Laser-based technique can quickly detect explosives

Scientists have developed a laser-based method that can accurately identify chemicals such as explosives and dangerous gases, an advance that can be used in airports security and monitoring environmental pollutants. The study, published in the journal Science, combined two techniques that speed up laser-based detection of chemicals while doing so accurately.

The first technique is based on the same idea as nuclear magnetic resonance spectroscopy, which uses radio frequencies to identify the structure of molecules. Researchers from University of Michigan in the US used a method called multi-dimensional coherent spectroscopy (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 colour – that specific gases absorb, researchers said. “If you have light going through the gas, and, for example, you use a prism to separate white light into coloured light, in the rainbow spectrum you’d see there’d be black stripes,” said Steven Cundiff, a professor at University of Michigan.

“Where the black stripes almost give you a barcode that tells you what kind of molecule is in the sample,” Cundiff said.

Previously, 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. “It is like trying to look at three people’s fingerprints on top of each other. This is a stumbling block for using these methods in a real-world situation.

Our method takes about 15 minutes to a few hours using traditional approaches to MDCS,” Cundiff said.

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

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, they said. “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. Researchers applied their method to a vapour of rubidium atoms that contained two rubidium isotopes.

The frequency difference between absorption lines for the two isotopes is too small to be observed using traditional approaches to MDCS, but by using combs, the team was able to resolve these lines and assign the spectra of the isotopes based on how the energy levels were coupled to each other. The method is general and can be used to identify chemicals in a mixture without previously knowing the makeup of the mixture.