A new type of explosive detector

October 19, 1999

EXPLOSIVES detectors that are used to search for bombs in luggage at airports can miss some devices because the concentration of the explosive material in sampled air is too low. But now a researcher in Maryland has developed a laser-based detector that may achieve the elusive goal of sniffing out single molecules of target substances.

Andrew Pipino, a chemical physicist at the National Institute of Standards and Technology in Gaithersburg, has already been granted a patent on his detector. A commercial prototype, which he hopes to complete in the next two years, would detect the minuscule explosives emissions from landmines, offering an alternative to more dangerous methods such as sniffer dogs. "With this detector we're making an electronic dog," says Pipino. "Next we want to go after chemical and biological weapons detection."

Pipino has demonstrated an experimental system, reported in the current edition of Physical Review Letters (vol 83, p 3093), which is based on an ultra-smooth cube of silica crystal with sides 1 centimetre in length and with a single curved convex face. Two prisms sit next to it on adjacent sides.

A laser pulse is fired into one of the prisms at such an angle that most of it is reflected, except for a minuscule portion of light called an evanescent wave, which passes into the crystal. Once inside the crystal, the tiny pulse undergoes total internal reflection off the cube's inner walls, but is redirected each time it is reflected from the curved side. When the pulse bounces off a wall, a tiny evanescent wave leaks out of the crystal, causing a slight drop in the energy of the pulse. The second prism focuses the evanescent wave leaking from the crystal at that face so that this energy loss can be measured.

After up to a million trips around the crystal, the pulse is all but expired. Researchers call the time this dissipation takes the "ring-down time"-and Pipino can measure it with incredible precision.

If a container of the sample gas is placed against one of the two sides of the crystal without a prism, sample molecules absorb energy from the pulse as it reflects off that face of the cube, decreasing the ring-down time. The difference between ring-down times with and without a sample is unique to each target substance. Different molecules absorb different amounts of light of a given wavelength, so test samples can be analysed over a range of wavelengths to provide a distinctive fingerprint for each.

Pipino claims that his method is about a hundred times more sensitive than rival techniques that also work with films of molecules, particles on surfaces, liquids and similar samples. Samples like these are used in explosives detection and other applications, such as those used to study increasingly miniaturised microchip components.

Pipino believes he can refine his method to achieve an additional 100-fold improvement in sensitivity. "The numbers suggest we might get single-molecule detection."

According to Richard Van Duyne, a chemist at Northwestern University in Evanston, Illinois, the new method has great potential. "This is really a major breakthrough," he says.
-end-
Author: Mark Schrope

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New Scientist

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