Thermodynamic pathways to melting, ablation, and solidification in absorbing solids under pulsed laser irradiation

Abstract

The thermodynamic pathways involved in laser irradiation of absorbing solids are investigated in silicon for pulse durations of $500\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$ and $100\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$. This is achieved by accounting for carrier and atom dynamics within a combined Monte Carlo and molecular-dynamics scheme and simultaneously tracking the time evolution of the irradiated material in $\ensuremath{\rho}\text{\ensuremath{-}}T\text{\ensuremath{-}}P$ space. Our simulations reveal thermal changes in long-range order and state of aggregation driven, in most cases, by nonequilibrium states of rapidly heated or promptly cooled matter. Under femtosecond irradiation near the ablation threshold, the system is originally pulled to a near-critical state following rapid $(\ensuremath{\lesssim}{10}^{\ensuremath{-}12}\phantom{\rule{0.3em}{0ex}}\mathrm{s})$ disordering of the mechanically unstable crystal and isochoric heating of the resulting metallic liquid. The latter is then adiabatically cooled to the liquid-vapor regime where phase explosion of the subcritical, superheated melt is initiated by a direct conversion of translational, mechanical energy into surface energy on a $\ensuremath{\sim}{10}^{\ensuremath{-}12}--{10}^{\ensuremath{-}11}\phantom{\rule{0.3em}{0ex}}\mathrm{s}$ time scale. At higher fluences, matter removal involves, instead, the fragmentation of an initially homogeneous fluid subjected to large strain rates upon rapid, supercritical expansion in vacuum. Under picosecond irradiation, homogeneous and, at later times, heterogeneous melting of the superheated solid are followed by nonisochoric heating of the molten metal. In this case, the subcritical liquid material is subsequently cooled onto the binodal by thermal conduction and explosive boiling does not take place; as a result, ablation is associated with a ``trivial'' fragmentation process, i.e., the relatively slow expansion and dissociation into liquid droplets of supercritical matter near thermodynamic equilibrium. This implies a liquid-vapor equilibration time of $\ensuremath{\sim}{10}^{\ensuremath{-}11}--{10}^{\ensuremath{-}10}\phantom{\rule{0.3em}{0ex}}\mathrm{s}$ and heating along the binodal under nanosecond irradiation. Solidification of the nonablated, supercooled molten material is eventually observed on a $\ensuremath{\sim}{10}^{\ensuremath{-}11}--{10}^{\ensuremath{-}9}\phantom{\rule{0.3em}{0ex}}\mathrm{s}$ time scale, irrespective of the pulse duration.