Ultrafast melting, spallation, and phase explosion in femtosecond laser processing on nickel film surface investigated by atomistic simulation and transient reflectivity microscopy

Phase change plays a fundamental role in the femtosecond laser micro/nano-processing of metals. Atomistic simulation combined with experimental observation of ultrafast melting, spallation, and phase explosion contributes to the comprehensive understanding of femtosecond laser processing of metals and remains urgent. This work aims to investigate ultrafast melting, spallation, and phase explosion on Ni film by molecular dynamics coupled two-temperature model simulation and pump–probe transient reflectivity microscopy experiment, as well as the quantitative connection between the simulation and experimental methods. Atom temperature and pressure evolutions projected on the phase diagram reveal evolutions from solid to liquid in spallation and from solid to supercritical fluid passing through liquid in phase explosion. Evolutions of atomistic configurations during the phase change are captured in atomistic snapshots and diffraction patterns. Above the ablation threshold, spallation and phase explosion are observed by the pump–probe transient reflectivity microscopy yielding different reflectivity mappings. This difference is explained by the optical response of ejected materials to probe beams. Oscillation of central reflectivity in spallation is induced by optical interference, which is dominated by the discrete ejection in the form of large liquid clusters. These results contribute to a comprehensive understanding and precise control of phase change in femtosecond laser micro/nano-processing of metals.