NIST’s new spectroscopy method monitors ‘greenhouse’ gases more sensitively

10 Oct 2024

Freeform dual comb spectroscopy creates images of methane plumes not visible to the human eye.

Scientists at the U.S. National Institute of Standards and Technology (NIST) have developed a new laser-based technique, which, they say, could significantly analysis of a variety of materials and gases, including greenhouse gases. The method, called “free-form dual-comb spectroscopy,” offers a faster, more flexible and more sensitive way to analyze substances in the air and other materials.

The technique, described in Nature Photonics, shows that the laboratory-based system could detect a single gas, in this case methane, with 22 times higher sensitivity than a traditional dual-comb system. NIST states that this increased sensitivity could help identify small leaks or emissions that might otherwise go unnoticed –potentially aiding in efforts to combat climate change.

Technological advancements

The NIST researchers have created an improved version of a laser-based measurement technique called dual-comb spectroscopy. Dual-comb spectroscopy is a particularly high-resolution form of spectroscopy that allows many colors of light to be examined at the same time and in fine detail.

The technique improves on older methods by allowing scientists to control the timing of laser pulses with high precision. This allows them to focus on the most important parts of a sample’s fingerprint. As a result, the smarter system can detect and measure substances much faster than before.

The new approach can be used in several ways. For example, scientists can use it to quickly create images showing how the target gas is distributed in space. Alternatively, if researchers do not know exactly what kind of gas is in the area they are investigating, they can use a generic identification technique called compressive sampling.

This is a “smart” method of making measurements, concentrating on areas likely to have important information and taking fewer measurements elsewhere. This strategy makes the whole process 10 to 100 times more efficient than traditional methods, says NIST.

Visualizing gas plumes

The approach can create fast, detailed images of a variety of gas clouds. In the Nature Photonics study, researchers created real-time images of methane plumes. By quickly generating images of methane plumes, scientists could quickly identify where gas is escaping.

Free-form dual-comb spectroscopy enables the making of videos, from which the above screen grab is taken. Dark areas mean there is little or no methane present, while brighter colors reveal where there is more methane. Because it can rapidly take pictures, this technology can show how the methane cloud creates swirling patterns and changes in real time, which was not possible previously. It can be easily adjusted to look for different gases, besides methane.

Two lasers better than one

At the heart of this method lies in the Nobel Prize-winning optical frequency comb, a laser tool that produces light at a series of equally-spaced, precise frequencies that resemble the teeth of a comb. Such combs are used for a variety of purposes, from precision timekeeping to medical diagnostics and even the search for elusive dark matter.

The “dual-comb” aspect of the new NIST technology refers to the use of two optical frequency combs working together. This approach enables rapid, precise measurements of substances by analyzing how they interact with the light from both combs. This technique is much faster than a single comb and can provide more detailed information than many traditional spectroscopy methods.

“Free-form” refers to the flexibility in highly precise frequency comb control that has recently become possible. The frequency combs emit light pulses that are just 100 fs in duration. Inside each of these brief light bursts, there’s an electric field that vibrates extremely rapidly.

The researchers are planning to continue improving their system in the laboratory, making it even faster and adapting their approach to work with a wide range of laser wavelengths.

“The flexibility of our system means it could be adapted for a wide range of applications,” said NIST researcher Esther Baumann. “In the future, we might see more versatile and efficient sensors based on this technology in everything from air quality monitors to food safety detectors to studying how materials burn or assessing muscle health noninvasively.”