Artist's rendition of how the frequency combs interact with atoms to produce a two-dimensional coherent spectrum. Photo Credit: Brad Baxley/UM

ANN ARBOR —University of Michigan Department of Physics Research Fellow Bachana Lomsadze and U-M Physics Professor Steven Cundiff have developed a novel optical technique that could potentially be used for rapid remote detection of chemicals such as explosives and dangerous gases.

Lomsadze and Cundiff’s goal was to develop a method that was simultaneously fast, had a high spectral resolution, and the ability to identify different species, with absolute certainty, in a cluttered environment. They have invented a clever way to satisfy these conditions by marrying two powerful optical techniques: multi-dimensional coherent spectroscopy (MDCS) and dual comb spectroscopy (DCS). This achievement is reported in the September 29 issue of Science.

Multi-dimensional coherent spectroscopy is an optical method based on concepts originated in nuclear magnetic resonance spectroscopy that enables the determination of molecular structure. It uses a sequence of ultrashort (femtosecond) laser pulses to excite the mixture of species. Analyzing the spectral features of the light emitted by the sample while varying the time delays between the laser pulses leads to identification of constituent species. However, current implementations of the method have long measurement times, or the limited ability to resolve closely spaced spectral features that makes the specimen identification process unreliable. Lomsadze and Cundiff have overcome this weakness by leveraging a technique known as dual frequency comb spectroscopy.

Frequency combs are laser sources that generate spectra consisting of hundreds of thousands of equally spaced sharp lines that are 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 any mechanical moving elements that usually limits the acquisition speed.

In the recently published Science paper, Lomsadze and Cundiff have shown how their newly developed method can be used for rapid chemical sensing. To demonstrate an unprecedented resolution, they applied their method to a vapor of rubidiuim atoms that contained both 87Rb and 85Rb isotopes. The natural linewidths of the hyperfine split transitions for Rb isotopes are ~6 MHz; however, at 100oC their spectral features are Doppler broadened (~580 MHz) and the hyperfine lines strongly overlap. Lomsadze and Cundiff were able to resolve these lines and assign the spectra of the isotopes (87Rb and 85Rb) based on how the energy levels were coupled to each other. Their method is general and can be used to identify chemicals in a mixture without prior knowledge of its constituent species.

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