Two spectroscopy techniques have been combined to develop a laser-based method that could be used to detect chemicals such as explosives and gases quickly and accurately. One technique, multidimensional coherent spectroscopy (MDCS), uses ultrashort laser pulses to read the specific wavelengths that are absorbed by each gas. It works almost like a barcode: “If you have light going through the gas, and, for example, you use a prism to separate white light into colored light, in the rainbow spectrum you’d see there’d be black stripes,” professor Steven Cundiff said. “Where the black stripes are almost gives you a barcode that tells you what kind of molecule is in the sample.” The University of Michigan research team combined MDCS with a second technique, dual comb spectroscopy, to speed up the process while preserving its accuracy. Frequency combs, whose spectra consist of a series of discrete, equally spaced frequency lines, can be used to measure the spectral features of atoms and molecules with precision. Dual-comb spectroscopy provides a way to acquire high resolution spectra rapidly. Dual laser frequency combs can rapidly measure high-resolution linear absorption spectra. However, one-dimensional linear techniques cannot distinguish the sources of resonances in a mixture of different analytes. Researchers overcame this limitation by acquiring high-resolution multidimensional nonlinear coherent spectra with frequency combs. The researchers applied their method to a vapor of rubidium atoms containing two rubidium isotopes. The frequency difference between absorption lines for the two isotopes was too small to be observed using traditional approaches to MDCS; but by using combs, researchers were able to resolve these lines and assign the spectra of the isotopes based on how the energy levels were coupled to each other. “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. With previous methods for identifying gases in a mixture, scientists have compared measurements against a catalog of molecules, a process that requires high-performance computers and a lot of time. “It’s 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,” Cundiff said. “Our method takes about 15 minutes to a few hours using traditional approaches to MDCS.” The team’s method can be used to identify chemicals in a mixture without previous knowledge of the makeup of the mixture. Cundiff believes that in the future the method could be used in systems placed in airports or even battlefields for the environmental monitoring of pollutants. The researchers plan to add a third laser that could further speed the ability to identify gases using this method. They also plan to use lasers based on fiber optics so that they can further investigate the use of IR light, which would expand the number of chemicals they would be able to identify. The research was published in Science (doi: 10.1126/science.aao1090).