Researchers at the Shanghai Institute of Optics and Fine Mechanics (SIOM) demonstrated how air lasing, a technique deployed in ambient air that is induced by femtosecond laser pulses and that generates cavity-free amplification in the air, could be used to assist coherent Raman spectroscopy in the remote sensing of pollutants in the atmosphere. The team’s method provided both multicomponent measurement capabilities and chemical specificity.

The air-lasing-assisted Raman spectroscopy technique offers a versatile way to detect gas pollutants and greenhouse gases with high sensitivity. It could potentially be used to rapidly detect infectious diseases, such as COVID-19, at a safe distance, the researchers said.

The researchers demonstrated that air lasing, created inside a femtosecond laser filament, could be used as a light source to probe Raman coherence excited by a femtosecond pump. Through the optical phenomenon of filamentation, high-energy femtosecond laser pulses propagated a long distance without diffraction.

When the Raman light was probed, it produced a coherent Raman signal with molecular vibrational signatures. The effect of pulse self-compression, combined with the air lasing that took place during filamentation, improved Raman excitation efficiency.

The researchers adopted an external-seed amplification mechanism, which improved lasing strength and the signal-to-noise ratio of the Raman scattering. To suppress the supercontinuum background, they chose a perpendicular polarization arrangement for their design. The external-seed mechanism paired with the perpendicular polarization to enable high detection sensitivity and signal stability.

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Graphic depicting greenhouse gas detection with air-lasing-based Raman spectroscopy. (a) The generation scheme of air lasing and coherent Raman scattering. (b) The original and the broadened spectra of the pump laser. (c) The spectrum and spatial profile of air lasing. Courtesy of Ultrafast Science.

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In the technique, a femtosecond laser excites ions of molecular nitrogen and achieves greater than 1000× seed amplification, resulting in 428-nm air lasing with a linewidth of 13 cm⁻¹. The spectral width of the pump laser is 3800 cm⁻¹ after nonlinear propagation, enabling the excitation of molecular coherent vibrations of most pollutants and greenhouse gases.

The researchers used the technique to quantitatively detect and measure greenhouse gases mixed in air. They were able to detect greenhouse gas concentrations as low as 0.1% for CO₂ and as low as 0.03% for SF₆. The minimum signal fluctuation was about 2%. The researchers also demonstrated that the technique can be used to simultaneously measure CO₂ and SF₆ and to identify ¹²CO₂ and ¹³CO₂.

By using the technique to simultaneously measure multiple pollutants, greenhouse gases, and CO₂ isotopes, scientists will be able to better trace the sources of air pollution and gather data for the study of carbon cycling.

The researchers consider this to be a significant advantage of the proposed air-lasing-assisted Raman spectroscopy technique for gas detection, compared to traditional remote sensing methods. However, for the application of trace gas remote detection, the researchers said it will be necessary to improve the detection sensitivity of the technique to the parts-per-million or even to the parts-per-billion level and extend the detection distance from the laboratory scale to the kilometer scale.

The research was published in Ultrafast Science (www.doi.org/10.34133/2022/9761458).

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