Several distinct mechanisms of femtosecond laser ablation of thin Ag films from a silica substrate are established in large-scale atomistic simulations and are mapped to the space of film thickness and absorbed fluence. For a fixed film thickness, the increase in fluence results in sequential transitions from melting with no ejection of the film, to film splitting or spallation, to an explosive decomposition of the top part of the film and generation of a residual layer in the lower part of the ablation plume, and to a complete phase decomposition of the film into small droplets and vapor. To facilitate the experimental validation of the computational predictions, the variation of the scattering and reflectivity of the ablation plume is calculated from atomic configurations predicted in the simulations and related to the results of pump-probe optical imaging of the ablation plume.
The mechanisms of picosecond laser fragmentation of gold nanoparticles in water are investigated in a closely integrated atomistic simulations and time-resolved X-ray probing. The results of this joint computational and experimental study reveals a sequence of nonequilibrium processes triggered by the laser irradiation, from heating, melting, and resolidification of nanoparticles proceeding under conditions of strong superheating and undercooling, to evaporation of Au atoms followed by condensation into atomic clusters and small satellite nanoparticles, and to the regime of rapid (explosive) phase decomposition of superheated nanoparticles into small liquid droplets and vapor phase atoms. The transition to the phase explosion fragmentation regime is signified by prominent changes in the small-angle X-ray scattering profiles measured in experiments and calculated in simulations. A good match between the experimental and computational diffraction profiles gives credence to the physical picture of the cascade of thermal fragmentation regimes revealed in the simulations.
The ability of short pulse laser ablation in liquids to produce clean colloidal nanoparticles and unusual surface morphology and microstructure has been employed in a range of practical applications. The results of large-scale molecular dynamics simulations help to uncover the key processes that control the structure of laser-modified surfaces and nanoparticle size distributions generated by pulsed laser ablation in liquids. The simulations are performed for one-component metals, bilayer thin films and alloy targets. The predictions of the simulations include the emergence of Rayleigh–Taylor hydrodynamic instability at the interface between ablation plume and liquid environment, as well as the limited elemental mixing in the colloidal nanoparticles generated by laser ablation of bilayer films.
Ultrashort pulse laser induced processes in the nanotechnology at interfaces are presented. A special focus is placed on femtosecond far field investigations of defect generation in solids [1-4] and on electrochemical in-situ techniques in graphene nanosheet synthesis [5]. Further, deterministic nanostructuring of solids and hot electron electrochemistry is discussed [6]. Apertureless scanning near-field nanolithography with a femtosecond Yb-doped fiber laser oscillator allows non-thermal electromagnetic energy transfer [7].
[1] W. Kautek and O. Armbruster, Springer Series in Materials Science 191 (2014) 43-66.
[2] O. Armbruster, A. Naghilou, M. Kitzler, W. Kautek, J. Phys. Chem. C 119 (2015) 22992−22998.
[3] O. Armbruster, A. Naghilou, M. Kitzler, W. Kautek, Appl. Surf. Sci. 396 (2017) 1736–1740.
[4] A. Naghilou, O. Armbruster, W. Kautek, Appl. Surf. Sci. 418 (2017) 487-490.
[5] M. Pfaffeneder-Kmen, I. Falcon Casas, A. Naghilou, G. Trettenhahn, W. Kautek, Electrochim. Acta 255 (2017) 160-167.
[6] O. Armbruster, H. Pöhl, W. Kautek, (2018), in publication.
[7] I. Falcón Casas, W. Kautek, Nanomaterials 8 (2018) 536
Coarse-grained molecular dynamics simulations are performed to investigate the origins of the surface features
observed in films deposited by the Matrix-Assisted Pulsed Laser Evaporation (MAPLE) technique. The simulations of
MAPLE are performed for polymer concentrations up to 6 wt.% and a broad range of laser fluences. The polymer
molecules are found to be ejected only in the ablation regime and are always incorporated into polymer-matrix
clusters/droplets generated in the process of the explosive disintegration of the overheated matrix. The entanglement of
the polymer molecules facilitates the formation of intricate elongated viscous droplets that can be related to the complex
morphologies observed in polymer films deposited by MAPLE. The effect of dynamic molecular redistribution in the
ejected matrix-polymer droplets, leading to the generation of transient "molecular balloons" in which polymer-rich
surface layers enclose the volatile matrix material, has been identified as the mechanism responsible for the formation
of characteristic wrinkled polymer structures observed experimentally in films deposited by MAPLE.
The main objective of this study is to explain the experimental observations. To simulate material ablation, plume
formation and its evolution, we developed a combined molecular dynamics (MD) and direct simulation Monte Carlo
(DSMC) computational study of laser ablation plume evolution. The first process of the material ablation is described by
the MD method. The expansion of the ejected plume is modelled by the DSMC method. To better understand the
formation and the evolution of nanoparticles present in the plume, we first used separate MD simulations to analyse the
evolution of a cluster in the presence of background gas with different properties (density, temperature). In particular, we
examine evaporation and growth reactions of a cluster with different size and initial temperature. As a result of MD
calculations, we determinate the influence of the background gas parameters on the nanoparticles. The reactions rates
such as evaporation/condensation, which are obtained by MD simulations, are directly transferred to the DSMC part of
our combined model. Finally, several calculations performed by using MD-DSMC model demonstrate both plume
dynamics and longer-time cluster evolution. Calculations results are compared with experimental findings.
The temperature dependences of the electron heat capacity and the electron-phonon coupling factor are
investigated for Au based on the electron density of states obtained from ab initio electronic structure calculations.
Thermal excitation of d band electrons leads to a significant (up to an order of magnitude) increase in the electronphonon
coupling factor and makes a considerable contribution to the electron heat capacity in the range of electron
temperatures typically realized in femtosecond laser material processing applications. Simulations performed with a
combined atomistic-continuum method demonstrate that the increase in the strength of the electron-phonon coupling at
high electron temperatures leads to a faster lattice heating, generation of stronger thermoelastic stresses, and a
significant decrease in the time of the onset of the melting process. The timescale of the melting process predicted in
the simulation accounting for the thermal excitation of d band electrons is in excellent agreement with the results of
recent time-resolved electron diffraction experiments. A simulation performed with commonly used approximations
of a constant electron-phonon coupling factor and a linear temperature dependence of the electron heat capacity, on the
other hand, significantly overpredicts the time of the beginning of the melting process, supporting the importance of the
electron density of states effects and thermal excitation of lower band electrons for realistic modeling of femtosecond
pulse laser processing.
The mechanisms of melting and photomechanical damage/spallation occurring under extreme superheating/deformation rate conditions realized in short pulse laser processing are investigated in a computational study performed with a hybrid atomistic-continuum model. The model combines classical molecular dynamics method for simulation of non-equilibrium processes of lattice superheating and fast phase transformations with a continuum description of the laser excitation and subsequent relaxation of the conduction band electrons. The kinetics and microscopic mechanisms of melting are investigated in simulations of laser interaction with free-standing Ni films and bulk targets. A significant reduction of the overheating required for the initiation of homogeneous melting is observed and attributed to the relaxation of laser-induced stresses, which leads to the uniaxial expansion and associated anisotropic lattice distortions. The evolution of photomechanical damage is investigated in a large-scale simulation of laser spallation of a 100 nm Ni film. The evolution of photomechanical damage is observed to take place in two stages, the initial stage of void nucleation and growth, when both the number of voids and the range of void sizes are increasing, followed by the void coarsening, coalescence and percolation, when large voids grow at the expense of the decreasing population of small voids. In both regimes the size distributions of voids are found to be well described by the power law with an exponent gradually increasing with time. A good agreement of the results obtained for the evolution of photomechanical damage in a metal film with earlier results reported for laser spallation of molecular systems and shock-induced back spallation in metals suggests that the observed processes of void nucleation, growth and coalescence may reflect general characteristics of the dynamic fracture at high deformation rates.
Microscopic mechanisms of short pulse laser damage to melanin granules, the strongest absorbing chromophores of visible and near -- IR light in the retina and skin, are studied using the molecular dynamics simulations. The pulse width dependence of the fracture/cavitation and vaporization processes within the small particles, their coupling to the surrounding medium and the resulting tissue injury are discussed based on the simulation results. The effect of laser irradiation on an isolated submicron particle at different laser fluences and pulse durations is first analyzed. The mechanical disruption of the particle due to the laser induced pressure is found to define the character of damage for short pulse widths (tens of picoseconds) at laser fluences that ar significantly lower than those required for boiling. Thermal relaxation and explosive disintegration of the overheated particle at higher laser fluencies are the processes that dominate at longer laser pulses (hundreds of picoseconds). Damage of an absorbing particle embedded into a transparent medium with different mechanical characteristics is then simulated. Coupling of the acoustic and thermal pulses generated within absorbing particles to the surrounding medium is studied and the possible cumulative effects from an ensemble of absorbing particles are discussed. The simulation results provide the basis for future work in which the microscopic and continuum descriptions are combined for multiscale modeling of laser tissue interaction.
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