Abstract
Short pulse laser irradiation can create conditions of strong electronic, thermal, phase, and mechanical non-equilibrium in the irradiated targets and trigger a cascade of processes leading to the modification of surface morphologies and formation of unique microstructures of interest to various practical applications. Materials processing techniques based on the laser-induced phase and structural transformations have been widely used and continuously developed for tailoring materials structures to meet desired properties, with the advantages of low manufacturing cost and high accuracy of laser-modified microstructures. Further advancement of the laser processing applications, however, is limited by incomplete fundamental understanding of the highly non-equilibrium processes driven by rapid laser energy deposition, which include fast melting and solidification, generation of crystal defects, collective materials ejection (ablation), and expansion of multi-phase ablation plume. In this dissertation, a combination of atomistic and continuum-level computational methods is used to reveal the mechanisms of the laser-induced modification of surface morphology and microstructure in Si and metal targets under a wide range of irradiation conditions.
The processes of laser-induced melting, resolidification, and generation of crystal defects in a Si wafer are investigated with a continuum-level model parametrized based on the results of atomistic simulations. The propagation of the solidification front under conditions of strong undercooling is found to produce high densities of vacancies and interstitials. The effects of laser parameters on the concentration of point defects in the transiently melted region, and the mitigation of point defects by means of following low power laser annealing, are examined and related to experimental observations. At a sufficiently high level of undercooling, the solidification velocity drops to zero, leading to the transition of the remaining molten region into amorphous phase. The irradiation regimes corresponding to different surface microstructure, with amorphous region formed either at the center or at the periphery of the laser spot, are established for a broad range of laser fluence, pulse duration, and spot size.
The laser-induced generation of crystal defects is also investigated for Ni and Ni-based binary single phase solid-solution alloys through atomistic simulations. The decrease in the thermal conductivity and strengthening of the electron-phonon coupling due to the intrinsic chemical disorder in the Ni-based alloys are found to have important implications on localization of the energy deposition and generation of thermoelastic stresses. The interaction of the laser-induced stress waves with the melting front is demonstrated to play a key role in the formation of dislocations during crystal growth. While the generation of high vacancy concentrations is observed in all irradiated targets, it is found to be partially suppressed in the alloy targets due to a combined effect of reduced solidification velocity and increased vacancy mobility. A detailed analysis of the mechanisms of vacancy generation at a rapidly advancing solidification front is carried out for Ni, Cr, and Ni-based alloys using molecular dynamics modeling of solidification at fixed levels of undercooling. The dependence of the vacancy concentration on material properties, crystallographic orientation of the solidification front, temperature and pressure are discussed based on the simulation results.
The formation of microbumps and nanojets on films composed of single and double Cu/Ag layers deposited on a glass substrate and irradiated by 60 fs laser pulses are investigated in atomistic simulations. Intra-layer cavitation and photomechanical spallation driven by the relaxation of the laser-induced stresses, as well as an explosive release of vapor at the interface with the substrate, are found to be responsible for the formation of frozen microbumps and nanojets observed in experiments.
Finally, the femtosecond laser ablation of a bulk Ag target in the regime of phase explosion is investigated. The processes leading to the explosive material decomposition are revealed and related to the characteristics of the ablation plume, including the spatial segregation of clusters and nanoparticles of different size in the plume. The dynamics of the nucleation, growth, and coalescence of subsurface voids, leading to the formation of a transient interconnected liquid structure, is used for explaining the origins of complex surface morphology featuring multiple frozen nanospikes of various shapes, commonly observed on laser-processed surfaces.
