Because of their unique benefits of high peak power and short pulse duration, ultrafast laser technology offers new ways to remote sense atmospheric pollutants and hazardous biochemical agent.
Air lasing is particularly promising for atmospheric remote sensing because it can generate cavity-free light amplification in the open air. It can be used as an atmospheric probe.
A research team from the Shanghai Institute of Optics and Fine Mechanics of the Chinese Academy of Sciences proposed an air-lasing assisted coherent Raman spectroscopy. This allows for quantitative measurement and simultaneous detection of both greenhouse gases as well as the identification of CO2 isotopes. The detection sensitivity is 0.03%, and the signal fluctuation is approximately 2%.
Ultrafast Science published the work on April 8.
The nonlinear interaction between the femtosecond and air molecules causes the optical gain of molecular nitro ions to be excited. This results in a seed amplification exceedingly high of over 1,000 times.
After nonlinear propagation, the pump laser's spectral width has increased to 3800 cm-1. This is more than one order-of-magnitude wider than the incident laser's spectrum.
This allows for the excitation molecular coherent vibrations that are common in greenhouse gases and pollutants. It will produce coherent Raman scattering when it encounters coherently vibrating molecules. The molecular identity information of a molecular can be determined by recording the frequency difference between Raman signal (and air lasing), namely the Raman fingerprint.
Air-lasing assisted coherent Raman spectroscopy combines both the benefits of femtosecond and air laser. Femtosecond laser is able to excite coherent vibrations from many molecules simultaneously due to its broad spectral coverage. It also has a shorter pulse duration. Air lasing is a narrow-spectral technique that allows for the identification of Raman fingerprints between different molecules. This technique is able to meet the requirements of chemical specificity and multi-component measurement.
The researchers also demonstrated that the technique could be used for simultaneous multi-component measurement and the distinction of 12CO2 or 13CO2. It is crucial to simultaneously measure various pollutants and greenhouse gases, as well as detect CO2 isotopes, in order to trace the source of air pollution and study carbon cycling.
To be able to use trace gas remote detection in a realistic manner, however, it is important to increase the detection sensitivity to ppm and even ppb levels, as well extend the detection distance beyond the laboratory scale to the kilometers scale.
