One of the many interesting features of semiconducting polymers is that they can be made to lase. By creating a population of excited electronic states, a situation can be achieved whereby light is amplified by the process of stimulated emission, giving rise to an intense beam of coherent light. Because semiconducting polymers can be laid down in a thin film from a simple solution, it’s tempting to dream of lasers that are fabricated by simple and cheap processes, like printing, or are simply painted on to a surface. The problem with this is that, so far (and as far as I know), the necessary population of excited states has only been achieved by illuminating the material with another laser. This optical pumping, as it is called, is obviously less useful than the situation where the laser can be pumped electrically, as is the case in the kind of inorganic semiconductor lasers that are now everyday items in CD and DVD players. But a paper in this week’s Nature (abstract free, subscription required for full article) demonstrates another neat use for lasing action in semiconducting polymers – as an ultrasensitive detector for explosives. See also this press release.
The device relies on the fact that lasing is a highly non-linear effect; if an optically-pumped polymer laser is exposed to a material which influences only a few molecules at its surface, this can still kill the lasing action entirely. The molecule that is being used in this work, done at MIT by Timothy Swager’s group, is particularly sensitive to the explosive TNT. This device can work as a sensor that would be sensitive enough (and this needs to be in the parts per billion range) to detect the tiny traces of TNT vapour that a buried land-mine would emit.
This work, rather unsurprisingly, is supported by MIT’s Institute for Soldier Nanotechnologies. The development of these ultrasensitive sensors for the detection of chemicals in the environment forms a big part of the research effort in evolutionary nanotechnology. On the science side, this is driven by the fact that detecting the effects of molecules interacting with surfaces is intrinsically a lot easier in systems with nanoscaled components, simply because the surface in a nanostructured device has a great deal more influence on its properties than it would in a bulk material. On the demand side, the needs of defense and homeland security are, now more than ever, setting the research agenda in the USA.