Sensitive gas measurement with a new spectroscopy technique

Scientists at the US National Institute of Standards and Technology (NIST) have developed a new laser-based technique that could dramatically improve our ability to analyse a variety of materials and gases, including greenhouse gases. The new method, called ‘free-form dual-comb spectroscopy’, offers a faster, more flexible and more sensitive way to analyse substances in the air and other materials.

In a study published in the journal Nature Photonics, the researchers have now demonstrated that their laboratory-based system can detect a single gas, in this case the potent greenhouse gas methane, with 22 times higher sensitivity than a traditional dual-comb system. This increased sensitivity could one day help identify small leaks or emissions that might otherwise go unnoticed, potentially aiding in efforts to combat climate change.

Technological advancements

Spectroscopy is a sophisticated technique that allows scientists to identify and measure different materials by observing how they interact with light. Just as a prism separates white light into a rainbow of colours, spectroscopy separates the light coming from or passing through a substance, revealing its unique ‘fingerprint’ and providing valuable information about its properties and composition.

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 colours of light to be examined at the same time and in fine detail.

The new laser measurement technique improves on older methods by allowing scientists to control the timing of laser pulses with incredible precision. This precise control lets them focus on the most important parts of a sample’s fingerprint and ignore areas that don’t provide useful information. As a result, the smarter system can detect and measure substances much faster than before.

This new approach can be used in several ways. For example, scientists can use it to quickly create images showing how the gas is distributed in space. Alternatively, if researchers don’t know exactly what kind of gas is in the area they are investigating, they can use a generic 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.

In the new study, researchers created real-time images of methane plumes, which wasn’t possible previously. Methane is a potent greenhouse gas that contributes to climate change, so being able to detect and address these leaks efficiently could one day help protect the environment and improve air quality. By quickly generating images of methane plumes, scientists could quickly identify where gas is escaping. Indeed, the technique is useful not only for detecting greenhouse gases but for any situation in which scientists need to identify and measure gases.

Two lasers are better than one

The heart of free-form dual-comb spectroscopy 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. These frequency 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 this technology refers to the use of two optical frequency combs working together. This approach enables rapid, precise measurements of substances by analysing 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 femtoseconds in duration. Inside each of these brief light bursts, there’s an electric field that vibrates extremely rapidly, millions of millions of times per second. The ability to quickly and accurately control this fast light allows researchers to improve and adjust how they take measurements.

Dual-comb’s next big leap

By enabling smarter detection of gases and other substances, this innovative laser technology could play a crucial role in protecting both public health and the environment in the years to come. The researchers plan 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 non-invasively.”

Image caption: Esther Baumann and her colleagues at NIST developed the improved version of dual-comb spectroscopy. Image credit: R Jacobson/NIST.