Researchers from the National Institute of Standards and Technology, Toptica Photonics AG, and the University of Colorado Boulder developed a device that can detect specific molecules within a sample every 20 nanoseconds. This is equivalent to billionths of a second. Researchers can now use frequency combs in order to understand better fast-moving processes, such as the functioning of hypersonic engines or the chemical reactions that control cell growth.
Frequency combs, laser systems that can detect COVID and monitor greenhouse gas concentrations with unmatched accuracy and sensitivity, can also identify molecules as complex as monoclonal antigens or as simple as CO2. Frequency combs are amazing, but they have limitations in terms of how quickly they can capture high-speed processes such as hypersonic flight or folding proteins into three-dimensional shapes.
Researchers at the National Institute of Standards and Technology, Toptica Photonics AG, and the University of Colorado Boulder developed a frequency-comb system that can detect specific molecules within a sample every 20 nanoseconds. This is billionths of a second. Researchers can now use frequency combs in order to understand better fast-moving processes, such as the functioning of hypersonic engines or the chemical reactions that control cell growth. The team published its findings in Nature Photonics.
The researchers used a dual-frequency setup that is now common. It consists of two laser beams that work together to detect a spectrum of colors that molecules absorb. The most common dual-frequency setups use two femtosecond lasers that send out ultrafast pulses at the same time.
Researchers used an inexpensive and simpler setup called “electrooptic combs” to split a continuous light beam into two beams. A modulator is electronic produces electric fields to alter each ray of light, forming them into individual “teeth.” Each tooth represents a particular color or frequency that can be absorbed into a molecule.
The researchers’ electrooptic comb had only 14 teeth in an average experiment. Conventional frequency combs have thousands, or even millions, of teeth. As a result of this, each tooth was much more powerful and had a frequency that was different from the others. This led to a strong, clear signal, which allowed the researchers to detect changes in light absorption at a 20-nanosecond timescale.
The researchers demonstrated the instrument by measuring supersonic CO2 pulses emerging from a small, air-filled nozzle. The researchers measured the CO2 mixing rate or the proportion of carbon dioxide in the air. Researchers were able to determine the pulse’s motion by observing the changing CO2 concentration. Researchers observed how CO2 reacted with air to create oscillations in air pressure. Even the most sophisticated computer models can’t always provide such accurate details.
“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 CO 2 , to observe the chemistry. We can also use this approach to measure things such as pressure, temperature or velocity by looking at changes in the signal,” said NIST research chemist David Long. The information from these experiments could provide insights that could lead to design improvements in combustion engines or a better understanding of how greenhouse gases interact with the atmosphere.
The optical parametric oscillator was used in this setup to move the comb teeth away from near-infrared and toward the mid-infrared colors that are absorbed by CO2. The optical parametric oscillator, however, can be tuned to detect molecules in other mid-infrared regions.
This paper contains information that can be used by other researchers to build a similar system in the laboratory, making this technique available across a wide range of research fields and industries.
The work of Greg Rieker, a former NIST researcher and professor at the University of Colorado Boulder, is a special one because it lowers the entry barrier for researchers interested in using frequency combs as a tool to study fast processes.
With this setup, you’ll be able to create any comb that you desire. This method is very flexible and fast, which allows for many different measurements.
The Air Force Office of Scientific Research has supported this work in part.