The Shanghai Institute of Optics and Computer of Chinese Academy of Sciences has made new progress in the application of air laser for long-range detection

Coherent Raman scattering is an important nonlinear spectroscopy technique, which has been widely used in substance detection, combustion diagnosis, biological microscopy, etc. The traditional coherent Raman spectroscopy technology usually requires multiple laser beams to excite and detect the coherence of molecular vibration, and requires a high requirement for the spatio-temporal control between multiple beams. Therefore, the development of single beam coherent Raman scattering technology is a very attractive research direction, the University of California, Berkeley, Texas A&M University, Israel Weizmann Institute of Science and other research institutions have carried out related research. However, the previous methods usually need to use space light modulator to shape the time, spectrum and polarization of femtosecond laser, which cannot be used for large-energy femtosecond laser. Moreover, the wavelength of Raman excitation light and detection light is similar, resulting in low signal-to-noise ratio of Raman signal, which is difficult to detect trace molecules sensitively.

To solve the above problems, a research team from the State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, proposed a novel single beam coherent Raman scattering technique based on the unique time-frequency properties and remote generation capability of air laser, and realized the quantitative measurement of greenhouse gas SF6 in air. The detection sensitivity reached a level of 4 parts per thousand [Opt.Lett. 47, 481 (2022)]. The team then combined air laser-based coherent Raman scattering with seed amplification and polarization filtering to achieve greenhouse gas detection at concentrations as low as 3ppm, demonstrating the unique advantages of the technique for simultaneous measurement of multiple components and accurate resolution of 12CO2 from 13CO2. He developed the initial application of air laser in the field of remote detection [Ultrafast Science 2022,9761458 (2022), selected journal cover papers and 10 high-impact papers in 2021-2022].

FIG. 1 Basic principle of coherent Raman scattering enhanced by electron resonance of a single beam

Recently, the research team further developed the electron resonance-enhanced single-beam coherent Raman scattering technique, using a femtosecond laser beam to simultaneously construct Raman and electron resonance double resonance conditions, improving the Raman signal by 1-2 orders of magnitude through electron resonance. Single beam electron resonance enhanced coherent Raman scattering not only requires the use of a laser beam to complete the process of excitation and detection of molecular coherent vibration, but also requires the excitation light or detection light to resonance with the electronic state transition of the substance to be measured, so it has not been reported until now. The team cleverly solved this problem by using air laser. The new technology developed not only takes advantage of the narrow spectrum of air laser and natural coincidence with the pump beam, but also perfectly matches the frequency of air laser and CO2+ transition to create electron resonance conditions for Raman scattering, providing a simple and effective method to greatly improve the efficiency of Raman scattering. The results are published in a recent Laser Photonics Review.

Figure 2 Raman spectra measured in CO2 gas with different concentrations. The shadow area is coherent Raman signal enhanced by electron resonance

The coherent Raman spectroscopy technology based on air laser reflects the unique advantages of air laser in the three dimensions of time, space and frequency. Combined with the dual advantages of femtosecond laser multi-component excitation and air laser hyperspectral resolution, it has the unique capabilities of multi-component simultaneous detection and isotope molecular screening, providing a new technical solution for sensitive detection of complex atmospheric molecules. In addition, the technology takes the naturally generated air laser as the detection light, simplifies the traditional multi-beam coherent Raman scattering into a single beam, the optical path is simple, without the need for multi-beam multi-color field time and space precision control, is suitable for high temperature and high pressure turbulence environment and complex atmospheric environment remote detection, is a simple and practical generic spectral technology.

Source: Shanghai Institute of Optics and Machinery, Chinese Academy of Sciences