Maxim V. Shugaev

Atomistic modeling of nanoparticle generation in short pulse laser ablation of thin metal films in water.

Laser ablation in liquids is actively used for generation of clean colloidal nanoparticles with unique shapes and functionalities. The fundamental mechanisms of the laser ablation in liquids and the key processes that control the nanoparticle structure, composition, and size distribution, however, are not yet fully understood. In this paper, we report the results of first atomistic simulations of laser ablation of metal targets in liquid environment. A model combining a coarse-grained representation of the liquid environment (parameterized for water), a fully atomistic description of laser interactions with metal targets, and acoustic impedance matching boundary conditions is developed and applied for simulation of laser ablation of a thin silver film deposited on a silica substrate. The simulations, performed at two laser fluences in the regime of phase explosion, predict a rapid deceleration of the ejected ablation plume and the formation of a dense superheated molten layer at the water-plume interface. The water in contact with the hot metal layer is brought to the supercritical state and transforms into an expanding low density metal-water mixing region that serves as a precursor for the formation of a cavitation bubble. Two distinct mechanisms of the nanoparticle formation are predicted in the simulations: (1) the nucleation and growth of small (mostly ⩽10nm) nanoparticles in the metal-water mixing region and (2) the formation of larger (tens of nm) nanoparticles through the breakup of the superheated molten metal layer triggered by the emergence of complex morphological features attributed to the Rayleigh-Taylor instability of the interface between at the superheated metal layer and the supercritical water. The first mechanism is facilitated by the rapid cooling of the growing nanoparticles in the supercritical water environment, resulting in solidification of the nanoparticles located in the upper part of the mixing region on the timescale of nanoseconds. The computational prediction of the two mechanisms of nanoparticle formation yielding nanoparticles with different characteristic sizes is consistent with experimental observations of two distinct nanoparticle populations appearing at different stages of the ablation process.

Growth Twinning and Generation of High-Frequency Surface Nanostructures in Ultrafast Laser-Induced Transient Melting and Resolidification

The structural changes generated in surface regions of single crystal Ni targets by femtosecond laser irradiation are investigated experimentally and computationally for laser fluences that, in the multipulse irradiation regime, produce sub-100 nm high spatial frequency surface structures. Detailed experimental characterization of the irradiated targets combining electron back scattered diffraction analysis with high-resolution transmission electron microscopy reveals the presence of multiple nanoscale twinned domains in the irradiated surface regions of single crystal targets with (111) surface orientation. Atomistic- and continuum-level simulations performed for experimental irradiation conditions reproduce the generation of twinned domains and establish the conditions leading to the formation of growth twin boundaries in the course of the fast transient melting and epitaxial regrowth of the surface regions of the irradiated targets. The observation of growth twins in the irradiated Ni(111) targets provides strong evidence of the role of surface melting and resolidification in the formation of high spatial frequency surface structures. This also suggests that the formation of twinned domains can be used as a sensitive measure of the levels of liquid undercooling achieved in short pulse laser processing of metals.

Atomistic simulation study of short pulse laser interactions with a metal target under conditions of spatial confinement by a transparent overlayer

The distinct characteristics of short pulse laser interactions with a metal target under conditions of spatial confinement by a solid transparent overlayer are investigated in a series of atomistic simulations. The simulations are performed with a computational model combining classical molecular dynamics (MD) technique with a continuum description of the laser excitation, electron-phonon equilibration, and electronic heat transfer based on two-temperature model (TTM). Two methods for incorporation of the description of a transparent overlayer into the TTM-MD model are designed and parameterized for Ag-silica system. The material response to the laser energy deposition is studied for a range of laser fluences that, in the absence of the transparent overlayer, covers the regimes of melting and resolidification, photomechanical spallation, and phase explosion of the overheated surface region. In contrast to the irradiation in vacuum, the spatial confinement by the overlayer facilitates generation of sustain...

Generation of Subsurface Voids, Incubation Effect, and Formation of Nanoparticles in Short Pulse Laser Interactions with Bulk Metal Targets in Liquid: Molecular Dynamics Study

The ability of short pulse laser ablation in liquids to produce clean colloidal nanoparticles and unusual surface morphology has been employed in a broad range of practical applications. In this paper, we report the results of large-scale molecular dynamics simulations aimed at revealing the key processes that control the surface morphology and nanoparticle size distributions by pulsed laser ablation in liquids. The simulations of bulk Ag targets irradiated in water are performed with an advanced computational model combining a coarse-grained representation of liquid environment and an atomistic description of laser interaction with metal targets. For the irradiation conditions that correspond to the spallation regime in vacuum, the simulations predict that the water environment can prevent the complete separation of the spalled layer from the target, leading to the formation of large subsurface voids stabilized by rapid cooling and solidification. The subsequent irradiation of the laser-modified surface is found to result in a more efficient ablation and nanoparticle generation, thus suggesting the possibility of the incubation effect in multipulse laser ablation in liquids. The simulations performed at higher laser fluences that correspond to the phase explosion regime in vacuum reveal the accumulation of the ablation plume at the interface with the water environment and the formation of a hot metal layer. The water in contact with the metal layer is brought to the supercritical state and provides an environment suitable for nucleation and growth of small metal nanoparticles from metal atoms emitted from the hot metal layer. The metal layer itself has limited stability and can readily disintegrate into large (tens of nanometers) nanoparticles. The layer disintegration is facilitated by the Rayleigh–Taylor instability of the interface between the higher density metal layer decelerated by the pressure from the lighter supercritical water. The nanoparticles emerging from the layer disintegration are rapidly cooled and solidified due to the interaction with water environment, with a cooling rate of ∼2 × 1012 K/s observed in the simulations. The computational prediction of two distinct mechanisms of nanoparticle formation yielding nanoparticles with different characteristic sizes provides a plausible explanation for the experimental observations of bimodal nanoparticle size distributions in laser ablation in liquids. The ultrahigh cooling and solidification rates suggest the possibility for generation of nanoparticles featuring metastable phases and highly nonequilibrium structures.

Laser-Rewriteable Ferromagnetism at Thin-Film Surfaces

Manipulation of magnetism using laser light is considered as a key to the advancement of data storage technologies. Until now, most approaches seek to optically switch the direction of magnetization rather than to reversibly manipulate the ferromagnetism itself. Here, we use ∼100 fs laser pulses to reversibly switch ferromagnetic ordering on and off by exploiting a chemical order-disorder phase transition in Fe60Al40, from the B2 to the A2 structure and vice versa. A single laser pulse above a threshold fluence causes nonferromagnetic B2 Fe60Al40 to disorder and form the ferromagnetic A2 structure. Subsequent laser pulsing below the threshold reverses the surface to B2 Fe60Al40, erasing the laser-induced ferromagnetism. Simulations reveal that the order-disorder transition is regulated by the extent of surface supercooling; above the threshold for complete melting throughout the film thickness, the liquid phase can be deeply undercooled before solidification. As a result, the vacancy diffusion in the resolidified region is limited and the region is trapped in the metastable chemically disordered state. Laser pulsing below the threshold forms a limited supercooled surface region that solidifies at sufficiently high temperatures, enabling diffusion-assisted reordering. This demonstrates that ultrafast lasers can achieve subtle atomic rearrangements in bimetallic alloys in a reversible and nonvolatile fashion.

Insights into Laser-Materials Interaction Through Modeling on Atomic and Macroscopic Scales

Computer simulations and theoretical analysis of laser-materials interactions are playing an increasingly important role in the advancement of modern laser technologies and broadening the range of laser applications. In this chapter, we first provide an overview of the current understanding of the laser coupling and transient variation of optical properties in metals, semiconductors and dielectrics, with the focus on the practical implications on the energy deposition and distribution in the irradiated targets. The continuum-level modeling of the dynamic evolution of laser-induced stresses, nonequilibrium phase transformations, and material redistribution within the laser spot are then discussed, and the need for the physical insights into the mechanisms and kinetics of highly nonequilibrium processes triggered by the laser excitation is highlighted. The physical insights can be provided by atomistic modeling, and several examples are discussed where large-scale molecular dynamics simulations are used for investigation of the mechanisms of the generation of crystal defects (vacancies, interstitials, dislocations, and twin boundaries) and the material redistribution responsible for the formation of laser-induced periodic surface structures in the single-pulse ablative regime. The need for the integrated computational approach fully accounting for the strong coupling between processes occurring at different time- and length-scales is highlighted.

Highly-regular laser-induced periodic surface structures: Experiment and atomistic modelling

Surface nano- and microstructuring with additive and subtractive lithography techniques are commonly used in nanotechnology and photonics, while nanoimprint lithography and nanosphere lithography are used for creating periodic structures for plasmonics applications. Single step, maskless optical patterning techniques also exist, notably multi-beam interference based techniques and Laser Induced Periodic Surface Structures (LIPSS). LIPSS have been demonstrated on metals, semiconductors, dielectric surfaces, polymers, and have been used in various applications including solar cells, plasmonics, colorizing metals, wettability and tribology applications. Until recently these optical techniques lacked the long-range periodicity required for some of the applications. Moreover, conventional optical lithography techniques require complex masks, while e-beam lithography and ion-beam lithography require expensive equipment. Recently, a method for direct writing of highly-regular LIPSS (HR-LIPSS) was developed for creating uniform approach overcoming all aforementioned lithography problems [1].