Model and simulation of laser pulse absorption in laser-induced plasma micro-machining (LIPMM)

Direct laser ablation removes material by directly focusing the laser on the material surface so its performance highly depends on the surface condition. Laser-Induced Plasma Micro-Machining (LIPMM) removes the material by focusing an ultrashort laser pulse in an auxiliary dielectric layer above the material and inducing dielectric breakdown to achieve more accurate energy deposition. The physical process of LIPMM can be divided into two stages: pulse absorption and material removal. This work focuses on modeling the pulse absorption stage by implementing the Finite Difference Time Domain (FDTD) method to provide a unified simulation framework for the ultrashort laser-matter interaction. The simulation model can be divided into three steps: 1) the evolution of the laser field, which is described by FDTD with current and a tightly focused beam with Gaussian entry utilized to approximate the unperturbed laser field near focus; 2) the metal absorption of a ultrashort laser pulse, where a two-temperature model is used and the current is described by the Drude model with mean free time approximated by a three stage model and 3) the water breakdown, which is described by Kennedy's first order model. Combining the above approaches, the unified framework demonstrates its utility on the example of the water-aluminum boundary in the LIPMM process.