air lasing is the coherent radiation generated by non-cavity amplification through the main component of air or its derivatives as gain medium. It uses the “femtosecond optical filament”, a low-temperature plasma channel generated by high-energy femtosecond laser pulse, as the carrier, which has the natural remote generation ability, and has the advantages of high brightness, narrow linewidth and transmission in a specific direction. Therefore, since the discovery of air laser, its application in the field of atmospheric remote sensing has attracted great attention from researchers at home and abroad. However, how to use this new “air laser” to accurately “diagnose” the atmosphere? A team of researchers at the State Key Laboratory of Innse Field Laser Physics at the Shanghai Institute of Optics and Mechanics, Chinese Academy of Sciences, has come up with the answer. The team reported on an air laser-assisted coherent Raman scattering technique and successfully recorded the “molecular fingerprint” of the greenhouse gases CO2 and SF6 in the atmosphere, achieved the detection of greenhouse gases in atmospheric concentrations as low as 3 PPM, and demonstrated the ability of the technique for simultaneous measurement of multiple components and CO2 isotope resolution.
Highly sensitive remote detection of air pollutants and biochemical agents is very important for environmental science and national defense security. The rapid development of ultra-strong and ultra-short laser technology provides a powerful tool for remote optical remote sensing. On the one hand, the high energy femtosecond laser filaments can transmit freely in the atmosphere over long distances without diffraction. On the other hand, a series of secondary radiation sources induced by femtosecond laser filamination, such as supercontinuous white light, air laser, molecular fluorescence, etc., provide a natural remote “probe” for atmospheric remote sensing. Therefore, optical remote sensing technology based on ultrafast laser has attracted much attention in the past two decades. In recent years, the discovery and extensive research of air laser have injected new vitality into ultrafast optical remote sensing. Air laser, which takes the ubiquitous atmosphere as the gain medium and the plasma channel generated by femtosecond laser as the carrier, has the advantages of high intensity, narrow spectrum, good spatial directivity and natural coexisting with pump beam, making it an ideal “probe” for atmospheric detection. However, the use of “air laser” as a new tool for atmospheric detection is still facing great challenges in principle, method, sensitivity and stability.
The research team of the State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Mechanics, Chinese Academy of Sciences has been committed to the research of high field physics and remote sensing application of air laser since the “air laser” phenomenon induced by high field ionization was first reported in the world in 2011 [Phys. Rev.A 84, 051802 (2011)]. Recently, the team developed a highly sensitive coherent Raman spectroscopy technique using an air laser, which enables quantitative detection of atmospheric greenhouse gas concentrations, simultaneous detection of multiple components, and identification of CO2 isotopes, with detection sensitivity up to 0.03% and minimum signal jitter up to 2%. The relevant research was published in Ultrafast Science on High Sensitivity Gas Detection with Air-Lasing-Assisted Coherent Raman Spectroscopy.
The basic principle of air laser assisted coherent Raman spectroscopy is shown in Figure 1. The extreme nonlinear interaction between femtosecond laser and air, on the one hand, excites the optical gain of air molecules, realizes the seed amplification of more than 1000 times, and generates nitrogen ion air laser with wavelength of 428 nm and linewidth of 13 cm-1. At the same time, the femtosecond laser transmits nonlinearly in the atmosphere, extending the spectrum bandwidth to 3800 cm-1, which is more than one order of magnitude wider than the incident spectrum, and is sufficient to excite coherent Raman vibration of most pollutant molecules and greenhouse gases in the air. When the air laser encounters coherently vibrating molecules, coherent Raman scattering is effectively generated. By recording the frequency shift between the coherent Raman signal and the air laser, known as a “Raman fingerprint”, the molecule’s “identity information” — its chemical composition — can be learned.
Figure 1. The basic principle of air laser assisted coherent Raman scattering technique: (a) Schematic diagram of the generation mechanism of air laser and coherent Raman scattering; (b) Comparison between the broadened pump light spectrum and the original spectrum; (c) Spectral and spatial distribution of airborne lasers.
Air laser-assisted coherent Raman spectroscopy combines the dual advantages of femtosecond laser and air laser: femtosecond laser has wide spectrum and short pulse width, which can simultaneously excite coherent vibration of many gas molecules. The narrow spectrum of air laser can be used as a probe with high spectral resolution, which can effectively distinguish the Raman fingerprints of different molecules. Therefore, the technology can meet the needs of multi-component measurement and chemical specificity. In this study, the signal-to-noise ratio of coherent Raman signals is effectively improved by using seed amplification and polarization filtering technology, and the background noise and signal jitter caused by supercontinuous white light generation are significantly suppressed, thus improving the detection sensitivity and stability. The research team used air laser-assisted coherent Raman spectroscopy technology to measure the quantitative relationship between the Raman signal intensity of CO2 and SF6 in the atmosphere and the corresponding gas concentration. The minimum detected concentrations of CO2 and SF6 were 0.1% and 0.03%, respectively, and the minimum signal jitter reached 2% (Figure 2).
Figure 2. Quantitative relationship between coherent Raman signal intensity measured by experiment and gas concentration. The illustration shows the Raman signals of CO2 and SF6 measured at minimum concentrations. The minimum detected concentrations of CO2 (1388 cm-1 Raman peak) and SF6 are 0.1% and 0.03%, respectively
Further, the technique can be used for simultaneous multicomponent measurements in a mixture of air, CO2, and SF6, as shown in Figure 3(a)-(c), benefiting from femtosecond laser multicomponent excitation and the ability of air laser multicomponent resolution. More importantly, air laser-assisted coherent Raman spectroscopy can be used to effectively distinguish between 12CO2 and 13CO2 isotopic gases, as shown in Figure 3(d).
Figure 3. Raman signals of (a) CO2 at a concentration of 0.5% in air, (b) SF6 at a concentration of 0.1%, (c) CO2 at a concentration of 0.5% and SF6 at a concentration of 0.1%, measured by air laser-assisted coherent Raman spectroscopy; (d) Raman signals of 12CO2 and 13CO2, both at 0.4% concentration in the air
Summary and prospect
Air laser-assisted coherent Raman spectroscopy, which combines the advantages of femtosecond laser and air laser, not only can be used for highly sensitive detection of common greenhouse gas concentrations in the air, but also has the ability of multi-component measurement and isotope resolution. The correlation measurement of various pollutants and greenhouse gases and the detection of CO2 isotope are of great significance for tracing the source of atmospheric pollution, studying the carbon cycle process and confirming the source and sink of carbon emissions, and are also important advantages of this technology over traditional remote sensing technology. However, to achieve high precision measurement of trace pollutants in the atmosphere, it is necessary to increase the detection sensitivity to ppm or even ppb level, and expand the detection distance from the laboratory scale to the kilometer scale. It is believed that through the innovation and development of high refrequency, high energy femtosecond laser technology and high sensitivity detection technology, this technology is expected to be significantly improved in detection distance and sensitivity, meet the practical application requirements of atmospheric detection, and serve the national “dual carbon” strategy.
The author introduced Shao
Zhihao Zhang (first author) is a doctoral candidate jointly trained by Shanghai University of Science and Technology and Shanghai Institute of Optics and Machinery, Chinese Academy of Sciences. His research interests are femtosecond laser interaction with matter. He has published 9 papers in journals such as Ultrafast Science and Science Bulletin.
Fangbo Zhang (co-first author) is a PhD candidate at the Shanghai Institute of Optics and Computer, Chinese Academy of Sciences. His research interests include high-field ultrafast optics and nonlinear spectroscopy. He has published 3 academic papers in Opt.Lett., Phys. Rev.A and other journals as the first author and co-first author. He was awarded Outstanding Student Scholarship and Merit Student honor of Shanghai Institute of Optics and Machinery.
As the first/corresponding author, he published 38 papers in Phys Rev. Lett., Science Bulletin, etc., and his research results were selected as “Important Achievements in Chinese Optics”, Rao Yutai Basic Optics Award and cover articles. The original work in the air laser direction has been cited nearly 200 times. Funded by the “Excellent Youth” project of the National Foundation Committee, he has been selected as Shanghai Excellent Academic Leader, Shanghai Youth Top-notch Talent, and Shanghai Young Science and Technology Star. Editorial board member of Laser China and Ultrafast Science Youth Editorial Board Member.
Cheng Ya (corresponding author) is a researcher at the Shanghai Institute of Optics and Machinery, Chinese Academy of Sciences, and a professor at the School of Physics and Electronic Science, East China Normal University. He is mainly engaged in the research of ultrafine nonlinear optics and laser micro-nano manufacturing. He has published more than 200 papers and more than 10,000 citations, with H factor greater than 50. He has been sponsored by the National Science Foundation for Outstanding Young People for more than 100 times. He has successively served as the chief scientist of the National 973 Plan project and key research and development Plan project. He has published 1 monograph in Chinese and 5 monographs in English. He is a member of the Optical Society of America, a member of the British Physical Society, and a member of the Chinese Optical Society.
Xu Zhi-zhan (corresponding author) is a researcher at the Shanghai Institute of Optics and Machinery, Chinese Academy of Sciences, Academician of the Chinese Academy of Sciences, and member of the Third World Academy of Sciences. He is one of the early leaders in the field of inertial confinement laser fusion in China, and a pioneer in the new field of super-strong and ultra-short laser science and strong field physics in China. He served as director of Shanghai Optical Machinery Institute and vice president of Chinese Optical Society. He presided over laser fusion research for a long time and made pioneering and outstanding contributions. It has made systematic scientific discoveries in important frontier research on the interaction between intense laser and matter. For the first time in the world, 8 new wavelength X-ray lasers have been obtained by lithium and sodium-like ion schemes, and the shortest wavelength has reached 46.8 angs. It has opened up new fields of super super short laser science and strong field physics in China and made breakthrough achievements. As the first winner, he has won 1 first prize of the National Science and Technology Progress Award, 2 second prizes of the National Natural Science Award, 1 second prize of the National Invention Award, 4 first prizes of the Chinese Academy of Sciences Natural Science Award and Science and Technology Progress Award, 2 first prizes of Shanghai Science and Technology Progress Award and Natural Science Award, etc. In 1996, he won the Shanghai Science and Technology Merit Award. He Liang Ho Li Foundation Award for Scientific and Technological Progress in 1998. In 2006, he was awarded the Gold Medal for “Outstanding Contribution to laser science” at the International Conference on Ultra Fast and Intense Laser Science.
Post time: Mar-09-2023