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A hyper-sensitive microscope detecting chemicals down to a single micron could have a future use scanning crowds and baggage at airports and other high-security places, according to a new series of demonstrations.

The technique of photothermal modulation of Mie scattering could be of use in detecting whether a person has been in contact with chemicals like explosives, gunpowder, or other trace substances of interest, as described in the journal Optics Letters.

Speed and its sensitivity to single particles mean it could be of wide use in the future, according to the Massachusetts Institute of Technology researchers.

“The most important advantage of our new technique is its highly sensitive, yet remarkably simple design,” said Ryan Sullenberger, lead author, from the MIT Lincoln Labs. “It provides new opportunities for nondestructive chemical analysis while paving the way towards ultra-sensitive and more compact instrumentation.”

The technique itself has been around since the 1980s – but improved digital instruments have now extended its potential.

An infrared laser illuminates the subject particles, heating them. A green laser then shines upon the particles as small as a picogram. The wavelengths of light are then measured as an identifying signature at a minute level.

“We’re actually imaging the area that we’re interrogating,” said Alexander Stolyarov, co-author of the paper. “This means we can simultaneously probe multiple particles that are extremely small and close together.”

The experiments involved detecting specially treated 3-micron fragments of silica and acrylic, according to the paper.

Traditional infrared spectroscopy only allows detail to the level of 10 microns. This new technique also does not permanently alter particles, so it preserves forensic evidence.

But hurdles remain. Better lasers need to be developed to probe wider swaths of objects, they said. Also, their trials were done with carefully-prepared samples, in a controlled and regulated environment – the real environment will contain more impurities that will make detection of the target chemicals more difficult.

“The presence of interferents is perhaps the biggest challenge I anticipate we will need to overcome,” said Stolyarov. “Although contamination is a problem for any technique measuring absorption from small amounts of materials, I think our technique can solve that problem because of its ability to probe one particle at a time.”

Multiple species of micron-sized particles are simultaneously illuminated by an infrared laser and a green laser beam. Absorption of the infrared laser light by the particles increases their temperatures, causing them to expand and slightly altering their optical properties. These changes are unique to the material composition of each particle and can be measured by examining the modulation of scattered green light from each particle. (Credit: Ryan Sullenberger, MIT Lincoln Laboratory)
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