Abstract
Rapid progress in the development of accessible sources of short (pico- and femtosecond) laser pulses opens up new opportunities for surface modification with high accuracy and spatial resolution. In particular, the shallow depth of the heat-affected zone in short-pulse laser processing of strongly absorbing materials can result in the confinement of the laser-induced structural modifications within a surface layer as small as tens of nanometers. The small size of the laser-modified region and the highly non-equilibrium nature of the transient processes occurring in the laser-irradiated targets, however, make time-resolved experimental evaluation of laser-induced structural transformations challenging and hinder the interpretation of the experimental results. In this work, we use atomic-level computer simulations to investigate the microscopic mechanisms of the ultrafast structural and phase transformations in the metal targets induced by the short pulse laser irradiation. The simulations are performed with a computational model that combines classical molecular dynamics method with a continuum description of the laser excitation and subsequent relaxation of the conduction band electrons.
Three sets of simulations are performed to address a range of research questions related to short pulse laser processing. In the first set of simulations, we investigate the atomic mixing and structural modification of an interfacial region in a target composed of layers of metals that are immiscible in the solid state. Using the Ag – Cu layered system as an example, we find that the region of atomic mixing generated by the fast melting and resolidification of the interface is substantially wider compared to the width of the equilibrium Cu–Ag interface and have a pronounced asymmetric shape that reflects the preferential melting of the Cu substrate. Moreover, the generation of a “runaway” lattice-mismatched interface shifted into the Cu substrate separated from the Ag–Cu mixing region by an intermediate pseudomorphic BCC Cu layer is observed in the simulations. The detailed structural analysis reveals that the new lattice-mismatched interface has a complex three-dimensional corrugated structure consisting of a periodic array of stacking fault pyramids outlined by stair-rod partial dislocations. The material systems optimized for stabilization of the metastable BCC Cu are explored in several targeted simulations and the experimental configuration for generation of a 5 nm BCC Cu layer sandwiched between two FCC-Ag(001)/BCC-Cu(001) interfaces is proposed based on the simulation results.
In the second set of the simulations, we investigate the processes responsible for an unexpected effect of surface “swelling” observed in recent experiments performed for Al and Ag targets irradiated by 100 fs laser pulses. A series of simulations of short pulse laser irradiation of Ag targets are performed to investigate the mechanisms of void formation and spallation (separation of a melted layer from the bulk of the target) due to the short pulse laser-induced photomechanical processes. We find that at laser fluences just below the spallation threshold, the voids can be captured by fast resolidification. The computational prediction of the generation of porous structure of a surface region of the irradiated target provides an explanation for the surface swelling observed in the experiments. Besides the surface swelling, the large-scale atomistic simulations reveal the generation of nanocrystalline structure of the surface layer of the irradiated target. The formation of the nanocrystalline structure is attributed to the homogeneous nucleation and growth of multiple randomly oriented crystallites under conditions of strong undercooling. The structural analysis on the nanocrystalline structure shows that the nano-grains have close-packed structure, with large number of stacking faults, twin boundaries and complex five-fold twinning structures. The grain boundaries, stacking faults, and twin boundaries are all likely to present strong barriers for dislocation propagation, resulting in the effective hardening of the surface region of the irradiated target.
Besides the direct modification of the surface region, transient thermoelastic stresses generated by short pulse laser irradiation can result in the emission of acoustic waves. The generation of photoacoustic waves has been widely used for material characterization, laser cleaning, and laser desorption. In the final part of this work, we combine theoretical analysis with molecular dynamics and kinetic Monte Carlo simulations to investigate the effect of laser-induced acoustic waves on surface diffusion. The substantial acoustic enhancement of surface diffusion (up to hundreds of percents) observed in the simulations suggests an attractive alternative to thermal activation in thin film growth on heat-sensitive substrates. It is found that, in addition to the diffusion enhancement, the surface self-structuring (spatial modulation of adatom concentration) can be effectively induced by standing surface acoustic waves. The predicted self-structuring phenomenon may open up opportunities for acoustic control of surface self-assembly without permanent modification of the substrate and growth conditions.
