Lasers are known to be extremely versatile tools suitable both for the synthesis and modifications of numerous nanomaterials with unique and extremely interesting optical properties suitable for a wide range of applications in various fields ranging from optics and photonics to medical applications. Efficient control over these processes is still challenging and often requires numerical simulations because of the complex interplay of many physical and chemical processes involved that depend on the combination of both material properties and laser parameters. To simulate these processes, multi-physical modeling should be used including electromagnetic, thermal, mechanical, and chemical effects taking place at several time and space scales. Depending on the experimental conditions, nanoparticles can be formed, grow, aggregate, or on the contrary decay, so that a set of transient variations often take place, particularly when multi-pulse laser irradiation is applied. In the case of short and ultra-short laser pulses, strongly non-linear and time-dependent processes play a role involving not only ionization but also phase transitions, acoustic vibrations, shock waves, as well as void formation, and cavitation. If a considerable energy is released in a very short time, firstly aggregates decay, then nanoparticles are fragmented. Here, based on numerical calculations, the roles of several above-mentioned effects are analyzed. The performed simulations can be used for a better understanding of laser interactions with nanoobjects.
Laser-induced structuring of nanoporous glass composites is promising for numerous emerging applications in photonics, plasmonics and medicine. In these laser interactions, an interplay of photo-thermo-chemical mechanisms is commonly activated and is extremely difficult to control. The choice of optimum laser parameters to tune the resulted optical properties remains extremely challenging. In this paper, we analyze the mechanisms involved and propose a way to control over not only structures formed by laser inside a nanoporous glass composites doped by metallic ions and nanocparticles, but also their plasmonic properties. For this, both experimental and numerical approaches are combined. The transmitted laser power is used to analyze the modification process. Spectral microanalysis provides plasmonic properties. Numerical effective medium modeling connects the measured data to the estimated size, concentration, and chemical composition of the secondary phase across the initial sample.
Generation of periodic arrangements of matter on materials irradiated by laser fields of uniform and isotropic energy distribution is a key issue in controlling laser structuring processes below the diffractive limit. Using three-dimensional finite-difference time-domain methods, we evaluate energy deposition patterns below a material's rough surface [1] and in bulk dielectric materials containing randomly distributed nano-inhomogeneities [2]. We show that both surface and volume patterns can be attributed to spatially ordered electromagnetic solutions of linear and nonlinear Maxwell equations. In particular, simulations revealed that anisotropic energy deposition results from the coherent superposition of the incident and the inhomogeneity-scattered light waves. Transient electronic response is also analyzed by kinetic equations of free electron excitation/relaxation processes for dielectrics and by ab initio calculations for metals. They show that for nonplasmonic metals, ultrafast carrier excitation can drastically affect electronic structures, driving a transient surface plasmonic state with high consequences for optical resonances generation [3]. Comparing condition formations of 2D laser-induced periodic surface structures (LIPSS) and 3D self-organized nanogratings, we will discuss the role of collective scattering of nanoroughness and the feedback-driven growth of the nanostructures.
[1] H. Zhang, J.P. Colombier, C. Li, N. Faure, G. Cheng, and R. Stoian, Physical Review B 92, 174109 (2015).
[2] A. Rudenko, J.P. Colombier, and T.E. Itina, Physical Review B 93 (7), 075427 (2016).
[3] E. Bévillon, J.P. Colombier, V. Recoules, H. Zhang, C. Li and R. Stoian, Physical Review B 93 (16), 165416 (2016).
Key issues of the controlled synthesis of nanoparticles and nanostructures, as well as laser-particle interactions are considered in the context of the latest applications appearing in many fields such as photonics, medicine, 3D printing, etc. The results of a multi-physics numerical study of laser interaction with nanoparticles will be presented in the presence of several environments. In particular, attention will be paid to the numerical study of laser interactions with heterogeneous materials (eg. colloidal liquids and/or nanoparticles in a dielectric medium) and the aggregation/sintering/fragmentation processes induced by ultra-short laser pulses.
Synthesis of transparent conductive coatings is a promising direction of modern nanotechnological research.
Thin nanostructured noble-metallic films demonstrate nonlinear optical effects in visible spectral range because of their
plasmonic properties [1]. In addition, optical characteristics of these thin films strongly depend on the period of the
formed surface structures [2]. If the distance between deposited particles almost equals their sizes, the optical properties
of the randomly deposited structures may considerably differ from these for periodical structures [3].
In this work, we have studied the degree of the morphology influence (particle diameter in the colloid, the
distance between the deposited particles, the number of layers etc.) on the optical and electrical properties of the
deposited thin film of bimetallic gold and silver clusters. In this work we used CW-laser with moderate intensity in liquid
(water or ethanol) for synthesis nanoparticles of noble metals. For the formation of quasi-periodically arranged clusters,
particle deposition from the colloidal systems is used. The optical properties of the deposited bimetallic films are shown
to change as a function of composition and geometry in agreement with the modeling of the optical properties.
Nanoparticles have found numerous applications in such areas as photonics, electronics, medicine, etc. Further development of these fields requires reliable and versatile methods of nanoparticle synthesis with well-controlled properties. Among promising synthesis techniques, both laser ablation and plasma discharges are considered. These methods provide numerous advantages that are unique in several cases. On one hand, the main advantage of the laser ablation method is in the possibilities of changing laser parameters and background conditions and in its capacity to preserve stoichiometry. Laser-based methods also yield bio-compatible nanoparticles and nano-colloids with unique chemical properties. Laser-induced fragmentation provides additional control ways over nanoparticle sizes. To better understand and to optimize these processes, detailed numerical modeling is performed. The involved stages are considered and analyzed. The resulting nanoparticle parameters are investigated as a function of the experimental conditions. Nanoparticle properties, such as mean size and mean concentration are analyzed. Differences and similarities between the considered synthesis methods are discussed.
We investigate femtosecond laser irradiation of dielectric materials containing randomly-arranged nanoparticles. For this, numerical modeling is performed based on three different methods: Mie theory, static solution of linear Maxwell's equations and a solution of nonlinear Maxwell's equations together with kinetic equations for free electron excitation/relaxation processes. First two approaches are used to define the static intensity distribution and to analyze the electromagnetic interaction between the nanoparticles. The third method allows us to investigate the complex dynamics of the laser-matter interaction. Multiphoton absorption is shown to be responsible for electron plasma generation in the regions of strong intensity enhancements in the vicinity of nanoparticles. The irradiation of the dielectric material leads to the elongation of nanoplasmas by the near-field enhancement perpendicular to the laser polarization and to their strong interaction resulting in periodic arrangement. Numerical results shed light on such effects as femtosecond laser-induced nanograting formation
Gold nanoparticles (Au NPs) attract particular attention because of their unique size-dependent chemical, physicochemical and optical properties and, hence, their potential applications in catalysis, nanoelectronics, photovoltaics and medicine. In particular, laser-produced colloidal nanoparticles are not only biocompatible, but also reveal unique chemical properties. Different laser systems can be used for synthesis of these colloids, varying from continuous wave (CW) to ultra-short femtosecond lasers. The choice of an optimum laser system is still a challenge in application development. To bring more light at this issue, we investigate an influence of laser parameters on nanoparticle formation from a gold target immersed in deionized water. First, an optical diagnostics of laser-induced hydrodynamic processes taking place near the gold surface is performed. Then, gold nanoparticle colloids with average particle sizes smaller than 10 nm and a very narrow dispersion are shown to be formed by CW laser ablation. The obtained results are compared with the ones obtained by using the second harmonics and with previous results obtained by using femtosecond laser systems.
Laser ablation (LA) is a unique tool for nanoparticle synthesis. The main advantages of this method are in its
green character and in the possibility of a control over particle size. In this study, we examine nanoparticle
formation by laser ablation under different experimental conditions and analyse the results based on the
developed models. The dynamics of the laser plume expansion is examined revealing the role of the background pressure and laser pulse parameters. As a result, the ablated material is compressed and a part of it becomes
supersaturated. The so-called "primary" nanoparticles are formed at this stage. Then, nanoparticle aggregation/fragmentation enters into play leading to the formation of the secondary particles. In addition, laser-
assisted fragmentation of nanoparticles is also examined. Based on numerical modeling we shed light on the above mechanisms by using different numerical approaches, such as molecular dynamics, Monte Carlo,
numerical hydrodynamics, and analytical analysis. Calculations are performed for metallic targets under
different background conditions. The obtained results explain recent experimental findings and help to predict the role of the experimental parameters. The performed analysis thus indicates ways of a control over
nanoparticle synthesis
Laser-induced electronic excitation, absorption and relaxation are the key issues in ultra-short laser interactions
with dielectric materials. To numerically analyze these processes, a detailed non-equilibrium model is developed
based on the kinetic Boltzmann equations without any appeal to the classical Drude model. The calculations are
performed including all possible collisional processes. As a result, electron energy distributions are obtained
allowing a better analysis of ultra-short laser interactions. A remarkable effect of the laser-field on collision
frequencies is demonstrated leading to smaller free-carriers absorption than the one predicted by Drude model
with a non-field dependent collision frequency. Both electron-electron and electron-phonon relaxation are then
examined, and the mean energy density of the electron sub-system is investigated as a function of laser fluence
and pulse duration. The developed model is useful for many laser applications including high precision in laser
treatment, laser-assisted atomic probe tomography, and for the development of new powerful laser systems.
The processes involved in nanoparticle and nanostructure formation by laser are analyzed. Relative contributions of several mechanisms involved are compared. First, we consider the formation of "primary" particles and discuss the difference between femtosecond and nanosecond regimes. Then, "secondary" particle/aggregate formation is discussed. In particular, attention is focused on (i) direct cluster ejection from a target under rapid laser interaction; (ii) condensation/evaporation; (iii) fragmentation/aggregation processes during cluster diffusion; (iv) diffusion, aggregation, and/or coalescence. In addition, routes of control over particle size distribution are proposed. Possibility of formation of colloidal nanoparticles with very narrow size distribution is proven numerically. The role of such parameters as ablation yield, laser wavelength and laser fluence, and surface tension are examined. Finally, controlled nanoparticle self-assembly is discussed as a potential technique for future development of nanomaterials.
We present experimental and theoretical investigations of interaction of a femtosecond laser (450 fs pulse at 1025nm)
with dielectric materials (fused silica) for the single-shot laser regime. The aim is to analyze and understand the complex
physical mechanisms of laser energy absorption yielding to damage and /or ablation. We outline the distinction between
the ablation and the damage thresholds for dielectric materials. The evolution of the reflection, transmission and
absorption signals is studied as a function of fluence. The experimental curves are accompanied by a modelling, which
takes into account the photoionization and avalanche ionization depicting absorption of the laser energy by the material.
The incident pulse propagation into the material, the temporal evolution of the electron density, reflection and
transmission illustrate the beginning and the duration of the laser pulse absorption. The magnitude of the absorption
process is energy density sensitive and, with the increase of the deposited fluence, the onset of absorption is moved
temporally to the beginning of the pulse. We show the influence of the effective electron collision frequency on the
calculated values of reflection, transmission and absorption. The results are particularly relevant to high micromachining
industrial processes.
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.
In this paper, we are focused on the understanding the underlying physical mechanisms of femtosecond laser interactions
with metallic and multi-layer optical materials. The results of the numerical modeling provide an estimation of damage
and/or ablation threshold for different laser parameters (pulse duration, fluence, angle of incidence, polarization) and
target material properties (metal, dielectric, or multilayer with variable metal layer thickness). These results are
compared with the experimental measurements of the thresholds obtained by using different techniques. In particular,
dielectric ionization and ablation mechanisms are analyzed based on the experimental findings.
In this work, spot-size dependence of surface femtosecond laser-induced damage threshold in fused silica is put in
evidence when the damage reach the micrometer scale. Measurements of the threshold with various numerical apertures
and different techniques are performed, revealing a noticeable threshold increase while decreasing the laser beam-focus
size below ~10 μm. This unexpected result could be explained by the presence of micrometer-sized defects pre-existing
in the SiO2 sample.
Numerical modeling is performed to study cluster formation by laser ablation. The developed model allows us to
compare the relative contribution of the two channels of the cluster production by laser ablation: (i) direct cluster ejection
upon the laser-material interaction, and (ii) collisional sticking, evaporation and coalescence during the ablation plume
expansion. Both of these mechanisms are found to affect the final cluster size distribution. Plume cluster composition is
correlated with plume dynamics. The results of the calculations demonstrate that cluster precursors are formed during
material ablation through both thermal and mechanical target decomposition processes. Then, clusters react in collisions
within the plume. In vacuum, rapid plume expansion and cooling take place leading to the overall decrease in the
reaction rates. In the presence of a gas, additional collisions with background gas species affect the cluster size
distribution. Growth of larger clusters can be observed at this stage. Calculation results explain several recent
experimental observations.
Femtosecond and nanosecond lasers are used to produce oxide nanoparticles by laser ablation of steel. The deposition of those particles on the surface strongly modifies its properties. The aim of this study is the understanding of the nanoparticle formation. The dynamics of the plume expansion and of the nanoparticle deposition processes are investigated by means of in-situ time resolved optical analysis. Scanning electron microscopy and atomic force microscopy are used to characterise the particle film morphology deposited on the surface. The influence of laser parameters such as pulse duration (ns, fs), wavelength (UV, visible, IR) and background gas pressure (10 mbar - 1 bar) on the processes of nanoparticle formation is studied. It is shown that a high density plasma favours the particle formation, and that the high temperature of the plume obtained with nanosecond JR irradiation impedes the nanoclusters nucleation and prevents an efficient nanoparticle formation.
The major bottleneck for the development of robust and cost-effective femtosecond amplification systems is the uncertainty concerning the damage threshold of Ti: Sapphire crystals. Up to now, Ti: Sapphire is the only material that supports the generation of temporally short pulses (few femtosecond) at high repetition rates, and overcoming this bottleneck will represent a major advance in laser performance for all the femtosecond community. Currently, when pumped at 532nm, the uncertainty on Ti:Sapphire damage threshold, is about a factor of ten. The empirically estimated threshold is 10J/cm2 but for safety reasons the femtosecond laser community (especially the companies producing the lasers) uses the conservative value of 1J/cm2. Such a low pumping fluency means low extraction efficiency during the amplification process and a great waste of pumping energy, the most expensive part of a Ti:Sapphire amplifier. In order to remove this bottleneck, we launch a complete analysis of all the factors that influence the damage threshold in Ti:Sapphire Crystals. Our program is to first measure the bulk threshold to define the upper threshold limit, and the influence of Ti ion concentration in the crystal garnet. Then, we will analyze all the surface effects that influence the value of the threshold. These effects depend on the polishing, on the cleaning process, as well as the type of anti-reflective coating. Only a complete understanding of all the mechanisms involved in threshold limitation will allow us to produce Ti:Sa crystals with the best performances. The study of the characteristics of the Ti:Sapphire damage threshold will not be complete and reliable without a complete characterization of the pump beams (temporal and spatial modulations), and this analysis will be done with nanosecond and picosecond pulses at 532nm. Finally, to complete the exploration of the the behavior of the titanium doped sapphire crystal, we will characterize the damage threshold with femtosecond pulses, at 800nm to reach the deterministic dielectric threshold and validate fundamentals models and simulation results. To our knowledges this is the first time that such a complete characterization is done for Ti:Sapphire laser crystals. We will present the first conclusions about the experiments as well as the methods we will employ in our systematic analysis.
We address the peculiarities of femtosecond laser ablation of both metallic and dielectric materials. The ablation process is investigated using two numerical models. For metals, a hydrodynamic model is used that describes the laser light absorption together with heat and pressure wave propagations and the material motion. This model is used to study laser ablation at different fluences for two metals with different strengths of the electron-ion coupling. In these calculations, the role of the temperature-dependent electron heat conduction is demonstrated. For dielectrics, material ionization and laser light absorption processes are modeled in both one and two dimensions. The saturation of the light absorption, and, hence, of the ablation depth, is shown to take place in dielectric materials at sufficiently large laser intensities. The role of this effect on the shape of the craters is examined. This saturation effect is demonstrated to be a consequence of the interplay between the ionization and the light absorption processes.
Femtosecond laser ablation of Ti, Zr and Hf has been investigated by means of in-situ plasma diagnostics. Fast plasma imaging with the aid of an intensified charged coupled device (ICCD) camera was used to characterize the plasma plume expansion on a nanosecond time scale. Time- and space-resolved optical emission spectroscopy was employed to perform time-of-flight measurements of ions and neutral atoms. It is shown that two plasma components with different expansion velocities are generated by the ultra-short laser ablation process. The expansion behavior of these two components has been analyzed as a function of laser fluence and target material. The results are discussed in terms of mechanisms responsible for ultra-short laser ablation.
The paper addresses different aspects of the numerical modeling of laser ablation. In particular, the advantages of microscopic and macroscopic numerical approaches are underlined for specific parts of the complex laser ablation process. Multi-scale, or hybrid models combining different methods are shown to provide most promising results. These models permit not only to diminish considerably the calculation time, but also to perform more realistic calculations facilitating the interpretation of experimental results. Numerical results are presented for the cases of both nanosecond and femtosecond laser pulses under conditions typical for such applications as reactive laser ablation and laser-induced plasma spectroscopy. Several experimentally observed physical phenomena taking place during laser plume formation and expansion are visualized and are explained by using the proposed combined models.
Pulsed laser ablation appears as a promising technique for depositing thin films. A large variety of successful experimental results were obtained in this field, including the growth of high-temperature superconducting films, ferroelectric films, oxides, semiconductors, diamonds, etc. One of the main advantages of this technology is the relative simplicity of the experimental set-up and the possibility to get good homogeneity, complex stoichiometry materials and well adhesive dense layers. The main drawback seems to be the production of macroparticles, their transfer to the growing film inducing inhomogeneity and roughness onto the surface, lowering the properties of the thin films. In a common configuration, the laser-generated flux is collected on a planar substrate positioned parallel to the irradiated surface. In order to improve the stoichiometry and the quality of the films, several modifications, like simultaneous generation of two plumes from different targets with different targets with different laser beams, were proposed. In this paper, we present some results concerning the production of cryolite thin films using the conventional pulsed laser deposition technique and the dual crossed beams pulsed laser deposition technique. Plasma plumes expanding in vacuum and interacting together are visualized. The different species ejected in the plumes are detected through narrow-band filters in order to determine their kinetic energies. The morphology and the composition of the films are compared with the thermal evaporation technique.
The application of the pulsed laser deposition technology requires a better understanding of the material evaporation mechanisms, of the gas and plasma plume formation, of its expansion and of its deposition onto the substrate. Experimental and numerical studies revealed the importance of the gas-phase collisions on the formation of a peaked angular distribution of particles described by cosp (theta) . Numerical calculations based on a Monte Carlo simulation of the desorption process were performed showing that the focusing effect of the angular distribution of particles increases with the number of collisions in the plume. This paper presents a 3D Monte Carlo simulation performed in vacuum and ambient gas in order to investigate the angular distribution of particles. Besides elastic collisions, the recombination-dissociation reactions proceed in the flow.
KEYWORDS: Particles, Chemical species, Molecules, Chemical reactions, Thin films, Monte Carlo methods, Tellurium, Pulsed laser deposition, Laser ablation, Helium
Pulsed laser ablation has attracted great attention over the past few years as promising technique for depositing thin films. A large variety of successful experimental results were obtained in this field, including the growth of high- temperature superconducting films, ferroelectric films, oxides, semiconductors, diamonds, etc. One of the main advantages of this technology is the simplicity of the experimental set-up. In a common configuration, the laser- generated flux of particles is collected on a planar substrate positioned parallel to the irradiated surface. Several modifications, like simultaneous generation of two plumes from different targets (double ablation), were proposed. Different lasers (e.g. KrF with (lambda) equals 248 nm, Nd-YAG with (lambda) equals 532 nm, etc.) with energy density 2 - 10 J/cm2 were used in the ablation experiments both in vacuum and into diluted ambient gas (pressure up to 750 mTorr). Monte Carlo simulation was found to be a successful technique for theoretical investigations of the laser ablation processes. This method has allowed us to investigate the influence of elastic collisions and chemical reactions in the laser ablated plume on the angular characteristics of the flow. The results of the simulation show that elastic collisions give rise to focusing of particles towards the surface normal and to the redirection of the velocities of the more energetic particles in the direction close to the surface normal. The chemical reactions are found to influence the angular distributions in the way opposite to the one of elastic collisions. The reaction heat contributes to the energy of particles and the velocity distributions are affected by reactions. As result of these processes, the angular distributions are broadened from the surface normal. Additional collisions with the particles of the ambient gas were shown to influence the composition and uniformity of thin films. The study of these processes is of a particular interest for the developing of pulsed laser deposition (PLD) technique.
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