Mathematical modeling of dynamics of fast phase transitions and overheated metastable states during nano- and femtosecond laser treatment of metal targets

For mathematical description of pulsed laser heating, melting and evaporation of aluminium target in ambient atmosphere was used one dimensional, multi front hydrodynamic Stephan problem, written for both phases (liquid and solid). On the boundary of solid and gaseous forms Stephan problem is combined with radiation gas dynamic equations, with thermo conductivity, and describes processes in evaporated material and surrounding gas. For numerical solution finite difference dynamic adaptation method, which gives opportunity of explicit tracking of inter phase boundaries and shockwaves. As a result, in the process of the solution the problem had 6 computation regions and 7 boundaries, 6 of them were moving, including 2 shockwaves and free boundary in atmosphere.
We used this model to calculate pulsed laser interaction with aluminum target with following parameters: $\lambda=0.8$, $\tau=10^{-8}\div10^{-15}$ s and $G_0=10^9\div10^{16}$ W/cm$^2$. Modeling revealed that in case of long $\sim1$ ns pulses greater part of the energy is spend on melting and heating of the liquid. Molten pool depth is about 1.2 $\mu$m. In case of femtosecond pulses greater part of the energy is spend on heating of the solid and for the formation of shockwave in solid. The depth of the molten pool does not exceeds 0.03 m. Although evaporated layers were almost the same thickness.
For nanosecond laser pulses with fluence $J$ less than 30 J/cm$^2$ there is no plasma formation in the evaporated material. For the same fluence for femtosecond laser pulse plasma is formed after the pulse and has thermal nature.