Heat transfer and phase change during picosecond laser ablation

An experimental and numerical study was conducted to improve understanding of heat transfer and phase change participating during the ablation of metals with a picosecond laser pulse. Numerical calculations of heat transfer and phase change were performed for the ablation of nickel, including melting and vaporization kinetics. Calculations showed that for picosecond laser heating, a significant molten surface layer, tens of nanometers thick, was formed. At the highest fluence investigated (508 mJ/cm2), the melt was superheated to near the thermodynamic critical temperature. However, even at this high temperature, the depth of mass removed by surface evaporation was calculated to be less than 0.1 nanometers. Experiments were performed with a Neodymium-doped Yttrium Aluminum Garnet (Nd:YAG) laser with a pulse width of 25 picoseconds (1 picosecond = 10 −12 seconds). A parametric study was performed which measured the surface damage and ablation depth in nickel as a function of incident laser fluence. This study found that the threshold for removal of a significant amount of surface material was 2.0 J/cm2. Below this threshold, surface damage was visible in the form of surface roughness with no material removal. Above this threshold, material was removed at approximately 10 nanometers per laser pulse, much higher than that predicted by the numerical model. It was concluded that the most likely cause of this discrepancy was the uncertainty in the physical properties used in the numerical model and also that the numerical model did not include the effects of homogeneous nucleation. Theoretical considerations showed that as the superheated melt approached the critical temperature, homogeneous nucleation will likely result and expel part of the superheated molten surface. A time-resolved experiment was developed for photographing the transient surface topography during and after laser pulse. This experiment allowed the surface to be imaged with a second visible probe laser beam, with picosecond time delay resolution between the beginning of ablating laser pulse and the probe pulse. Photographs showed strong attenuation of the probe beam pulse as long as 800 picoseconds after the ablating laser pulse. However it was not clear if phase explosion participated during the time scale investigated