EVANSTON, Ill. --- The difficulty of detecting the presence ofexplosives and chemical warfare agents (CWAs) is once again all tooapparent in the news about the London bombings.
In a significant breakthrough, researchers at Northwestern University'sCenter for Quantum Devices have demonstrated a specialized diode laserthat holds promise as a weapon of defense in both civilian and militaryapplications. Once optimized, the tiny laser could quickly detectexplosives and CWAs early and warn against possible threats.
The Northwestern team, led by center director Manijeh Razeghi, becamethe first to create a quantum cascade laser (QCL) that can operatecontinuously at high power and at room temperature with an emissionwavelength of 9.5 microns and a light output of greater than 100milliwatts.
Existing standard diode lasers, such as those used to read compactdiscs or barcodes, do not operate effectively in the longer wavelengthsthat are required to detect CWAs. The challenge for researchers aroundthe world has been to develop a portable laser that operates in thefar-infrared (wavelengths of 8 to 12 microns). Every chemical has aunique "fingerprint" because it absorbs light of a specific frequency,and most CWAs fall in the 8 to 12 micron region.
"Our achievement is critical to building an extremely sensitivechemical detection system," said Razeghi, Walter P. Murphy Professor ofElectrical and Computer Engineering. "One of the key elements in asuccessful system is the laser source. Both mid- and far-infrared diodelasers need to operate at room temperature, have high power -- greaterthan 100 milliwatts -- and be extremely small in order to keep thesystem portable. We have now demonstrated such a laser in thefar-infrared wavelength range."
This research is part of a four-year program called LaserPhotoacoustic Spectroscopy (LPAS) funded by the Defense AdvancedResearch Projects Agency (DARPA). The goal of the program is to developa man-portable system that can warn against a large number of potentialthreats using mid- and far-infrared diode lasers. Once optimized, suchlasers would be a very reliable means of detecting explosives andchemical warfare agents while distinguishing them from benign chemicalspresent in the atmosphere.
During the next two years Razeghi and her team will work to puttogether a detection system based on the center's far-infrared laser.The system will then be evaluated by DARPA for use by the military.
Northwestern is a world leader in high-power QCL research. The Centerfor Quantum Devices was the first university research lab in the worldto successfully grow, fabricate and test quantum cascade lasers back in1997. By utilizing quantum mechanical design principles and advancedcrystal growth techniques, the QCL is able to demonstrate high-powerand high-temperature operation.
After the initial demonstration of room-temperature pulsed lasers in1997, the primary efforts of Razeghi and her colleagues over the pastseveral years have been to increase the laser's operating temperature,power output and efficiency in order to achieve the continuousoperation necessary for sensitive chemical analysis.
In 2003 the center was the first to demonstrate high-powermid-wavelength infrared continuous wave QCLs operating above roomtemperature. (Like the far infrared, standard diode lasers cannotaccess this mid-infrared range.) At present, individual devices withoutput powers of several hundred milliwatts have been demonstrated inthe 3 to 5 microns wavelength range.
Razeghi's research is supported by the Defense Advanced ResearchProjects Agency, U.S. Air Force Office of Scientific Research, the ArmyResearch Office and the Office of Naval Research.
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