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 fs and 100 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-T-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 ͑Շ10 −12 s͒ 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 ϳ10 −12-10 −11 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 ϳ10 −11-10 −10 s and heating along the binodal under nanosecond irradiation. Solidification of the nonablated, supercooled molten material is eventually observed on a ϳ10 −11-10 −9 s time scale, irrespective of the pulse duration.

Droplet evolution in expanding flow of warm dense matter

“…Of course, such procedure cannot account for the complexity of non-equilibrium situations encountered in experiments or simulations [9,11,[17][18][19]30]. The two-phase regime exists only for temperatures T < T c , and for an average density between the value at the liquid and gas binodals, as seen on Fig.…”

Nonthermal graphitization of diamond induced by a femtosecond x-ray laser pulse

“…11,32,33 First, the photoabsorption promotes electrons from the bound states of the valence band or deep atomic shells (K shell for carbon) to the conduction band. This process occurs during the action of the laser pulse.…”

“…35,36 In contrast, the elastic phonon scattering can lead to significant electron energy losses only at much longer, typically picosecond, time scales. 32,37 The exchange of the kinetic energy during interparticle collisions leads to rapid changes of the transient state of the electronic subsystem, which, in turn, induces a change of potential energy. To explain, in covalently bonded materials, the interatomic bonds are strongly dependent on the specific state of the electronic system.…”

Transient dynamics of the electronic subsystem of semiconductors irradiated with an ultrashort vacuum ultraviolet laser pulse

“…The kinetics of the excitation and relaxation of the target can be divided into a set of processes separated temporally. Due to the mass difference between electrons and ions, excitation of the electronic subsystem by a laser pulse and the subsequent creation of second-generation free electrons occur much faster (some femtoseconds, ∼10 −15 s, or the duration of a pulse) [14,21] than other processes, such as energy exchange with the lattice and the cooling of excited electrons, which both take up to ∼10 −11 s [22]- [27]. The processes in the electronic subsystem play a fundamental role because they provide the initial conditions for subsequent energy 3 dissipation.…”