New Frequency Comb Can Identify Molecules in 20-Nanosecond Snapshots

“In a more complicated system like an aircraft engine, we could use this approach to look at a particular species of interest, such as water or fuel or CO2, to observe the chemistry,” says NIST research chemist David Long. “We can also use this approach to measure things such as pressure, temperature, or velocity by looking at changes in the signal.” The information from these experiments could provide insights that lead to design improvements in combustion engines, or a better understanding of how greenhouse gases interact with the atmosphere.

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A special component in the setup, known as an optical parametric oscillator, was used to shift the comb teeth from the near-infrared to the mid-infrared colors absorbed by CO2. But the optical parametric oscillator can also be tuned to other regions of the mid-infrared so that the combs can detect other molecules absorbing light in those regions.

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This work was supported in part by the U.S. Air Force Office of Scientific Research.

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A new frequency comb setup can capture the moment-by-moment details of carbon dioxide gas escaping from a nozzle at supersonic speeds in an air-filled chamber, followed by rapid oscillations of gas due to complex aerodynamics within the chamber. The data plot shows the absorbance of light (vertical) over time (horizontal left to right) across a range of frequencies (horizontal forward to back). Credit: G. Mathews/University of Colorado-Boulder.

From monitoring concentrations of greenhouse gases to detecting Covid in the breath, laser systems known as frequency combs can identify specific molecules as simple as carbon dioxide and as complex as monoclonal antibodies with unprecedented accuracy and sensitivity. Amazing as they are, however, frequency combs have been limited in how fast they can capture a high-speed process such as hypersonic propulsion or the folding of proteins into their final three-dimensional shapes.

In their demonstration, the researchers used the instrument to measure supersonic pulses of CO2 emerging from a small nozzle in an air-filled chamber. They measured the CO2 mixing ratio, the proportion of carbon dioxide in the air. The changing concentration of CO2 told researchers about the motion of the pulse. The researchers saw how the CO2 interacted with the air and created oscillations of air pressure in its wake. Such details are often hard to accurately obtain even with the most sophisticated computer simulations.

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Whereas conventional frequency combs can have thousands or even millions of teeth, the researchers’ electro-optic comb had only 14 in a typical experimental run. However, as a result, each tooth had much higher optical power and was far apart from others in frequency, resulting in a clear, strong signal that enabled the researchers to detect changes in the absorption of light at the 20-nanosecond time scale.

“With this setup, you can generate any comb you want. The tunability, flexibility, and speed of this method opens the door to lots of different types of measurements,” Long says.

Paper: David A. Long, Matthew J. Cich, Carl Mathurin, Adam T. Heiniger, Garrett C. Mathews, Augustine Frymire, and Gregory B. Rieker. “Nanosecond time-resolved dual-comb absorption spectroscopy.” Nature Photonics. Published online Oct. 30, 2023.

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In their experiment, the researchers used the now-common dual-frequency comb setup, which contains two laser beams that work together to detect the spectrum of colors that a molecule absorbs. Most dual-frequency comb setups involve two femtosecond lasers, which send out a pair of ultrafast pulses in lockstep.

Published: Wednesday, December 20, 2023 – 12:01

The paper includes information that other researchers can use to build a similar system in the lab, making this new technique widely available across many research fields and industries.

In this new experiment, the researchers used a simpler and cheaper setup known as “electro-optic combs,” in which a single continuous beam of light first gets split into two beams. Then, an electronic modulator produces electric fields that alter each light beam, shaping them into the individual “teeth” of a frequency comb. Each tooth is a specific color or frequency of light that can then be absorbed by a molecule of interest.

Published Oct. 30, 2023, on NIST News.