Studies On Filamentation Characteristics Of Femtosecond Laser In Fused Silica

When femtosecond laser pulses propagate in a transparent medium,the laser beam can overcome diffraction and propagate over extended distances much longer than the typical diffraction length without the help of any external guiding mechanism if the peak power of the laser pulse is larger than the critical power of self-focusing.In the meantime,a plasma channel,also called a filament,can be formed due to the combined action of linear and nonlinear effects,such as Kerr self-focusing,plasma defocusing and so on.Typically,some by-products such as supercontinuum radiation,third harmonics generation,terahertz radiation,and light bullets can be generated along with a filament.Femtosecond laser filamentation and its associated products have received widespread attention due to their important applications in many fields,such as artificial rainfall,remote detection,generation of few-cycle pulses,micro-nano processing,laser spectroscopy technology,etc.Although a large number of studies have been carried out in the field of femtosecond laser filamentation in solids,there are still some problems that need to be further investigated,such as the role of free-electron plasma during femtosecond filamentation under different conditions,the role of the higher-order Kerr effect during femtosecond filamentation,the influence of the spatio-temporal distribution of laser pulses on the dynamics of filamentation,the dynamic evolution and control of spatialtemporal solitons,and so on.Based on numerical simulation of femtosecond laser pulse propagating in a solid,this paper investigates the filamentation dynamics of femtosecond laser pulses in solids under different conditions.The main work of this article includes the following three parts:First,by numerically solving the unidirectional pulse propagation equation,we investigate the filamentation dynamics and supercontinuum generation of ultraviolet laser pulses in fused silica.The results show that,when ultraviolet femtosecond laser pulse propagates in fused silica,the splitting dynamics exhibit quasisymmetrical pattern,which is different from laser pulses filamentation at 800 nm in solids.The main reason is that the physical factor of balancing the self-focusing effect is multiphoton absorption rather than plasma defocusing.Besides,with the help of time-frequency analysis,we also investigate the mechanism of supercontinuum generation and find that,self-phase modulation mainly dominates the supercontinuum generation and spectral broadening induced by plasma is not obvious.Considering that the collision time of the electrons is controversial and it would afect the contribution of avalanche ionization,we investigate the influence*of avalanche ionization on the supercontinuum generation.By changing the electron collision time,it is find that the plasma enhancement induced by avalanche ionization would not lead to blue shift of the spectrum,on the contrary,the increasement of plasma density causes energy absorbtion of the laser pulse,which suppressed the self-phase modulation effect and thus limited the broadening of the supercontinuum spectra.This work help us clarify the role of plasma on the supercontinuum generation during UV laser filamentation in solids.Second,the light bullet dynamics are retrieved by studying the dispersive wave,or resonance radiation(RR)when mid-infrared laser pulses propagate in fused silica.During the mid-infrared laser pulse filamentation in fused silica,RR occurs associated with the formation of a light bullet.Thereafter,we can retrieve the dynamics of light bullets by investigating RR.We firstly reproduce RR by conducting an experiment in fused silica at a central wavelength of 2080 nm and find that the RR spectrum exhibits obviously asymmetrical feature.Numerically,we also repeat the asymmetry of RR by solving the unidirectional pulse propagation equation.Immediately after,we retrieve the formation process of RR through time frequency analysis.There are two main findings:First,the formation of RR is due to the mutual interaction of nonlinear effects such as selffocusing effects,plasma effects,nonlinear absorption,and self-steepening effects.Second,the asymmetric feature of the RR spectrum originates from the temporal chirp feature.Through the analysis of an effective three-wave mixing model,we find that the temporal chirp characteristics of RR can be attributed to the variation of phase matching condition,which reflects the accelerated propagation of the light bullet.This study provides a deeper understanding of formation and evolution of RR,and accelerated propagation of light bullets.Third,by numerically solving the unidirectional pulse propagation equation,we investigate the filamentation dynamics of mid-infrared super-Gaussian laser pulses in fused silica.It is found that,in the anomalous dispersion region of the medium,the superGaussian pulse and Gaussian pulse filamentation showed significant differences.This is mainly manifested in the following two aspects.Firstly,compared with Gaussian pulse filamentation at the same laser fluence,the distance for super-Gaussian pulse filamentation is shorter,and the steeper the front and back edges of the super-Gaussian pulse,the earlier the filament will be formed;Secondly,in the case of lower initial pulse power,the superGaussian pulse filament dynamics are similar to the Gaussian pulse filament dynamics,and a single light bullet is formed in the process of filamentation,and in the case of higher initial pulse power,the super-Gaussian pulse and the Gaussian pulse both split into two sub-pulses.However,the mechanisms of pulse splitting between them are different,the splitting of Gaussian pulses stems from spatial refocusing and self-steepening,while the splitting of super-Gaussian pulses is due to imbalances in self-phase modulation effects and dispersion.This study revealed filamentation dynamics of different pulse shapes in solids and provide a scheme for generating specific spatialtemporal wavepackets.