First Principles Simulation of the Dynamics of Transient Warm Dense Matter during the Formation of Ultrashort Laser Pulse induced Damage using the Particle-in-Cell Method

Understanding the warm dense matter (WDM) state is of fundamental importance in the modeling of femtosecond laser damage because laser electron coupling and subsequent electron lattice coupling can rapidly increase the material temperature at the laser focal region to on the order of an eV, producing WDM not well described by standard liquid and solid equations of state. We have developed a simulation approach based on the particle-in-cell (PIC) method capable of modeling the formation of warm dense matter via an ultrashort pulse on a mesoscopic scale by utilizing two temperature interionic potentials. The dynamics are simulated via two sequential stages, the first of which models the femtosecond laser-target interaction directly by solving Maxwell's equations and the Lorentz force law, along with a sophisticated scheme for properly modeling the short range collisionality of particles. The second simultaneously models electron diffusion and electron-ion relaxation via the two temperature model and material ablation using the PIC pair potential model. Our simulation enables us to calculate the temporal and spatial dynamics of particles over the entirety of the laser affected material and to determine a crater profile which can be used to compare to experimental results.