Atomic cascade broadens laser

By Kimberly Patch, Technology Research News

Ever since ruby red light was coaxed into forming the coherent, or synchronized waves of a laser 40-odd years ago, researchers have been expanding the ranges of wavelengths that certain types of lasers can emit, which, in turn, has expanded their uses.

Researchers from Lucent Technologies' Bell Labs have inched forward the utility of the laser with a device that emits coherent light at a consistent intensity over a broad range of wavelengths in the mid-infrared, or heat, range.

The laser is made of 650 very thin, alternating layers of two types of semiconductor material. The thinnest layers are only a few atoms deep, and the entire stack measures 1.65 microns, or a little less than twice the thickness of a bacterium.

What makes the laser especially useful is that the layers are grouped into 36 active regions that emit different wavelengths of light, said Claire Gmachl, a technical staff member at Lucent Technologies' Bell Labs. "[They] are all designed differently and cooperate to generate a broadband spectrum."

This type of laser has the potential to transmit information over optical data communications lines faster than a terabit per second, and could also be used in very precise metrology and spectroscopy applications, according to Gmachl. A terabit of information is equivalent to about 62 hours of full-motion video. Metrology is the branch of science concerned with measuring mass, length and time; spectroscopy measures the way materials interact with light in order to determine their chemistry, energy levels, and molecular structures.

Key to the broadband design is quantum cascade, a technology pioneered a few years ago by some of the same researchers.

Electrons orbit atoms at various energy levels, and must gain or lose energy, usually in the form of photons, to change levels. The materials lasers are made from ordinarily emit light as electrons drop to a lower energy level, releasing a single photon each. In a quantum cascade, an electron falls, or cascades, down several energy levels at once, giving off a photon at each level.

In this type of laser, the optical properties of the light-emitting regions depend more on layer thickness and less on chemical properties, said Gmachl.

The layers contain quantum wells, which are essentially electrical dimples that trap electrons. The wavelength of light emitted or absorbed by a quantum well depends on the width of the well.

The researchers used indium gallium arsenide and aluminum indium arsenide as the semiconductor layers. The more conventional semiconductor material used in today's computer chips, silicon, would not work in this type of laser because it strongly absorbs light, according to Gmachl.

The technical challenge in making the laser was putting together all the layers, said Gmachl.

The researchers' prototype laser emits lightwaves ranging from 6 to 8 microns, or 10 to 100 times wider than the same type of laser that has just one type of active region. "The wavelength range can in principle be made much wider, or also narrower," said Gmachl. "Eventually, one will be able to custom-tailor the broadband spectrum to the application."

Currently the laser shines light all the time. The researchers are now working on locking all the emitted modes together, said Gmachl. Locking the modes means synchronizing the peaks and troughs of the different wavelengths, or colors. "This would provide ultrashort light pulses which are at the same time very powerful and still spectrally broadband," she said. Ultimately, these could be used in fiber-optic communications, she said.

This type of laser could be used as a gas sensor in a few years, said Gmachl. "Applications to the fiber optic wavelength range are [much] farther out," she said.

Semiconductor lasers that use quantum wells and have an extremely wide tuning range were reported as early as 1989, said Amnon Yariv, a professor of physics and applied computing at the California Institute of Technology. What is new, however, is building a broadband quantum well laser using quantum cascades, "which allows operation in the mid-infrared wavelengths, albeit at the cost of requiring cryogenic cooling," he said.

Gmachl's research colleagues were Deborah L. Sivco, Raffaele Colombelli, Federico Capasso and Alfred Y. Cho. They published the research in the February 21, 2002 issue of the journal Nature. The research was funded by Lucent Technologies and the Defense Advanced Research Projects Agency (DARPA).

Timeline:   < 3 years; > 5 years
Funding:   Corporate; Government
TRN Categories:  Optical Computing, Optoelectronics and Photonics; Telecommunications
Story Type:   News
Related Elements:  Technical paper, "Ultra-Broadband Semiconductor Laser," Nature, February 21, 2002.


March 6, 2002

Page One

Interactive robot has character

Atomic cascade broadens laser

Biology harbors hidden complexity

Heat engines gain quantum afterburner

Nanotubes branch out


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