May 26, 1997
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FOR RELEASE TUESDAY, MAY 20, 1997
MURRAY HILL, N.J. -- In a major breakthrough, Bell Labs scientists have demonstrated the world’s first laser-based semiconductor sensor that operates at room temperature and at high power to detect minute amounts – potentially parts per billion -- of trace gases or pollutants by scanning for their optical-absorption "fingerprints."
Gases or pollutants are identifiable by their absorption wavelengths, which depend on their chemical nature. The invisible but telltale fingerprints can be detected by focusing the sensor on an area and precisely tuning the laser’s wavelength until its light is absorbed.
"This is a major accomplishment," said Alastair Glass, director of the Bell Labs Photonics Research Laboratory. "The tuning range and peak power of these prototype laser sensors are unprecedented for mid-infrared semiconductor lasers – about 10 and 100 times better, respectively, than commercial lasers of this type – all of which must be cooled."
The experimental sensor is based on the novel quantum-cascade (QC) laser invented at Bell Labs just three years ago and demonstrated at room temperature operation last year. The newest versions can be continuously tuned to operate at any of a wide range of single frequencies in the mid-infrared region of the electromagnetic spectrum, the region in which light is invisible and causes heat.
The laser’s wavelength is determined by several factors. In manufacture, varying the thicknesses of the material layers of the laser sets its wavelength, and adding a grating (etched material) on top of the laser makes the wavelength more precise. In use, adjusting the temperature or the electrical current applied to the laser changes its wavelength.
Using the sensor to detect pollutants involves scanning the area over a smokestack, for example, and shifting the laser’s wavelength until the light crossing the smokestack hits the pollutant’s "fingerprint" and is affected by it.
"This work opens up an entire field of uncooled, tunable mid- and far-infrared laser sensors," said L.N. Durvasula, program manager, Defense Sciences Office, U.S. Defense Advanced Research Projects Agency (DARPA), which partly funded the work. "This is a revolutionary development for sensor applications."
The Bell Labs research team includes Jerome Faist, Claire Gmachl, Federico Capasso, Carlo Sirtori, Deborah Sivco, James Baillargeon and Alfred Cho, in collaboration with Professor Edward Whittaker of the Stevens Institute of Technology in Hoboken, N.J.
"We’re happy to have moved very quickly from basic research to practical application," said Federico Capasso, head of the Quantum Phenomena and Device Research department. "The enabling technology behind all this is the marriage of optics, quantum physics, and crystal growth, in the QC laser, and we foresee a number of market opportunities for these portable, robust devices."
Potential environmental applications include pollution monitoring, automotive emission sensing and combustion diagnostics; law-enforcement possibilities, such as the detection of explosives or of fugitive emissions from illicit drug-manufacturing sites; military applications like portable battlefield sensing of toxic gases and biological toxins; as well as industrial process control, collision-avoidance radar and medical applications.
Conventional semiconductor lasers, which operate at wavelengths from near-infrared to visible, are widely used in other applications such as lightwave communications and compact-disk players.
Detailed information about the distributed-feedback QC laser sensor appears in this week’s issue of the journal Applied Physics Letters. Gmachl will present the research team’s latest experimental results in a post-deadline talk on Thursday at the joint Conference on Lasers and Electro-Optics (CLEO '97) and Quantum Electronics and Laser Science Conference (QELS '97) in Baltimore.
Lucent Technologies designs, builds and delivers a wide range of public and private networks, communications systems and software, consumer and business telephone systems and microelectronics components. Bell Labs is the research and development arm for the company. Further information about the company is available at http://www.lucent.com.
BELL LABS DISTRIBUTED-FEEDBACK QUANTUM-CASCADE LASERS
ADDITIONAL TECHNICAL INFORMATION
Bell Labs researchers have demonstrated continuously tunable, single-mode, high-power room-temperature QC distributed-feedback lasers operating at mid-infrared wavelengths (5 and 8.5 micron) in pulsed mode. The single-mode tuning range is typically 50 nanometers in wavelength, and the peak powers are 60 milliwatts, one and two orders of magnitude better, respectively, than commercially available mid-infrared lead-salt lasers.
The lasers’ high peak power, 50 milliwatts at 300 degrees Kelvin, allows the use of uncooled detectors and enables LIDAR (radar using light) applications. They are particularly well suited for portable, robust sensors in applications such as the point detection of trace gases and remote sensing applications.
QC lasers are made using molecular beam epitaxy (MBE), a materials-growth process from Bell Labs that makes it possible to build structures with layers only a few atoms thick. The QC laser’s emission wavelength is determined initially by quantum-confinement effects: the fact that its layers are so thin – typically a few nanometers, or about 100 billionths of an inch – that electrons are squeezed and change their quantum-mechanical properties, allowing a range of possible wavelengths.
The distributed-feedback lasers incorporate a grating that makes it possible to further refine the laser’s wavelength, making them continuously tunable.
QC lasers were invented at Bell Labs in 1994. Their operation is unlike that of all other laser. They operate like an electronic waterfall: When an electric current flows through a QC laser, electrons cascade down an energy staircase; every time they hit a step, they emit an infrared photon, or light pulse.
At each step, the electrons make a quantum jump between well defined energy levels. The emitted photons are reflected back and forth between built-in mirrors, stimulating other quantum jumps and the emission of other photons. This amplification process enables high output power.
Bell Labs has a long history of laser invention and innovation, beginning in 1958 with publication of the scientific paper describing the concept of the laser by Nobel Laureates Arthur Schawlow and Charles Townes. Both worked for Bell Labs at the time, Schawlow as a researcher and Townes as a consultant.
Applications of Quantum Cascade Lasers
Important potential commercial applications of the QC laser -- based on the spectroscopic detection of molecular species – include environmental sensing and monitoring, with particular emphasis on pollution monitoring and measurement of air quality, in particular:
· remote sensing (over a range from hundreds of meters to a few miles) of chemicals such as toxic gases and vapors emanating from industrial smokestacks, landfills and other hazardous waste sites.
· point sensors, based on multipass absorption cells, of the local concentration of hazardous gases and vapors and short-range sensing (a few to tens of meters) for uses such as monitoring of automobile emissions on the entrance and exit ramps of highways, etc., combustion diagnostics via fast on-line monitoring of gases in automobile exhausts, collision avoidance radar, industrial process control, ammonia- and water-vapor sensing in agriculture to monitor dosages of fertilizer.
The enactment of the Clean Air Act Amendments (CAAA) of 1990 has resulted in a strong increase in these environmental monitoring needs. Most of the toxic chemicals (gases and vapors) included in the CAAA have strong absorption features in the 3-to-5-micron and 8-to-13-micron wavelength atmospheric transparency windows.
Other applications include medical diagnostics, molecular clocks, laser radar heterodyne detection.
Military applications include sensors for biological toxins and toxic gases such as nerve and mustard gas, countermeasures and infrared scene projection, treaty verification, etc. In the law enforcement area, the detection of explosives and of illicit drug production sites are among the possible uses.
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