Molecular-dynamics study of ablation of solids under femtosecond laser pulses

The ablation of solids under femtosecond laser pulses is studied using a two-dimensional molecular-dynamics model. The simulations show that different expansion regimes develop as a function of the injected energy. The origin of these regimes lies in changes of the thermodynamical relaxation path the material follows when the intensity of the laser increases. The shape of the pressure waves generated as a result of the absorption of the pulse is shown to vary from bipolar at low fluence to unipolar at high fluence, as a result of the decrease of the tensile strength of the material with temperature. By combining these results with an analysis of the thermodynamical trajectories for different portions of the target, we show that four different mechanisms can account for ablation at fluences below the threshold for plasma formation, namely spallation, phase explosion, fragmentation, and vaporization. These mechanisms are characterized in detail; it is demonstrated that they can occur simultaneously in different parts of the target.