Multiscale Modeling of Short-Pulsed Laser Ablation of Metal Targets

Short pulse laser ablation is a phenomenon that forms the basis for a broad range of material processing and synthesis techniques. The ability of pulsed lasers to confine the energy deposition in time and space can create highly nonequilibrium processing and synthesis conditions that are hardly accessible with other methods. For the synthesis of nanoparticles, the nonequilibrium conditions have implications for the phase composition, defect structures, shapes, and size distributions of nanoparticles. The chemical purity of nanoparticles produced by laser ablation makes them suitable for biomedical applications, while the high density of crystal defects has been shown to significantly enhance catalytic activity in some of the practically relevant reactions. While an improved theoretical understanding of the nanoparticle generation mechanisms can potentially guide the selection and/or optimization of the synthesis processes, the far-from-equilibrium conditions of laser ablation present a significant challenge for theoretical analysis and computational description of this complex phenomenon. The primary goal of the research reported in this dissertation is to design and implement a multiscale computational model suitable for the simulation of nanoparticle formation in laser ablation, to verify the model against time-resolved experimental measurements, and to apply it for investigation of the nanoparticle synthesis mechanisms in pulsed laser ablation in liquids.
To model the laser-material interactions, I developed a state-of-the-art multiscale approach combining the two-temperature model (TTM) to describe the laser excitation and fast electronic heat transport, classical molecular dynamics (MD) to model phase transformations in the metal target, a coarse-grained (CG) MD representation of the liquid environment, non-reflecting boundary (NRB) conditions to simulate pressure wave propagation, and compressible hydrodynamics (HD) for tracking the long-term propagation of shock waves in the liquid environment and providing reliable feedback from the surrounding liquid to the expanding cavitation bubble modeled with CG MD. Additionally, electromagnetic (EM) calculations are used for making connections between the computational predictions and measurements of transient optical properties of the ablation plume in pump-probe experiments.
The model was validated in two collaborative projects involving in-situ probing experiments, demonstrating its accuracy in predicting key characteristics of the ablation dynamics. The first project studied evolution of target reflectance during the initial stage of laser ablation. Experiments utilized a strong pump laser pulse to generate the ablation plume, followed by a weak probe pulse to measure reflectance changes, providing insights into plume dynamics. We established direct quantitative links between the results of large-scale atomistic simulations and measured time-resolved reflection profiles. Key findings include the evaluation of the relative contributions of temperature dependent and temperature independent mechanisms of the electron scattering affecting the optical properties of molten FeNi alloy, as well as the identification of optical signatures of the transitions to the spallation and phase explosion regimes of laser ablation. The emergence of oscillations of reflectance due to the interference of parts of the probe beam reflected from the surface of the target and the spalled layer is observed at the spallation threshold, while the disappearance of the interference pattern at higher fluences is found to signify the transition to the regime of phase explosion. The calculation of complex refractive index of a transient spongy structure generated between the spalled layer and the target suggests that the variation of the refractive index can significantly affect the interference conditions at the initial stage of laser spallation.
The second project probed laser-induced ablation plumes with intense femtosecond X-ray pulses delivered by an X-ray free-electron laser. By comparing experimental wide- and small-angle X-ray scattering patterns measured in a time-resolved manner on thin gold films irradiated by intense femtosecond laser pulses with the results of large-scale atomistic simulations, we linked the characteristics of the phase decomposition dynamics to their corresponding scattering signatures. The processes predicted in the simulations and confirmed experiments include rapid melting of the irradiated films, nanovoid formation and growth, transient appearance of molten filaments, and nanodroplet ejection in a range of laser fluences covering the regimes of photomechanical spallation and phase explosion.
The experimental validation of the multiscale computational model through the comparison of the computational predictions with the results of time-resolved optical and X-ray probing of the ablation dynamics gives us confidence to extend the computational study to a more complex phenomenon of pulsed laser ablation in liquids (PLAL). The mechanisms of PLAL are investigated through a series of large-scale atomistic simulations of FeNi targets irradiated in a liquid environment by picosecond laser pulses across a broad range of fluences. The simulations reveal several distinct mechanisms of nanoparticle formation activated in different fluence regimes: (1) in the low fluence regime, small nanoparticles form through condensation of metal atoms evaporated from the irradiated target; (2) in the medium fluence regime, nanoparticles form via roughening and decomposition of the top part of the ablation plume decelerated by the liquid environment; and (3) in the high fluence regime, nanoparticles form primarily at the phase separation front propagating through the plume cooled from a supercritical state. The results of the simulations are mapped to local conditions realized within a laser spot irradiated by a beam with a Gaussian spatial profile, where different ablation regimes are activated simultaneously in different parts of the laser spot. The spatially and temporally resolved maps of transient nonequilibrium states predicted in the simulations provide a comprehensive picture of the ablation dynamics and a solid foundation for interpretation of the results of time-resolved experimental probing of the initial stage of the ablation process.