Ultrafast laser ablation of copper with GHz bursts: A numerical study on the effects of repetition rate, pulse number and burst fluence on ablation efficiency and heat-affected zone

Abstract

Ultrafast laser burst ablation (ULBA) with high repetition rates up to GHz has emerged as a promising method for improving ablation efficiency and reducing heat-affected zone (HAZ). However, the effects of different laser parameters are not well understood. This article presents a numerical study by an improved two-temperature model on the influences of repetition rate, pulse number and burst fluence on ablation efficiency and HAZ in ULBA of copper. Due to thermal accumulation, increasing the repetition rate enhances the maximum ablation efficiency, by over 150% at 3456 MHz. Interestingly, large pulse numbers also assist this effect by extending the period of thermal accumulation. However, if the burst fluence is too low, increased pulse numbers will reduce the ablation efficiency due to insufficient pulse fluences. Burst duration and fluence are determined to have the greatest impact on HAZ thickness. To minimize HAZ, the burst duration needs to be short, by raising repetition rates or reducing pulse numbers, to decrease the thermal diffusion time. The dependences of ablation optimum and threshold fluences on pulse numbers and repetition rates are analyzed and summarized in simple formulae. This study unveils the impacts of laser parameters in ULBA and provides guidelines for ablation efficiency and HAZ optimization.