Molecular Dynamics Simulations Of Femtosecond Laser Ablation Of Silicon

In this paper, a molecular dynamics program has been written out to handle the femtosecond laser ablation of crystal silicon. The program is written in FORTRAN, parallelized by the mean of decomposing atoms, and optimized by the neighbor-list method to reduce the time expended in the force calculation. The results agree with the experimental and other numerical results, which indicate the molecular dynamics method is valuable for problems of laser ablation.Comparative study of five empirical potentials for silicon was performed. Influences of bond angles and distances between atoms on the system energy were investigated. The results show SW potential is widely used because of its moderate cutoff radius, less parameters, simpler form and physical foundations. Thus, SW potential was chosen to describe the interactions between atoms in this paper. Equations of force calculation were educed based on SW potential.Evolutions of initial systems to NVT and NVE ensembles besides the free evolution were simulated in order to debug the program and investigate the characteristics of the balancing processes. The FREE process can conserve the total energy well and drive the system to reach a reasonable equilibrious state quickly, so is usually used in the balancing process after a dynamics loading. The NVT process can keep the temperature constant and is suitable for the balancing process at a given temperature. Both of the two processes were used in the simulations of laser ablation.A 3-D"x-section"model was developed to describe the deposition process of laser energy. Based on the 3-D"x-section"model, femtosecond laser ablations of Si(100) were simulated under two conditions which were total covering of the laser spot on the target surface and less laser spot than the target surface. Through the simulations, common characteristics of the femtosecond laser ablation were gotten. The ablation begins from the emergence of bubbles and is eventually induced by developments of the bubbles. The ablated material is composed of crystal, melted and ablated regions in which atoms move just like particles in solid, liquid and gas respectively. Moreover, two kinds of laser-induced stress waves were captured. One in the melted region is caused by the dispersion of surface atoms; the other in the crystal region propagates with the sound velocity.Influences of laser parameters (such as pulse width, intensity, spatial spectrum and wave length) and loading styles on ablation phenomena were analyzed systemically. The results show: with the same energy flux, the short pulse produces more serious ablation phenomena than the long pulse; at a given pulse width, ablation becomes more and more serious with the increase of laser intensity, the least intensity with which the laser can ablate the target is called ablation threshold; the spatial spectrum determines the spatial distribution of laser energy, and will lead specific ablation phenomena; the laser with longer wave length has photons with lower energy and a larger thermal-affected region in the ablation process. There are two loading styles of the incident laser, one is long-time loading and the other is instantaneous loading. The difference between them is whether the deposition process of laser energy is accompanied with the balancing process of the system. The ablation phenomena of femtosecond lasers under the two loading styles are almost same, so the instantaneous loading can be used in femtosecond laser ablations to reduce the simulation time. However, it is not effective to the picoseoncd laser ablation because of magnifying the ablation phenomena. Decomposing the long laser pulse into femtosecond lasers may be used to simplify the simulation.