UC Berkeley researchers are developing plasmon laser sensors that could soon give bomb-sniffing canines some serious competition, even get them jobless.

Led by Xiang Zhang, UC Berkeley professor of mechanical engineering, the researchers has found a way to dramatically increase the sensitivity of a light-based plasmon sensor to detect incredibly minute concentrations of explosives. They noted that the sensor could potentially be used to sniff out a hard-to-detect explosive popular among terrorists.

The sensors were put to test with various explosives—including 2,4-dinitrotoluene (DNT), ammonium nitrate and nitrobenzene—and found that the device successfully detected the airborne chemicals at concentrations of 0.67 parts per billion, 0.4 parts per billion and 7.2 parts per million, respectively. One part per billion would be akin to a blade of grass on a football field. They noted that these results are much more sensitive than those published to date for other optical sensors.

"Optical explosive sensors are very sensitive and compact," said Zhang, who is also director of the Materials Science Division at the Lawrence Berkeley National Laboratory and director of the National Science Foundation Nanoscale Science and Engineering Centre at UC Berkeley. "The ability to magnify such a small trace of an explosive to create a detectable signal is a major development in plasmonsensor technology, which is one of the most powerful tools we have today."

Latest generation of plasmon sensors

The nanoscale plasmon sensor used in the lab experiments is much smaller than other explosive detectors on the market. It consists of a layer of cadmium sulfide, a semiconductor, that is laid on top of a sheet of silver with a layer of magnesium fluoride in the middle.

In designing the device, the researchers took advantage of the chemical makeup of many explosives, particularly nitro-compounds such as DNT and its more well-known relative, TNT. Not only do the unstable nitro groups make the chemicals more explosive, they also are characteristically electron deficient, the researchers said. This quality increases the interaction of the molecules with natural surface defects on the semiconductor. The device works by detecting the increased intensity in the light signal that occurs as a result of this interaction.

The ability to increase the sensitivity of optical sensors traditionally had been restricted by the diffraction limit, a limitation in fundamental physics that forces a trade-off between how long and in how small a space the light can be trapped. By coupling electromagnetic waves with surface plasmons, the oscillating electrons found at the surface of metals, researchers were able to squeeze light into nanosized spaces, but sustaining the confined energy was challenging because light tends to dissipate at a metal's surface.

The new device builds upon earlier work in plasmon lasers by Zhang's lab that compensated for this light leakage by using reflectors to bounce the surface plasmons back and forth inside the sensor – similar to the way sound waves are reflected across the room in a whispering gallery – and using the optical gain from the semiconductor to amplify the light energy.

Zhang said the amplified sensor creates a much stronger signal than the passive plasmon sensors currently available, which work by detecting shifts in the wavelength of light. "The difference in intensity is similar to going from a light bulb for a table lamp to a laser pointer," he said. "We create a sharper signal, which makes it easier to detect even smaller changes for tiny traces of explosives in the air."

The sensor also could be developed into an alarm for unexploded land mines that otherwise are difficult to detect, the researchers said. According to the United Nations, landmines kill 15,000 to 20,000 people every year. Most of the victims are children, women and the elderly.

The U.S. Air Force Office of Scientific Research Multidisciplinary University Research Initiative programme helped support this work. The research's findings were published in online publication of the journal Nature Nanotechnology.

By: DocMemory
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