Numerical Simulation Of Femtosecond Laser Nonlinear Propagation In Fused Silica

Femtosecond laser enables fabrication of3D microstructures with low damagethreshold, low heat effect and high precision due to its characteristics of ultra peakpower and ultrashort pulse duration. In recent years, femtosecond laser has beenwidely used for producing various optical elements inside transparent materials, suchas waveguide,3D micro-channel, micro-gratings and photonic crystals. Since thesignificance of nonlinear effects between femtosecond laser and materials becomevery notable, it is an ergent need for carrying out the theoretical study of nonlineartransmission process. The corresponding theoretical study will possibly promote thedevelopment of the physical mechanism of new experimetal discoveries. In thisdissertation, a number of studies are carried out in this field, our mainly work is tostudy the nonlinear propagation of femtosecond laser inside fused silica which isrepresentative of the transparent materials.Firstly, this paper introduces the mechanism of the interactions betweenfemtosecond laser and fused silica, the research progress of femtosecond lasernonlinear propagation in the medium. Then we study the femtosecond lasernonlinear propagation in the fused silica, and introduce the generalized nonlinearSchr dinger equation for the following numerical simulations. The equation hasincluded mostly current known nonlinear effects, such as Kerr effect andRaman-Kerr delay effect, group velocity dispersion, plasma defocusing and inversebremsstrahlung absorption, photoionization absorption. According to this equation,we simulate the nonlinear propagation with different experimental parameters tostudy the dependence of induced microstructures on single pulse energy, pulseduration and focus depth.In the next, our research focuses on the electron momentum relaxation timewhich plays an important role in the nonlinear propagation. It is surprising that lots of research groups bring up various values for this parameter without a clear origin,however we think that it is signficant for our numerical simulation. In theexperiments, we irradiated a silica glass with a tightly focused femtosecond laser,then we polished the sample and etched it in hydrofluoric acid, and finally a damagetrack caused by femtosecond laser was observed from a scanning electronmicroscope. We simulated the propagation by a (3+1)-dimensional generalizednonlinear Schr dinger equation, and the results show a good agreement with theexperiments in the aspects of plasma density distribution, energy flow distribution,and peak intensity distribution. We finally propose a reasonable value for thiselectron momentum relaxation time.By carrying out the research on simulation of nonlinear propagation offemtosecond laser pulses in transparent materials, it is very helpful for us tounderstand the formation mechanism of microstructure induced by laser and graspthe characteristics of various types of nonlinear effects. Meanwhile current resultsalso can provide theoretical guidance in the area of femtosecond laser inducedfunctional micro-optical structures.