Tiny laser sensor increases bomb detection sensitivity

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.

Potential use to sense a hard-to-detect explosive
“We think that higher electron deficiency of explosives leads to a stronger interaction with the semiconductor sensor,” said study co-lead author Sadao Ota, a former Ph.D. student in Zhang’s lab who is now an assistant professor of chemistry at the University of Tokyo.

Because of this, the researchers are hopeful that their plasmon laser sensor could detect pentaerythritol tetranitrate, or PETN, an explosive compound considered a favorite of terrorists. Small amounts of it pack a powerful punch, and because it is plastic, it escapes x-ray machines when not connected to detonators. It is the explosive found in Richard Reid’s shoe bomb in 2001 and Umar Farouk Abdulmtallab’s underwear bomb in 2009. 

U.S. Attorney General Eric Holder Jr. was recently quoted in news reports as having “extreme, extreme concern” about Yemeni bomb makers joining forces with Syrian militants to develop these hard-to-detect explosives, which can be hidden in cell phones and mobile devices.

“PETN has more nitro functional groups and is more electron deficient than the DNT we detected in our experiments, so the sensitivity of our device should be even higher than with DNT,” said Ma.

Latest generation of plasmon sensors
The sensor represents the latest milestone in surface plasmon sensor technology, which is now used in the medical field to detect biomarkers in the early stages of disease.

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 tradeoff 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 researchers noted that the sensor could have applications beyond chemical and explosive detection, such as use in biomolecular research.

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

— Read more in Ren-Min Ma et al., “Explosives detection in a lasing plasmon nanocavity,” Nature Nanotechnology (20 July 2014) (doi:10.1038/nnano.2014.135)