A Numerical Study Of The Solid-liquid Phase Change In Copper Nanofilms Induced By Ultrafast Lasers

Ultrashort lasers have been widely used in micro/nano-systems, e.g., thin-film micromachining, micro-processing of materials, plasma deposition of thin films, etc. Three two-temperature models, e.g., parabolic (PTTM), hyperbolic (HTTM) and dual-hyperbolic (DHTTM), are widely employed or modified to simulate the non-equilibrium thermal transports and phase changes of the irradiated nano-films. There are two distinctly different kinds of methods to solve heat conduction equations in the above models. Finite Difference Method, Finite Element Method, and the Material Point Method are continuum-based methods while hybrid method coupling the two-temperature model and molecular dynamic (PTTM-MD) is atomic-based. The precisions of the thermo-physical and optical properties used in the models are key factors that governs the solution accuracies of the simulations. The thermo-physical properties control the thermal transports and temperature distributions in the heated films, while the optical properties (reflectivity and absorption coefficient) dictate laser energy depositions that influence the thermal responses.In this work, the Implicit Difference Method has been used to solve electron and lattice temperature equations in the PTTM. The Explicit Difference Method has been used to solve the electron temperature equations, and the Verlet-velocity algorithm has been used to solve the lattice temperature equations in the PTTM-MD. The PTTM-MD and PTTM have been used to investigate the interactions between lasers and nano-films. In the simulations, copper films (h=200nm) were irradiated, respectively, by kinds of lasers (tp= 200 fs,1 ps,5ps,10ps, Jo=30007/m2,5500J/m2). To distinguish the differences between the two methods, the numerical results have been compared and discussed. The thermal responses of the copper films induced by different ultrashort lasers have been calculated, considering constant optical property and dynamic optical property, respectively. The method used in the simulations is the PTTM-MD. Based on the analysis of the numerical results, the feasibility of the constant optical property has been explored.The results show that, the surface electron temperatures calculated by the PTTM-MD are slightly higher than that by the PTTM. The surface lattice temperatures calculated by the PTTM-MD are obviously higher than that by the PTTM when the films irradiated by high fluence lasers, while they are almost the same when irradiated by low fluence lasers. The PTTM could considerably underestimate the melting depths than the PTTM-MD, due to the absence of the lattice deformation and pressure relaxation, and homogeneous melting process. It can be known from the numerical results that thermal-mechanical coupling is not the factor resulting in the underestimation of the melting depths. In order to calculate efficiently, it is suggested to replace dynamic reflectivity by the average one to calculate the film surface electron and lattice temperatures, but it would slightly overestimate the melting depth.