Phase explosion under ultrashort pulsed laser ablation: modeling with analysis of metastable state of melt

We present a model of femtosecond laser ablation for a metal target, including three sets of equations for different time scales and ablation conditions: a two-temperature model of laser radiation absorption by conduction electrons and electron–lattice relaxation; the thermal conductivity equation to describe the subsequent temperature evolution; the Euler equations supplemented with generalized van der Waals equation to describe vapor expansion. The solid–liquid interface is followed in the calculations. A metastable state of the molten layer has been shown to occur for regimes both below and above the ablation threshold that may not lead to phase explosion. For phase explosion to be put into effect, an external action has to be applied. For laser fluences below the ablation threshold, a probe laser pulse can play the role of the external action, whereas for high fluences the recoil pressure of the expanding vapor causes explosive boiling.