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Laser Sensor Could Lead Way To Handheld Bomb-detectors

Tiny plasmon-based sensor detects minute traces of explosives in the air

A team at University of California, Berkeley led by Xiang Zhang, professor of mechanical engineering, has shown that a plasmon laser sensor can be used to detect minute concentrations of explosives in the air, including a hard-to-detect plastic explosive called PETN popular among terrorists. The results were published in the  journal Nature Nanotechnology.

Plasmon lasers work by coupling electromagnetic waves with the electrons that oscillate at the surface of metals to squeeze light into nanoscale spaces far past its natural diffraction limit of half a wavelength. The UC Berkeley  plasmon laser is based on a cadmium suphide semiconductor square measuring around 50nm thick and 1000nm long, placed on a silver surface and separated by a 8nm gap of magnesium fluoride. The most intense electric fields of the device reside in the magnesium fluoride gap.

In designing the sensor 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. Their unstable nitro groups are characteristically electron deficient, which increases the interaction of the molecules with natural surface defects on the semiconductor. The sensor works by detecting the increased intensity in the light signal that occurs as a result of this interaction.

The engineers put the sensor to the test with various explosives - 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. These results, which are much more sensitive than those published to date for other optical sensors, were published in the advanced online publication of the journal Nature Nanotechnology.

The researchers hope that their plasmon laser sensor could detect pentaerythritol tetranitrate, or PETN, a plastic explosive favoured by terrorists because small amounts of it pack a powerful punch and it escapes x-ray machines when not connected to a detonator. It is the explosive found in Richard Reid's shoe bomb in 2001 and Umar Farouk Abdulmtallab's underwear bomb in 2009.  PETN has more nitro functional groups and is more electron deficient than the DNT we detected in our experiments, so the sensitivity of their device should be even higher than with DNT, say the researchers.

"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, " said Zhang.

The sensor device builds upon earlier work on plasmon lasers by Zhang's lab that uses 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."

Co-lead author Ren-Min Ma, an assistant professor of physics at Peking University who did this work when he was a postdoctoral researcher in Zhang's lab, said: "Our technology could lead to a bomb-detecting chip for a handheld device that can detect the tiny-trace vapour in the air of the explosive's small molecules."

The sensor also could be developed into an alarm for unexploded land mines that otherwise are difficult to detect, the researchers said.

The US Air Force Office of Scientific Research Multidisciplinary University Research Initiative program helped support this work. 

'Explosives detection in a lasing plasmon nanocavity' by Ren-Min Ma et al, appears in Nature Nanotechnology (2014), doi:10.1038/nnano.2014.135



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