CHARACTERIZING THE ROLE OF ELECTRON-PHONON INTERACTIONS IN ULTRAFAST LASER ABLATION OF METAL

Abstract

This thesis investigates electron-phonon interactions in ultrafast laser ablation of nickel using hybrid two-temperature model with molecular dynamics (TTM-MD) simulations. The functional definition of subsystem properties and electronic-lattice coupling influences the accuracy of these simulations. The first phase determines the optimal description of thermophysical properties in the electron subsystem by comparing empirical definitions and Density Functional Theory (DFT) based values within the Two-Temperature Model (TTM). Results showed that with simplistic Beer-Lambert’s optical absorption modeling, empirically derived temperature-dependent parameterizations matched previous studies. Subsequently, hybrid TTM-MD ablation simulations were performed using two electron-phonon coupling approaches: temperature difference scaled coupling and Langevin thermostat. Analyses indicated that temperature difference scaled coupling leads to increased tensile stress due to artificial enhancement of the collision cascade, based on the deterministic nature of the coupling force. In contrast, the Langevin thermostat’s probabilistic nature predicts ablation threshold (115 mJ/cm², absorbed fluence) and phase explosion onset (270 ~ 300 mJ/cm²) for 1 picosecond laser pulses more accurately. Building on these insights, the second phase employs a sophisticated TTM-MD framework incorporating Generalized Langevin Dynamics (GLD) for wavevector-dependent coupling and a Helmholtz solver for precise optical absorption modeling. This optical model addresses the inferior performance of DFT-derived electronic properties, revealing that Beer-Lambert's law concentrates energy deposition near the surface, causing inaccurate elevated temperatures. The primary contribution of this research is the first implementation of TTM-MD with GLD coupling for laser ablation modeling, revealing anisotropic effects in ablation characteristics through phonon mode-dependent interactions. This approach predicts an ablation threshold of 300 mJ/cm² (incident fluence), closely aligning with experimental results. The model shows a yield decrease of approximately 20% when the laser propagates along the <110> versus <100> crystallographic direction at near-threshold fluences. These findings enhance understanding of electron-phonon interactions in ultrafast laser processes and highlight the crucial role of crystallographic orientation in ablation outcomes, with potential implications for optimizing laser ablation processes.