzers."

Semiconductor lasers are very convenient light sources. They are compact, rugged, often portable, and powerful. However, semiconductor lasers are typically narrowband devices, emitting light of a single color at a characteristic wavelength.

Multilayer Sampling
An ultrabroadband laser offers significant advantages in that it allows for sampling of a wide swath of wavelengths at the same time. Making a reliable one that can operate under a wide variety of operating conditions has been a goal for scientists for a long time.

To fabricate the new laser, Bell Labs scientists stacked more than 650 different layers of standard semiconductor materials used in photonics on top of one another, much like in a multilayered club sandwich. These layers are grouped into 36 stacks. Each stack has slightly different optical excitation properties and generates light over a short but characteristic wavelength range, while remaining transparent to the rest. When all the stacks are combined, they cooperate to produce broadband laser emission.

The new laser belongs to a class of high-performance semiconductor lasers -- known as quantum cascade (QC) lasers -- that were invented at Bell Labs in 1994 by Federico Capasso, Alfred Cho and their collaborators. A QC laser operates much like an electronic waterfall. When an electric current flows through the laser, electrons cascade down an energy staircase; every time they hit a step, they emit a photon of infrared light. The emitted photons are reflected back and forth inside the semiconductor resonator that contains the electronic cascade, stimulating the emission of other photons. This amplification process enables high output power.

Adjustable Range
The ultrabroadband laser emits 1.3 watts at peak power over the mid-infrared range of 6 to 8 micrometers. (A micrometer is one-millionth of a meter, or roughly one-hundredth the diameter of a human hair.)

"The wavelength range can in principle be made much wider, or also narrower," said Gmachl. "We picked the range of 6 to 8 micrometer for laser action as a good range for a convincing demonstration of the idea. In the future, we may be able to tailor the laser to the specific needs of individual applications, including fiber optics."'

In addition to Gmachl, the interdisciplinary team that designed and fabricated the latest QC laser included Bell Labs researchers Deborah Sivco and Raffaele Colombelli, as well as Federico Capasso, vice president of physical research, and Alfred Cho, adjunct vice president of semiconductor research.