| Intense femtosecond laser propagating in gas with a series of linear and nonlinear effects has many potential applications in reality, such as super-continuum light detection and ranging techniques, high-voltage discharge controlling and lightning protection, generation of single cycle pluses and high order harmonics. Because of these promising potential applications, intense femtosecond laser propagating in gas gets many researchers' attention. So far, there are two physical models to describe the propagation of intense femtosecond laser. One is the classical model adopted since 1990s, the other one is the so-called full model considering the higher-order Kerr effects. The filamentation produced by the intense femtosecod laser propagating in gas is described as a dynamic balance between the Kerr self-focusing and the plasma defocusing in classical model while between Kerr self-focusing and defocusing by the higher-order Kerr effects wihout considering the plasma defocusing in full model. Five aspects of the intense femtosecond laser propagating in gas are investigated by the above two models in this doctoral dissertation.The higher-order Kerr effects becomes a hot topic since the measurements of the higher-order Kerr refractive index of the air by V. Loriot et al. in 2009. The Kerr refractive effects dominate the self-focusing and defocusing processes in the full model proposed by P. Bejot et al, which means that the self-guiding is independent on the plasma. Then a large number of studies on the dominant position of the classical model and the full model are conducted, but there is no uniform conclusion. We use the above two models to simulate the intense femtosecond laser propagating in air medium, the same initial parameters are selected. Results tell that the peak intensity by the classical model is three times the one by the full model and the length of the filament by the full model is about twice the one by the classical model. The spatial and temporal distributions of the two models in the cross-section of filament are also different.Because of the limit of the experimental conditions, the latest nonlinear refractive index containing higher-order Kerr is approximately 10% of floating up and down. The nonlinear refractive index will affect the properties of the femtosecond filament. We study the influence with four different groups of refractive index by full model. The simulation results tell that the peak intensity and the on-axis electron density decrease with the diminishing refractive index and the less the refractive index is, the slower is the decreasing velocity. The length and radius of the filament both decrease with the increasing refractive index. The shape of the spatial and temporal distributions in different propagating positions shows that the spatial and temporal distributions are widely disprepant with the variant refractive index.The wavelength of the intense femtosecond laser covers from ultraviolet region to infrared region, the laser with variant color propagating in gas will produce different filaments. Up to now, only the higher-order nonlinear refractive index of the nitrogen, the oxygen and the argon for 800 nm laser pulse has been measured, so we use the classical model to simulate the filamentation produced by laser pulses with different color. In addition, the noble gas is mono-atomic and there is no Raman effect nor any complication such as molecular association and fragmentation, so we choose argon as the propagation medium. We investigate the filamentation in argon by fixed incident peak power and fixed ratio of the incident peak power to the critical power of self-focusing. The shorter the laser pulse wavelength is, the higher the peak intensity and electron density in propagating are. The filament channel formed by the intense femtosecond laser with short wavelength is not only fine and long but also stable, what's more, the ratio of energy within the beam radius to the total energy along the propagating direction is flat which is conductive to long-distance propagating.The temporal and spatial distribution of the electron density changes when the intense femtosecond laser propagates in gas. The electron density is mainly determined by four kinds of effects, which are photo ionization effect, the avalanche ionization effect, the electron attachment effect and the electron recombination effect. Most of the simulation researches consider the photo ionization but takes parts of the next three processes into account, especially ignoring the last two processes. The durations of the incident laser pulse range from 10 fs to 300 fs, the electronic attachment and recombination effect should be different, more importantly, the roles of the attachment and recombination effect in high pressure environment are also significant. We investigate the effects of the electron density on the femtosecond filament under variant pressures and durations of the incident laser pulse in air and argon by the full model. In the air and argon medium, considering the pressure factor, the influence of the above last three effects increase from zero to a constant. Among the three effects, the influence of the avalanche ionization is a little higher than the electron attachment effect and the electron recombination effect. These influence on the on-axis intensity in air are less sensitive than those in argon, the three effects should be taken into account in high pressures. As for the incident duration factor, although it covers a such large range from 10 fs to 300 fs, these influence on the femtoseond filament is still little and can be neglected.In standard atmospheric conditions, many scholars use the classical model and the full model containing the higher-order Kerr effects to study the intense femtosecond laser filamentation; in the changeable pressure environment, previous scholars mainly use the classical model. We investigate the influence of the pressure and delayed Kerr effect on the femtoseond filamentation by comparing the full model and the classical model. The results show the pressure is very significant. For the two models, the most significant numerical results is that the length of filament is approximately inversely proportional to the pressure, and the radius is approximately inversely proportional to the square root of pressure. At the same time, the energy in the filament also decreases with the increasing pressure. The effect of pressure is severer for the laser pulse with shorter duration, and the filament may even not be formed when the duration of pulse is too short. On the other hand, the lengths of the filament predicted by the full model at various pressure from 0.5 atm to 4 atm are about twice those of the classical model, and this may provide a method to determine which of HOKE and plasma dominates the defocusing effect. For the two models, the delayed Kerr effect doesn't change the intensity clamping but change the location it appears, i.e. delay the position. It doesn't impact the length and radius of the filament but decrease the electron density. |
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