Lasers generally use the ablation process to aid material removal. A femtosecond laser, for example, induces various changes in metals such as bond hardening and structural changes. Recently, non-thermal ablation process has attracted significant interests amongst the industrialists and researchers due to its capability of reducing thermal damage in metals. Unfortunately, non-thermal ablation has not been fully explored therefore making it hard to predict through simulations.
Generally, laser irradiation results in a significant change in the electron subsystems of metals which has been effectively investigated by the two-temperature model (TTM). Although various studies have tried to explain the cause of ablation in metals, most of the work has been based merely on theoretical calculations with no supportive experimental results. For instance, in the previously published literature, several studies have suggested the possibility that large repulsive forces between atoms come from the kinetic energy of the delocalized electrons as the cause of ablation. However, the explanations are yet to be fully clarified and proved experimentally.
Recently, University of Tokyo researchers Yuta Tanaka and Shinji Tsuneyuki investigated the physical mechanism of non-thermal ablation of metal-induced by femtosecond laser irradiation. They purposed to determine the main cause of non-thermal ablation and to develop a model to experimentally describe the data, especially in the low-laser-fluence region that experiences significant electronic entropy effect. Their work is currently published in the research journal, Applied Physics Express.
Briefly, the authors commenced their work by investigating the mechanism of non-thermal ablation of copper using the first-principles calculations. The calculations were based on the TTM and finite-temperature density functional theory (FTDFT). The non-thermal ablation process was described based on the newly proposed electronic entropy-driven (EED) mechanism. Furthermore, the ablation depth was simulated using the developed mathematical model. They eventually compared the obtained simulation results and experimental results.
From the FTDFT calculation results, the authors observed that the electronic entropy effect was the main contributor to the instability of the condensed copper at high electron temperature. This was due to the volume-dependence of the electron states near the chemical potential. Furthermore, the EED mechanism and the mathematical model effectively described the non-thermal ablation mechanism in metals with the possibility of predicting the experimental data in the low-laser-fluence region.
Based on the FTDFT results, Yuta Tanaka and Shinji Tsuneyuki successfully explained non-thermal ablation in metals using the proposed EED mechanism. Consequently, the mathematical model that took into consideration the electronic entropy effects described the obtained experimental data successfully in the low-laser-fluence region. This is the region where the effect of the electronic entropy is especially significant. Also, the mutual agreement in the simulation and experimental results confirmed the dominance of the effect of the electronic entropy in non-thermal ablation as compared to other ablation processes. The study presents an in-depth understanding of non-thermal ablation in metals which is a promising solution to most of the industrial ablation processes using laser.
Tanaka, Y., & Tsuneyuki, S. (2018). Possible electronic entropy-driven mechanism for non-thermal ablation of metals. Applied Physics Express, 11(4), 046701.Go To Applied Physics Express