Researchers Discover New Tool for Atmospheric Detection

Due to their distinctive characteristics of high peak power, short pulse length, and broad spectral coverage, ultrafast laser technologies provide novel options for remote sensing of atmospheric contaminants and dangerous biological agents.

By its capacity to generate cavity-free light amplification in the open air, air lasing has become particularly attractive in atmospheric remote sensing. It is the perfect probe for diagnosing the atmosphere.

A research group from the Chinese Academy of Sciences (CAS) Shanghai Institute of Optics and Fine Mechanics (SIOM) recently suggested an air-lasing-assisted coherent Raman spectroscopy that allows for quantitative measurement and instantaneous sensing of two greenhouse gases, as well as the recognition of CO₂ isotopes. The minimum signal fluctuation is roughly 2%, and the detection sensitivity is 0.03%.

On April 8^th, 2022, the research was published in the Ultrafast Science journal.

The highly nonlinear interaction of a femtosecond laser with air molecules increases the optical gain of molecular nitrogen ions, culminating in seed amplification of over 1,000 times, which then results in 428 nm air lasing with a linewidth of 13 cm^-1.

Conversely, after nonlinear propagation, the pump laser’s spectral width has reached 3800 cm^-1, which is more than one order of magnitude wider than the spectrum of the incident laser.

As a result, most pollutants and greenhouse gases can have their molecular coherent vibrations excited. When air lasing collides with coherently vibrating molecules, coherent Raman scattering is produced. The chemical “identity information” can be determined by measuring the frequency difference between the Raman signal and air lasing, referred to as the “Raman fingerprint.”

The benefits of a femtosecond laser and air lasing are combined in air-lasing-assisted coherent Raman spectroscopy. The femtosecond laser has broad spectral coverage and short pulse duration, allowing it to simultaneously stimulate coherent vibrations in numerous molecules.

Air lasing has a limited spectrum width, allowing it to discern between distinct molecules’ Raman fingerprints. As a result, this approach can handle multi-component measurements as well as chemical specificity.

The scientists also proved that the approach could be used to assess many components simultaneously and distinguish between ¹²CO₂ and ¹³CO₂. For determining the sources of air pollution and investigating carbon cycling, simultaneous measurements of numerous pollutants and greenhouse gases, as well as the identification of CO₂ isotopes, are critical.

Furthermore, for a realistic application of trace gas remote identification, the detection sensitivity must be improved to the ppm or even ppb level, and the detection distance must be extended from laboratory scale to kilometer scale.

Journal Reference:

Zhang, Z., et al. (2022) High-Sensitivity Gas Detection with Air-Lasing-Assisted Coherent Raman Spectroscopy. Ultrafast Science. doi.org/10.34133/2022/9761458.