Lasers Find Distant Hidden Explosives

A new method uses laser light to detect chemicals inside a container from a distance of more than 100 meters.

Specific substances scatter laser light in a very specific way. This is the basis of Raman spectroscopy and can be used to analyze the contents of a nontransparent container without opening it.

Scientists at Vienna University of Technology (TU Vienna) used Raman spectroscopy to discover what was in certain containers, irradiating samples with a laser beam. When the light is scattered by the molecules of the sample, it changes its energy. For example, the photons transfer energy to the molecules by exciting molecular vibrations, changing the wavelength of light — and thus its color. By analyzing the color spectrum of the scattered light, the researchers determined what kind of molecules scattered it.

Bernhard Zachhuber mounts optical elements of the spectrometer (Images: TU Vienna)

“Until now, the sample had to be placed very close to the laser and the light detector for this kind of Raman spectroscopy,” said Bernard Zachhuber of TU Vienna.

Due to his technological advancements, measurements can now be made over long distances.

“Among hundreds of millions of photons, only a few trigger a Raman-scattering process in the sample,” Zachhuber said.

These light particles are scattered uniformly in all directions. Only a tiny fraction of them travel back to the light detector, and from this very weak signal, as much information as possible must be extracted. This can be done using a highly efficient telescope and extremely sensitive light detectors.

The TU Vienna scientists collaborated with private companies and with partners in public safety, including The Spanish Guardia Civil, to put their method to the extreme. With the help of the Austrian military, they tested frequently used explosives such as TNT, ANFO and RDX on their testing grounds.

The results proved successful: Even at a distance of more than 100 meters, the substances could be detected accurately and reliably, said Engelene Chrysostom of TU Vienna.

The scientists also discovered that Raman spectroscopy works even if the sample is hidden in a nontransparent container. Although the laser beam is scattered by the container wall, a small portion of the beam can penetrate the box. Inside the sample, it can still excite Raman scattering processes.

The Raman spectroscope at TU Vienna.

“The challenge is to distinguish the container’s light signal from the sample signal,” said scientist Bernhard Lendl.

This can be done using a simple geometric trick: The laser beam hits the container on a small, well-defined spot. The light signal emitted by the container stems from a very small region, while the light that enters the container is scattered into a much larger region. If the detector telescope is not aimed exactly at the point at which the laser hits the container, but rather just a few centimeters away, the characteristic light signal of the contents can be measured instead of the signal coming from the container.

The researchers’ new method could make security checks at airports a lot easier, but they said that the area of application is broader than that. It could also be used wherever it is hard to get close to the subject of investigation — for studying icebergs or for geological analysis on a Mars mission, for example. A host of chemical industry applications also could be possible.

For more information, visit: www.tuwien.ac.at