Research On The Propagation Of Femtosecond Laser In Air And The Spectral Characteristic During Filamentation

During the propagation of femtosecond laser pulses in air, the laser intensity keeps increasing due to the Kerr self-focusing effect, and as it increases to a certain value (ionization threshold of atoms or molecules), it will make the atoms and molecules in air ionized, generating a large number of plasmas. Since the plasmas play a defocusing role in the femtosecond laser propagation, it makes the laser intensity decrease. Kerr self-focusing effect and plasma defocusing effect exist at the same time, when they reach the dynamical balance, a long plasma channel can be formed, which is also known as filamentation.Femtosecond laser filamentation process is accompanied by several nonlinear phenomena like photoionization, supercontinuum generation, terahertz radiation, high-order harmonic generation, conical emission and fluorescence emission, etc.. Studying these phenomena not only has important academic value in the understanding of the mechanism of interaction of ultra-short laser pulses with atoms and molecules, and the transmission rule of laser pulses in different medium environments, but also has important application prospects in the astronomy, meteorology, national defense and other fields, such as remote sensing, lightning protection, rainmaking and air waveguide, etc.. These attractive and promising applications have made the femtosecond laser filamentation become a research focus in recent years, and in-depth investigation in theory experiment and applications has been carried out, obtaining many important results.During my PhD, we carried out the research on filamentation of femtosecond laser in air from the aspect of numerical simulation and experimental measurement. In the first three chapters, we make a systematic introduction and summary of the fundamental work in femtosecond laser filamentation. In the introduction part, we first review the research history and progress of the laser propagation and the mechanism of the femtosecond laser filamentation, then introduce its applications in combination with the significance of femtosecond laser filamentation, and finally summarize some effective way to control filamentation. In the second chapter, starting from the models used to describe the femtosecond laser filamentation, we introduce common phenomena that occur during filamentation, including higher-order Kerr effect, filament induced fluorescence and other ones which have attracted great attention recently. In the third chapter, we discuss the theoretical and experimental methods that we adopt to study the femtosecond laser filamentation in this dissertation.Theoretically, the filamentation process of femtosecond laser pulses can be investigated by numerically solving the propagation equation. Currently, various methods have been developed to numerically solve the propagation equation, and theoretical works involved in this dissertation are based on the finite difference method. There are a great many theoretical studies on the femtosecond laser filamentation, however, they mainly focus on the normal and low pressures environment, and rarely involves in high pressure, strong electromagnetic radiation and other extreme environments. The theoretical works of this dissertation are mainly devoted to this area.In the fourth chapter, we study the propagation of femtosecond laser pulses in relatively high pressure, and find that the group velocity dispersion effect (GVD) has a great influence on femtosecond laser filamentation process, leading to the fact that the semi-empirical Marburger formula does not work any more. By utilizing the classical optical imaging principle, we introduce the dispersion length into the Marburger formula and propose a revised one, which can not only well explain the influence of GVD on the collapse distance, but also is in good agreement with the numerical results. This study changes generally accepted viewpoint that GVD effect are not strong enough to affect the pulse collapse in gaseous media, which will contribute to a more comprehensive understanding of GVD effect during laser propagation.In the fourth chapter, we take a look ar the high-orfer Kerr effects (HOKE) that have been paid great attention in recent years. We mainly study the influence of the uncertainty of multiphoton ionization (MPI) cross section on the applicability of higher-order Kerr model, and find that in the case that the MPI cross section is relatively large, the electron density and clamped intensity calculated via the model with HOKE being considered and neglected are nearly in agreement with each other, under this circumstance, even if the higher-order nonlinear terms do exist, the free-charge generation and the associated defocusing in a filament are enough to mask their effects, which demonstrates that it is ionization that results in the difference between the two models. The different behaviors of the maximum intensity and on-axis electron density at the collapse position with the pulse duration provide an approach to determine which effect plays the dominant defocusing role.In the upper atmosphere, there exists electromagenetic wave radiation covering a wide wavelength range, and particularly, under the radiation of electromagenetic wave with high frequency, the gaseous components in atmosphere, e.g. the oxygen molecules, are easy to be excited to the higher energy-level. Due to the large photo-ionization cross section of excited molecules, they are quite easy to be ionized, which will affect the propagation of femtosecond laser pulses in it. In Chapter 6, we numerically simulate the propagation of femtosecond laser pulse in air containing a certain amount of excited molecules. It is found that as the gas contains a certain amount of excited molecules, the propagation of femtosecond laser pulse in it is affected, and the more excited molecules, the greater the influence on the spatial and temporal distribution of laser intensity is, thereby exhibiting phenomena different from the filamentation in neutral gas which does not contain excited molecules.After filamentation of femtosecond pulses in air, the plasmas left behind undergo complex transitions, emitting characteristic fingerprint luminescence. By utilizing the fluorescence, we can not only qualitatively characterize of filamentation process, but also extract the plasma density, electron temperature as well as laser intensity inside filaments. In addition, we can also get insight into the mechanism of the interaction of ultrashort pulsed laser with atoms and molecules, as well as the radiation characteristics of atoms and molecules in plasma environment.In Chapter 7, by utilizing the plasma fluorescence to characterize filamentation, the filamentation induced by collinear femtosecond double pulses with wavelengths of 400 and 800nm in air is investigated. Though the two pulses do not overlap in time, the filament generated by the previous pulse will interact with the latter one, thus affecting the filamentation process. Each pulse plays a different role when the time delay and input energy are different, thereby affecting the fluorescence intensity and filament length during filamentation. This study provides an effective way to control filamentation.Nitrogen is the main component of air, and is also the main working matter in air laser. However, nitrogen fluorescence generation mechanism during the femtosecond filamentation has not been fully understood. From Chapter 8 to Chapter 10, we discuss the mechanism of nitrogen fluorescence.The influence of laser polarization state on fluorescence emission is investigated in Chapter 8. It is found that under different polarizations, the emission behavior of the 391-nm signal is different from those of the other lines. We make an explanation of the phenomenon through the analysis of the source of each line.337,357,380,428 nm and other spectral lines mainly come from N2 and N2+, and their emission behavior will stronger due to the opening-up of impact excitation channel cased by circularly polarized laser pulses. While for 391 nm spectral line, both N2+ and N+ contribute to its emission. It is the competition between the emission from the N+ and impact excitation that makes the difference between the emission behavior of the 391-nm signal and those of the other lines under different polarizations.In the ninth chapter, we experimentally measure the radial angular distribution of plasma fluorescence emission during the filamentation of linearly polarized femtosecond laser pulses. It is found that the angular distribution of the fluorescence from N2 and that of N2+ are quite different from each other:the fluorescence from N2 shares the same intensity in all directions, while that from N2+ is stronger in the direction parallel to laser polarization direction than that in the direction perpendicular to laser polarization direction. The isotropic emission behavior of luminescence from N2 illustrates that the formation of excited does not result from the dissociative recombination; on the other hand, the linear increase of the strength of 337 nm signal with pressure indicates the collision assisted intersystem crossing scheme plays the dominant in the formation of N2(C3∏u+). Taking the two factors into account, the direct intersystem crossing scheme is proposed. Though no direct evidence to support this mechanism has been provided at present, several previous works have proven it possibility.In Chapters 8 and 9, we discuss the generation mechanism of nitrogen fluorescence by measuring the sidewise fluorescence. However, in practical applications (e.g., remote detection), what is needed is the backward scattering fluorescence. In Chapter 10, the generation mechanism of nitrogen fluorescence is discussed by measuring the backward scattering fluorescence during femotosecond filamentation. It is found that when focusing lens focal length is relatively short, the fluorescence from N2 is much stronger as the laser pulse is linearly polarized, while that from N2 is much stronger as the laser pulse is circularly polarized. The phenomenon indicates that in formation of N2(C3∏u+), dissociation recombination mechanism does not play a leading role, which further supports the conclusion in chapter 9. These researches will be helpful to the understanding of mechanism of nitrogen fluorescence emission during femtosecond filamentation.