Dynamics of ultrashort pulse-laser ablation: Equation-of-state considerations

Ultrafast time resolved microscopy of femtosecond laser irradiated surfaces reveals a universal feature of the ablating surface on nanosecond time scale. All investigated materials show rings in the ablation zone, which were identified as an interference pattern (Newton fringes). We present a theoretical analysis taking into account the unique characteristics of short pulse excitation and semiempirical equation-of-state data. Before ablation starts the irradiated material can be described as a hot, pressurized fluid at solid density, since energy deposition and melting occur much faster than expansion. Considering the sharp drop of the sound velocity in the liquid-gas coexistence regime, the solution of the hydrodynamic equations yields a density profile with an extremely steep ablation front. Subsequently the ablation plume develops a bubble-like structure: a low density two-phase region is delimited by the substrate and a thin moving shell of expanded liquid material. We conclude, that the observed interference results from the superposition of light reflected at the shell and at the non-ablating substrate. In agreement with the observed disappearance of the Newton-rings at higher fluences the calculated density profiles start to smooth out for increasing excitation strength. Our model predicts the ratio between the two fluence limits approximately equal to the ratio of critical temperature to boiling temperature at normal pressure. This is experimentally confirmed on different materials.