Thermodynamics of absorbing solids during short-pulse laser ablation

摘要

The fundamental mechanisms of matter removal involved in the interaction of short laser pulses with absorbing solids have been investigated using molecular-dynamics/Monte Carlo simulations. This is accomplished under the two following assumptions: (?) the elementary thermodynamic properties of targets (metals and semiconductors) are adequately described by empirical potentials; (ⅱ) in the regime where ablation is thermal, the complete time evolution of the system can be followed in ρ-T-P space and the result mapped onto the equilibrium phase diagram of the material. We find remarkable similarities in the physical pathways to ablation in metals and semiconductors for pulse durations ranging from 200 fs to 400 ps: (ⅰ) under conditions of isochoric heating and rapid adiabatic cooling with femtosecond pulses, several mechanisms can simultaneously account for matter removal in the target: spallation, phase explosion, vaporization, and fragmentation; the latter is identified for the first time in the context of laser ablation, (ⅱ) Under nonadiabatic cooling with picosecond pulses, ablation is driven by a "trivial" fragmentation process in the metallic, supercritical fluid; this suggests a pulse duration upper limit for phase explosion of ～ 10~(11) s.