The new detector contains a lasing nanocavity that is a coherent light source, explains team member Renmin Ma. It consists of a 50 nm-thick layer of semiconducting cadmium sulphide atop a sheet of silver with an 8 nm-thick layer of magnesium fluoride in between. The detector lases at the nanoscale thanks to surface plasmons and the device can be used to sense minute concentrations of airborne vapours by measuring changes in the light signal emitted when gas molecules settle on the semiconductor.

Plasmons are quantized collective oscillations of conduction electrons on the surface of metallic nanostructures that interact strongly with light. Such enhanced interaction allows them to concentrate light into subwavelength volumes, well below the diffraction limit of light. However, keeping the focused light in the nanocavity is no easy task because the light energy tends to spread out of the cavity and dissipate into so-called Joule heating very fast.

Bouncing surface plasmons

The researchers, led by Xiang Zhang, have now found a way to prevent this light from leaking by employing reflectors that “bounce” the surface plasmons back and forth inside the nanocavity. They then used an “optical gain” from the semiconductor to compensate for the light energy dissipated. The result is a sensor that produces a much stronger signal than the “passive” plasmon sensors currently available, says Zhang. “The difference in intensity is like going from an ordinary table top light bulb to a laser pointer, he explains. “We create a sharper signal, which makes it easier to detect even smaller changes for tiny traces of explosives in air.”

Zhang and his colleagues tested their sensor by exposing it to three different types of explosive: 2,4 dinitrotoluene (DNT), ammonium nitrate and nitrobenzene. The device was able to detect airborne concentrations of the chemicals at concentrations of 0.67, 0.47 and 7.2 part per million respectively. Compounds such as DNT are explosive because of the unstable nitro groups they contain, but these groups also make the explosives electron deficient – something that allows them to interact with the natural surface defects on the cadmium sulphide semiconductor. The detector works by detecting the more intense light signal produced as this interaction take place.

Small vapour pressures

“Being able to sense dangerous explosives is important in a number of situations, but these compounds are notoriously difficult to detect because their vapour pressures are small and can be reduced even further depending on how they have been packaged,” Ma tells nanotechweb.org. “Our detector with its increased sensitivity could come into its own here.”

Apart from detecting dangerous chemical vapours, the sensors might also be used to detect biomarkers implicated in a variety of diseases, such as cancer and diabetes – especially in their early stages.

The team says that it is now looking at further boosting its sensor’s sensitivity and how selective it is to different molecules.

The device is detailed in Nature Nanotechnology.