Bell LaboratoriesMurray Hill, N.J.

Room-temperature quantum cascade laser

In 1994 Bell Laboratories invented a significantly new semiconductor laser, the quantum cascade (QC) laser. Unique to the QC laser was the ability to tailor it to emit light at specific wavelengths by altering layer thickness of the laser -- not by altering laser materials -- and the laser's ability to emit wavelengths in the 3-to-5-micron and 8-to-13-micron spectral ranges, making it viable for detecting pollutants and toxic gases in the atmosphere. Bells Labs has continued to advance the QC laser, primarily because those initial devices were limited by operating temperatures of lower than -300 F, and this year demonstrated the world's first room-temperature devices that operate in those same spectral ranges and provide about 25 times more pulsed power than predecessor QC lasers (power extends the distance over which the wavelengths can be projected). At 5-micron wavelength, the new QC lasers generate pulsed peak power of 200 milliwatts at 77 F and 100 milliwatts at 127 F. Conventional semiconductor lasers at comparable and greater wavelengths operate only up to temperatures of about 60 F and produce just one-one hundredth the power. Federico Capasso, head of the Quantum Phenomena & Device Research Dept. at Bell Labs and co-inventor along with Jerome Faist, says that while conventional semiconductor layers can operate in the mid-infrared region, they need substantial cooling, typically have low power, and emit a fluctuating wavelength. "They tend to emit several wavelengths that are closely spaced," says Capasso, "which means it's very difficult to resolve a single molecule of one particular kind of material or gas." The new QC lasers have no such difficulty. "In these two windows [3-to-5-micron and 8-to-13-micron] the atmosphere is basically transparent," explains Capasso. "If you put a laser through corresponding to these wavelengths, the light is not absorbed -- transparent means the light of the laser goes through without being activated. It turns out that in these two regions, many molecules -- particularly molecules of toxic gases and gases of pollutants, such as nitric acid and compounds containing sulfur and carbon monoxide -- absorb precisely in those wavelength regions where the atmosphere does not activate. . . . Traces of pollutant gases or other toxic substances will activate a laser going through at precisely those wavelengths that are characteristic of the polluting material. So you can detect signatures of these pollutants by passing a laser beam through the atmos-phere at certain wavelengths over distances that could range from a few tens of meters up to a kilometer." Applications in those two spectral windows could include remote sensing of smokestacks or hazardous landfills or detection of gas emissions from industrial processes. Applications specific to 2-to-5-micron wavelengths include monitoring of auto exhausts, collision avoidance radars, molecular clocks, military applications, medical diagnostics, and molecular spectroscopy. The next stage of development for the room-temperature QC lasers at Bell Labs, a division of Lucent Technologies, will be working with a two-year grant from the Defense Dept.'s Advanced Research Programs Administration (ARPA) to develop continuous-wave -- not pulsed-wave -- QC lasers that operate at temperatures at which thermo-electric coolers can be used (above -100 F), making the applications commercializeable. The present models can emit continuous wavelengths in addition to pulsed power, but they must be cooled with bulky, expensive equipment. The research team achieved the breakthrough to pulsed-power room-temperature operation by optimizing heat dissipation, and Capasso expects that by 1998 the continuous-wave temperature goals sought by ARPA will be achieved by similar techniques.

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