Computer modeling of ultrashort pulsed laser ablation of diamond and graphite with experimental verification

Abstract

Ultrashort pulsed lasers create a fundamentally different ablation mechanism than that of conventional pulsed lasers because of the ultrashort laser pulse's extreme intensity (>10^W/cm^ and time duration ( 10's). Consequently, a thermally excited plasma is generated in a cool lattice. Assumptions used in conventional pulsed laser ablation, such as the assumption that absorption processes are governed by the Beer-Lambert Law and that thermal equilibrium of the electrons and lattice is achieved, are invalid for ultrashort pulses. In this work, computer modeling of ultrashort pulsed ablation was performed for large band-gap insulating solids such as diamond. A two-step model was developed in which a heat transfer, finite-difference model was formulated and tightbinding molecular dynamics simulations were performed to evaluate the dynamics of ablation events. The heat transfer model incorporated intensit\^ dependent absorption of the laser light by the electrons and predicted the thermal profiles within the electrons from the start of a laser pulse to 1 ps. The tight-binding molecular dynamics predicted the threshold electron temperatures (room temperature lattice) and overall equilibrium temperature (electron and lattice at the same temperature) required for changes in structure and ablation to occur in the material. The results of both simulations were then used to predict ablation threshold, ablation volume, and the size of the heat-affected zone within the material. Ultrashort pulsed laser ablation experiments were performed on chemical vapor deposited and on single crystal diamonds, as well as on highly-oriented pyroKndc graphite, in order to verify the model predictions. Scanning electron microscopy, atomic force microscopy, profilometry, and micro-Raman spectroscopy were employed to characterize the ablated surfaces.