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Quantum Cascade Laser to Play Key Role in Gas Detection

Photonics Spectra
Jul 1997
Michael D. Wheeler

MURRAY HILL, N.J. -- A new laser-based sensor could mark a breakthrough in the detection of minute quantities of trace gases and pollutants.
The exceedingly sensitive device is capable of detecting pollutants in parts per billion, according to one of its developers, Federico Capasso.
This sensitivity lends itself to a host of potential applications including detecting emissions from illicit drug manufacturing sites, portable battlefield sensing of toxins and remote pollution monitoring.
Developed by Capasso and several colleagues at Lucent Technologies' Bell Laboratories, the sensor detects foreign elements in the atmosphere by their absorption of infrared wavelengths.
These elements, or gases, have different chemical compositions, so each absorbs different IR wavelengths. In effect, each gas leaves behind its own optical absorption "fingerprint," enabling users to immediately identify, for example, whether a gas is carbon monoxide or radon.

Room temperature operation
While lead salt diode lasers can detect in the mid-IR, they have several drawbacks; namely, they require cooling and operate at low powers. Bell Labs' laser generates 50 mW at 5.4 µm at 80.6 °F (300 K). In addition, the device is tunable from 4.3 to 11.7 µm.
No other semiconductor laser emitting in the mid-IR can operate at room temperature, Capasso said, and that could determine the sensor's commercial success.
The laser's robust nature and wide tunability are by-products of years of research on the "quantum cascade" phenomenon.
To produce the quantum cascade laser, electrons are injected through tunneling between minibands. They cascade between barriers and quantum wells, ultimately recycling and generating between 20 and 25 photons from every electron.
Other research is being done on quantum cascade lasers. One member of Capasso's research group, Carlo Sirtori, plans to work in Europe on a gallium-arsenide laser that is expected to push close to the 1.55-µm range.
Plans to further develop the quantum cascade sensor are also on track. Capasso said the group's short-term strategy includes partnering with outside groups to produce a commercially viable device.
Capasso said he may even spin off a company to manufacture the sensors, possibly from the current affiliate, Lucent.

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