Multiscale Modeling And Experimental Study Of Ultrafast Laser Processing

Ultrafast laser micro/nano fabrication is a frontier research area. Different from the traditional laser processing, ultrafast laser fabrication is a multiscale process, whose time scales from femtosecond to nanosecond, and space scales from nanometer to micrometer, causing a variety of new physical phenomena, such as multiple ionization, tunneling ionization, Coulomb explosion and phase explosion. In order to describe this process, it is necessary to introduce a multiscale model. The quantum mechanics need to be introduced for explaining the non-linear effect of electronic excitation. The plasma model needs to be introduced for calculating thr depth of crater and being compared with the experimental values. The hydrodynamic model needs to be introduced for explaining the phenomenon of phase transition.Firstly, the density functional theory which is derived from the Schrodinger equation is used to calculate the electron excitation and energy absorption of atoms, moleculars and solids. The main research work and achievements as follow:1). We investigated the absorbed energy and spin polarization of an Fe-graphene system using first-principles calculations. As the angle between the electric field and the graphene plane increased from θ=0° to 90°, the ionized electrons and absorbed energy are reduced, while the spin polarization first undergoes a decrease and then rises rapidly. As the laser intensity increases, the spin polarization decreases continuously. The results show that it is possible to control the energy distribution and the spin polarization of the electron beam by fine-tuning the laser parameters. The Fe-graphene system may be a source of spin-polarized electrons.2). TDDFT-based first-principles method has been used to study the ionization process of alpha-quartz under intense femtosecond laser irradiation. The 6-photon absorption cross section and the underlying ultrafast multi-electron dynamics are presented. Using the 6-photon absorption cross section from first-principles calculation, the damage threshold fluences as a function of pulse width are calculated with the help of plasma model. The good agreement between the calculation results and the experimental data validates the results of first-principles calculation.3). A one-dimensional radiation hydrodynamics simulation code, MULTI-fs, has been used to study the interaction of ultrashort laser pulses with metals, in which the electron-ion collision frequency was taken as an adjustable parameter to match the absorbed laser energy with the experimental data. The model calculation is employed to investigate the ablation depth and the dependence of the threshold fluence of gold film on pulse width and wavelength. For estimating the ablation depth, two methods have been introduced with their respective scope of application. The dependence of the threshold fluence of gold film on pulse width of the laser with a center wavelength of 1053 nm is good agreement with experimental data. It is also observed that for pulses shorter than approximately 200 ps the threshold fluence shows the linear dependence on the logarithm of pulse width and increases with the wavelength.We could use the above models to guide experimental design, and Deepen the understanding of fabrication.