Ablation by ultrashort laser pulses: Atomistic and thermodynamic analysis of the processes at the ablation threshold

Abstract

Ultrafast laser irradiation of solids may ablate material off the surface. We study this process for thin films using molecular-dynamics simulation and thermodynamic analysis. Both metals and Lennard-Jones (LJ) materials are studied. We find that despite the large difference in thermodynamical properties between these two classes of materials—e.g., for aluminum versus LJ the ratio $T_c/T_{\text{tr}}$ of critical to triple-point temperature differs by more than a factor of 4—the values of the ablation threshold energy $E_{\text{abl}}$ normalized to the cohesion energy, ${ϵ}_{\text{abl}}=E_{\text{abl}}/E_{\text{coh}}$, are surprisingly universal: all are near 0.3 with $\pm 30\%$ scattering. The difference in the ratio $T_c/T_{\text{tr}}$ means that for metals the melting threshold ${ϵ}_m$ is low, ${ϵ}_m<{ϵ}_{\text{abl}}$, while for LJ it is high, ${ϵ}_m>{ϵ}_{\text{abl}}$. This thermodynamical consideration gives a simple explanation for the difference between metals and LJ. It explains why despite the universality in ${ϵ}_{\text{abl}}$, metals thermomechanically ablate always from the liquid state. This is opposite to LJ materials, which (near threshold) ablate from the solid state. Furthermore, we find that immediately below the ablation threshold, the formation of large voids (cavitation) in the irradiated material leads to a strong temporary expansion on a very slow time scale. This feature is easily distinguished from the acoustic oscillations governing the material response at smaller intensities, on the one hand, and the ablation occurring at larger intensities, on the other hand. This finding allows us to explain the puzzle of huge surface excursions found in experiments at near-threshold laser irradiation.