Technique that detects explosives, dangerous gases remotely

Researchers from University of Michigan have developed laser-based method used to detect chemicals. Such as explosives and dangerous gases quickly and accurately.

The development of a technique with the ability to detect trace quantities of explosives at a distance is of critical importance. This method used in systems placed in airports, for the environmental monitoring of pollutants or even in battlefields.

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Multi-dimensional coherent spectroscopy

The researchers use a method called multi-dimensional coherent spectroscopy, or MDCS. MDCS uses ultrashort laser pulses to read types of gases like a bar code. When the scientists bounce the laser pulses through the mixture of gases. Those pulses can read the specific wavelengths of light or color that specific gases absorb.

Lomsadze and Cundiff"s method combines two techniques that speed up laser-based detection of chemicals while doing so accurately. The first technique based on the same idea as nuclear magnetic resonance spectroscopy. It uses radio frequencies to identify the structure of molecules.

Scientists have been working on similar simpler methods. Many important molecules have a very rich spectra for certain colors of light. Although the colors may actually be in the infrared. So not visible the human eye which makes them easily identifiable. But this becomes difficult when scientists try to identify gases in a mixture. Scientists relied on comparing what they measured against a catalogue of molecules. A process that requires high performance computers and a significant amount of time.

Dual comb spectroscopy

To speed up the process while preserving its accuracy. The U-M researchers combined MDCS with another method called dual comb spectroscopy. Frequency combs are laser sources that generate spectra consisting of equally spaced sharp lines. Used as rulers to measure the spectral features of atoms and molecules with extremely high precision.

In spectroscopy, using two frequency combs, known as dual comb spectroscopy, provides an elegant way to rapidly acquire a high resolution spectrum without mechanical moving elements such as a "corner cube," which is three mirrors arranged to make one corner, used to reflect a laser beam directly back on itself. This element usually limits how long it takes for the researchers to measure a spectrum.

"This approach could allow the method of multidimensional coherent spectroscopy to escape the lab and be used for practical applications such as detecting explosives or monitoring atmospheric constituents," Cundiff said.

Lomsadze and Cundiff applied their method to a vapor of rubidium atoms that contained two rubidium isotopes. The frequency difference between absorption lines for the two isotopes is too small to observe using traditional approaches to MDCS.

To resolve these lines and assign the spectra of the isotopes based on how the energy levels coupled to each other. Their method is general and used to identify chemicals in a mixture without previously knowing the makeup of the mixture.

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Next, the researchers plan to add a third laser that could even greater speed up their ability to identify gases. They also plan to use lasers based on fiber optics so that they can look further into infrared light. Expand the number of chemicals they would be able to identify.