Study On Generation,Mechanism And Application Of Air Lasing

Air lasing as a new nonlinear optical phenomenon induced by ultrafast laser,can generate far-field cavity-free coherent radiation with the air components as gain media,which has attracted extensive attention in the past ten years,especially the N₂⁺lasing discovered in 2011.The reasons why the N₂⁺lasing is so attractive can be summarized as follows.On the one hand,it shows important application potential in remote detection of atmospheric pollutants and greenhouse gas,and in research of the atomic and molecular physics.On the other hand,the optical gain mechanism of the N₂⁺lasing is complicated,which includes rich physical processes,such as molecular ionization,coupling of energy states and quantum coherence.Although a large number of theoretical and experimental studies on the N₂⁺lasing have been carried out,its mechanism has not been clear.What's more,there are still some related physical phenomena which have not been fully interpreted.And the study on the application of N₂⁺lasing is still in its infancy.In this thesis,we presented our systematic study on the generation,machanism and application of air lasing,especially the N₂⁺lasing.The ionization process induced by the intense field and coupling process of the energy states are explored on some aspects,including their influences on the population of N₂⁺in the electronic and vibrational states and their contributions to the establishment of optical gain.In addition,we realized the optimization of N₂⁺lasing signal by modulating the ionization and coupling processes.Then,the new-type air-laser-based remote Raman spectroscopy technology was developed.The specific innovation achievements are as follows:1.Research on generation mechanism of the N2⁺lasing phenomenon.A pair of pulses,i.e.E_y and E_x,with polarization directions orthogonal to each other were constructed based on the multi-order quarter-wave plate(MQWP)modulated laser field,and the establishment mechanism of optical gain in the N₂⁺lasing was explored through the pump-probe scheme.The modulation of N₂⁺lasing signals at wavelengths of 391nm and 428 nm was achieved by varying the ellipticity of the pump laser to adjust the amplitude ratio of the E_y and E_x in the optical field.Based on the post-ionization optical coupling model,and combined with the analysis of the variation of fluorescence signal,it was revealed that the intensity variation of the 391-nm lasing with the ellipticity is originated from the competitive balance between the ionization process dominated by the E_y and the coupling process dominated by the E_x,and the difference between the428-nm and 391-nm cases is due to the difference between the population in the X²?_g⁺(v''=1)state and the population in the X²?_g⁺(v''=0)state after E_y ionization.In addition,by changing the order of MQWP to modulate the relative delay between the E_y and E_x,the direct control of the optical coupling efficiency after ionization was realized,based on which the 391-nm lasing signal was further optimized.The results of numerical simulation based on the post-ionization optical coupling model were consistent with the experimental observations,which indicates that the theory of post-ionization optical coupling is useful for interpreting the generation process and polarization effects of N₂⁺lasing.At the same time,the asymmetric dependence of the391 nm lasing signal on the ellipticity of the MQWP-modulated laser field was demonstrated experimentally,which can be attributed to the order reversal of ionization and coupling pulses in the time domain induced by the birefringence effect,leading to different optical coupling efficiencies in the same ellipticity case.We also demonstrated the manipulation of self-seeded N₂⁺lasing through the MQWP-modulated laser field.This series of work is important for clarifying the generation mechanism and polarization effects of N₂⁺lasing and for effectively modulating and optimizing the N₂⁺lasing.2.Research on optimization of the N2⁺lasing under atmospheric condition.Through precisely adjusting the spatiotemporal overlap of two 800-nm near-infrared laser pulses(pump and control)in the filamentary plasma grating,the intensity of self-seed 428-nm lasing along the pump beam can be enhanced by?4 orders of magnitude in the ambient air.Interestingly,the enhancement of N₂⁺lasing can be modulated effectively by changing the relative delay time and polarization direction between the two laser pulses.Combined with the variation of fluorescence signal under the same experimental condition and theoretical analysis,it was revealed that the enhancement of 428-nm lasing is due to the increased population in the B²?_u⁺(v=0)state through the plasma grating,which directly increases the density of population-inversed N₂⁺and results in a giant optical gain since the population in the X²?_g⁺(v''=0)state is insensitive to the formation of the plasma grating.By comparing the enhancement factors of 428-nm and 391-nm lasing signals in the plasma grating with the single pump case,it was verified that the difference of population inversion caused by the different population in the lower level of transition is important for the enhancement effect of the N₂⁺lasing.Our results verified the key role of population inversion in the gain generation of N₂⁺lasing and paved the way for generating high-intensity air lasing under the atmospheric condition,which lays a foundation for the application of air lasing in remote atmospheric detection.3.Research on application of the N2⁺lasing.Through the N₂⁺lasing generated from the nonlinear transportation of intense femtosecond laser pulses in nitrogen gas,a new-type remote Raman spectroscopy technique was developed,through which the Stokes and anti-Stokes,rotational and vibrational,time-and frequency-domain,non-resonance,single-resonance and double-resonance Raman spectra of the atmospheric components,i.e.N₂ and O₂,can be obtained.Based on this technique,the dispersion processes of rovibrational wave packets of N₂ and O₂ and the temporal evolutions of electronic,vibrational and rotational states of N₂⁺were simultaneously demonstrated.In addition,combined with the Raman spectroscopy in time and frequency domains,it is found that the couplings of N₂⁺laser with the rotational states of atmospheric molecules can generate second-order laser sources for vibrational Raman processes.On the other hand,the species discrimination and concentration detection of volatile organic macromolecules were realized based on this technique.Benefiting from the high resolution of the coherent anti-Stokes Raman spectrum provided by this technique,the measurement and calibration of Raman processes of carbon-hydrogen bonds and carbon-oxygen single bonds in methanol and ethanol molecules can be realized.And the Raman frequency shifts of the same chemical bonds in different molecular structures were analyzed.Finally,the detection sensitivity of the concentration was calculated and discussed based on the vibrational Raman spectra of ethanol molecules with different heights.In this work,we developed a new-type air-laser-based Raman spectroscopy technique,which expanded the application range of the air lasing,and opened up a new route for the identification and detection of remote atmospheric pollutants,as well as for the analysis of atomic and molecular dynamics in biological and chemical reactions.