Laser ablation of covalent materials

Abstract

Ultra-fast laser ablation is the process of material removal from solid surfaces by pulsed sub-picosecond laser irradiation. In contrast to longer pulse durations, ultra-fast laser ablation shows the distinguishing feature of the timescale of excitation being below the timescale of consequent material heating. Excited charge carriers distribute the thermal energy over a larger volume than the optical penetration depth suggests, while the lattice remains in a cold state. Spatial energy distribution is followed by a fast carrier-lattice energy relaxation, which induces overheated and meta-stable states of matter. These meta-stable states are induced simultaneously in the laser-affected zone, forcing the material to relax in a variety of mechanisms, ranging from ultra-fast melting over hydrodynamic expansion to material ejection in a complex mixture of chunks, droplets or vapor. While a multitude of publications successfully study the laser irradiation induced material dynamics of metals, we investigate laser ablation of covalent materials. In contrast to metals, covalent materials show a band gap, excitation-dependent carrier heat conduction and strong excitation-dependent interatomic bonding strengths, rendering the theoretical description of such materials a difficult task. However, it also gives rise to a number of unique dynamics like non-thermal melting, Coulomb explosions and altered carrier heat conduction due to charge carrier confinement. In this work we choose silicon as our prototypical covalent material and perform molecular dynamics simulations of laser irradiated silicon, while applying an excitation-dependent interatomic potential. We present new parametrizations of the optical properties, as well as the extension of established charge carrier transport models for silicon, which are both tailored for the application on large scale massive multi-parallel high-performance computers. Finally we observe and characterize the novel and non-thermal ablation mechanics of laser irradiated silicon.