Thermo-elasto-plasto-dynamics of ultrafast optical ablation in polycrystalline metals. Part I: Theoretical formulation

Abstract

Ultrafast laser-induced ablation is a complex function of many coupled parameters including laser intensity, pulse duration, optical wavelength, material thermophysical properties, and grain size. The predominant belief is that ultrafast laser ablation involves either heterogeneous or homogeneous nucleation that results in rapid phase transition or the material been superheated. However, surface morphology and fragmentation ejection by ultrafast heating with low laser fluence along with the fact that the corresponding lattice temperature is lower than the melting temperature suggest that a different removal mechanism other than phase transition and explosion is at work. Hot carrier emissions and their impact on the dissipation of the electron subsystem are also commonly ignored when describing electron and electric transports. A non-thermal ablation mechanism underlying femtosecond laser–polycrystalline material interaction and the induced surface morphology are formulated for low optical intensity using a comprehensive ablation model. The formulation follows the time scales characteristic of each of the underlying dynamics and obeys the principle of energy conservation when establishing the energy transports of the electron and lattice subsystems. The implications for properly describing low-intensity ultrafast ablation include providing the knowledge base essential for mitigating fracture and fragmentation ejection.