Researcher Qizhong Liang in Optica Fellow Jun Ye’s laboratory at JILA on the University of Colorado Boulder’s campus. [Image: Patrick Campbell, CU Boulder]
As the saying goes, the nose knows. But sometimes the human olfactory system needs a boost—and a new interferometric technique can distinguish the faintest traces of gas molecules.
Researchers at a US laboratory have modified cavity ringdown spectroscopy to enhance the method’s sensitivity in the mid-infrared region, where many interesting molecules have their spectral signatures (Nature, doi: 10.1038/s41586-024-08534-2 ). The team’s experiment employs a frequency comb in a laser cavity with a constantly changing length. The resulting broad spectral coverage enables the experiment to detect multiple gases inside the cavity at better than 1-part-per-trillion sensitivity.
Cavity ringdown spectroscopy
In cavity ringdown spectroscopy, scientists measure the decay times of light pulses inside a high-finesse optical cavity filled with a light-absorbing material, such as a gas. Since each pulse makes hundreds or thousands of trips between the reflective surfaces at each end of the cavity, the long path length results in extremely high sensitivity.
The usual ringdown technique, however, doesn’t work as well in the mid-infrared (2 to 20 μm) as in shorter wavelength regions. In particular, today’s detector arrays have an integration time that is too slow to pick up on the ringdown process in the mid-infrared. The need to match laser wavelengths to cavity resonance frequencies also limits the number of trace gases that can be detected in one test.
The need to match laser wavelengths to cavity resonance frequencies also limits the number of trace gases that can be detected in one test.
Developing “breathomics”
Scientists at JILA, the US National Institute of Standards and Technology and the University of Colorado Boulder say their method, which they call modulated ringdown comb interferometry (MRCI), consists of two major components: the cavity, swept-locked to the mid-infrared frequency comb, and a Michelson interferometer that detects and reads out the transmission bursts from the comb lines.
For the experiments, the team used two intersecting high-finesse cavities, one designed for spectroscopy at wavelengths between 3 and 3.7 μm and the other for the range of 4.5 to 5.4 μm. The researchers collected and tested samples of breath exhaled by humans as well as samples of ambient air. The spectroscopic analysis picked up 20 different molecular species in the breath samples, notably nitric oxide, which is important in asthma monitoring—and which tends to blend into the atmospheric water background signal in the mid-infrared.
The team hopes the new technique can lead to new studies of “breathomics,” or medical diagnoses based on exhaled gases.