Pulsed Laser Ablation Of Metals Research Articles

Real-Time In-Process Evaluation of Spatter Area and Depth of Aluminium Surface in a Pulsed Laser Ablation Process Using Received Radio Frequency Power from Plasma Plumes

During the pulsed laser ablation of metals, as well as other materials, the development of a plasma plume close to the ablated surface leads to the emission of radio frequency energy. In this paper, we describe a process for analysing the received radio frequency power (RFP) for an aluminium (Al) surface ablation process in atmosphere using picosecond laser pulses at a wavelength of 1064 nm. The analysis of the RFP was carried out on two sets of experiments, where two parameters of the laser (repetition rate of laser (RRL) and power of laser (PL)) were varied while other parameters remained constant. In addition to the RFP measurement during the laser processing, the spatter area (SA), which is defined in this paper, and the depth of the ablated hole were measured post-process using a 3D microscope. It was observed that there is a direct relationship between (RFP)2 and SA. Accordingly, an appropriate RF calibration was performed, which leads to the definition of a quantity called the RF regulation % (RFR%). By comparing the RFR and PL/RRL variations, to which the laser beam fluence is proportional in these experiments, a diagnostic process (i.e., flowchart) for real-time depth evaluation was proposed and experimentally confirmed. This diagnostic process can indicate if the depth of the laser ablated crater is less than or exceeds a predetermined depth, which in this study was set to 15 µm. It is also demonstrated that the SA variation can be estimated in real-time by analysing the received RF power and, secondly, the depth of ablation can be measured in real time using a combination of information from the received RF power and laser parameters.

Morphology modelling and validation in nanosecond pulsed laser ablation of metallic materials

Nanosecond pulsed lasers are widely used in the surface treatment of metallic materials due to their low cost and simple operation. During pulsed laser processing, many parameters such as laser and material properties are involved. Consequently, the existing mathematical models for pulsed laser ablation of metals are complex and computationally difficult. In this paper, a simple and novel model was proposed for nanosecond pulsed laser processing based on heat transfer and geometrical mathematics. The geometrical parameters functions of the recast layer under pulsed laser single ablation and continuous superimposed ablation are presented, respectively. The feasibility of the models is validated by experiments. Errors in diameter and depth of single-pulse laser ablation craters were 2.56%–7.14% and 6.82%–18.91%. The recast layer depth error is about 3.47%–12.47% for successive pulses of superimposed ablation. It predicts the shape of the recast layer on pulsed laser ablated metal surfaces. The model is a guide to the surface morphology of laser woven materials and the preparation of surface functionalities, particularly the selection of process parameters for microstructure machining.

Deformations of biological tissues with photothermal nanoparticles under laser irradiation

In this work, optical coherence tomography (OCT) of gel phantoms and cartilage tissue of the joint impregnated with nanoparticles was performed under laser exposure to an erbium fiber laser with a wavelength of 1.56 m. Multifunctional "magnetite + gold" nanoparticles with a "core-shell" structure were obtained by pulsed laser ablation of metals in a liquid, and oxide bronzes KxTiO2 and NaxTiO2 are formed in the processes of self-propagating high-temperature synthesis and mechanical activation. An improvement in the efficiency of the diagnostic method by introducing antireflection additives and absorbing nanoparticles due to a decrease in the irradiation intensity and an increase in the photothermal effect is shown. OCT elastography data indicate the dependence of tissue deformation on the preliminary history of tissue exposure.

Influence of stress confinement, particle shielding and re-deposition on the ultrashort pulse laser ablation of metals revealed by ultrafast time-resolved experiments

The ablation rate in double-pulse material processing is strongly influenced by the pulse separation. For pulse separations exceeding 3 ps a significant decrease in ablation volume has been observed. This was attributed to three mechanisms: rarefaction wave interaction, shielding by ablation plume and material re-deposition. Here we present carefully designed double-pulse ablation experiments on three industrially relevant metals: stainless steel, aluminum and copper and interpret them based on ablation dynamics derived from ultrafast time-resolved pump-probe microscopy and ellipsometry. Adjustment of the fluence distribution within the double-pulse allows us to separate effects arising from rarefaction wave suppression and shielding/re-deposition. We found that the rarefaction wave contribution to the double-pulse ablation volume is about 25% for aluminum and stainless steel and about 40% for copper. A pronounced re-deposition was observed for stainless steel and aluminum, while for copper shielding by the ablation plume plays a dominant role. It was revealed that the ablation volume stays maximal, if the pulse energy is deposited within the mechanical relaxation time of (2–5)ps. According to our findings the ablation process exerts maximum efficiency and precision, when energy is coupled into the material before mechanical relaxation or after material surface equilibration.

Atomistic simulation of ultra-short pulsed laser ablation of metals with single and double pulses: An investigation of the re-deposition phenomenon

The demand for higher throughput in the processing of materials with ultra-short pulsed lasers has motivated studies on the use of double pulses (DP). It has been observed in such studies that at relatively high time delays between the two pulses, the ablated volume is lower than that for a single pulse (SP). This has been attributed to the shielding of the second pulse and the re-deposition of the material removed by the first pulse. The investigation of re-deposition in copper with the aid of atomistic simulations is the main objective of this study. Nevertheless, a computational investigation of SP-ablation and experimental measurement of the SP-ablation depths and threshold fluence are also covered. The applied computational apparatus comprises a combination of molecular dynamics with the two-temperature model and the Helmholtz wave equation. The analysis of the simulation results shows that the derived quantities like the SP-ablation threshold fluence and the ratio of DP-ablation depth to SP-ablation depth are in agreement with the experimental values. An important finding of this study is that the characteristics of the re-deposition process are highly dependent on the fluence.

Atomistic simulations of ultra-short pulse laser ablation of aluminum: validity of the Lambert-Beer law

Abstract Based on hybrid molecular dynamics/two-temperature simulations, we study the validity of the application of Lambert-Beer’s law, which is conveniently used in various modeling approaches of ultra-short pulse laser ablation of metals. The method is compared to a more rigorous treatment, which involves solving the Helmholtz wave equation for different pulse durations ranging from 100 fs to 5 ps and a wavelength of 800 nm. Our simulations show a growing agreement with increasing pulse durations, and we provide appropriate optical parameters for all investigated pulse durations.

Simulation of femtosecond pulsed laser ablation of metals

In this paper a mathematical model for femtosecond laser ablation of metals is proposed, based on standard two-temperature model connected with 1D hydrodynamic equations. Wide-range equation of state has been developed. The simulation results are compared with experimental data for aluminium and copper. A good agreement for both metals with numerical results and experiment shows that this model can be employed for choosing laser parameters to better accuracy in nanoparticles production by ablation of metals.

Dynamics of liquid nanodroplet formation in nanosecond laser ablation of metals

The laser ablation mechanisms of metallic targets leading to liquid nanodroplet ejection are of wide interest both from a fundamental point of view and for applications in various fields, especially when nanoparticle synthesis is required. The phase explosion process was recognized as the driving mechanism of the expulsion of a mixture of vapor and liquid nanodroplets in the short pulse laser ablation of metals. A model based on thermodynamics that links the theory of homogeneous vapor bubble nucleation to the size distribution of the generated liquid nanoclusters has been recently proposed. The present work aims to take a step ahead to remove some assumptions made in previous work. Here an improved computational approach allows us to describe time-dependent nucleation in a homogeneous system with no temperature spatial gradients under nanosecond laser irradiation. Numerical results regarding the size distribution of formed liquid clusters and the time evolution of the process are shown for aluminum, iron, cobalt, nickel, copper, silver and gold. Connections with experimental data and molecular dynamics simulations, when available from literature, are reported and discussed.

Stability of micron-sized spheres formed by pulsed laser ablation of metals in superfluid helium and water

It has been shown that micrometer-sized balls, resulting from the condensation of products of laser ablation of fusible metals in both superfluid helium and water, are in the state of strong internal tension counterbalanced by external compression. By radiation-induced or chemical damage to the integrity of their surface, they break up, ejecting a plurality of nanoparticles. The empty shells of the microspheres, which nonetheless remain intact, are identical to the “hollow spheres” of unclear origin that have been observed previously under laser ablation in usual liquids. The metastability of the microparticles produced by ablation in a liquid should be taken into consideration in their use in engineering and medicine.

An improved model for nanosecond pulsed laser ablation of metals

A model is presented for the ablation of metals by nanosecond laser pulses, based on one-dimensional heat flow with temperature dependent material properties. A numerical optical calculation is introduced to account for laser beam absorption in the target, utilizing established matrix methods for electromagnetic plane wave propagation in multi-layered media. By including the effects of reflection from the dielectric-metal interface, the fall in reflectivity of aluminum during nanosecond laser pulses above the phase explosion threshold is found to be approximately twice that calculated in previous works. A simulated shielding coefficient is introduced to account for reflection and absorption of the incident laser beam by the ablation products. With these additions to foregoing models, good agreement between calculated and published experimental ablation data is attained for aluminum, both in terms of ablation threshold and depth. An investigation is subsequently carried out into the effects of laser wavelength, pulse duration and target thickness on the phase explosion threshold of aluminum.

Properties of plasmas produced by short double pulse laser ablation of metals

We investigate the composition of plasmas produced by laser ablation of metals with two time-delayed short laser pulses using fast imaging and time- and space-resolved optical emission spectroscopy. The ablated material is deposited on mica substrates and analyzed by atomic force microscopy. The laser-produced craters are inspected by optical microscopy to evaluate the ablated material quantity. It is shown that the fraction of nanoparticles in the ablation plume is strongly altered when a second laser pulse of sufficiently large delay is applied. Comparing the results obtained for different metals, we observe a significant nanoparticle reduction for interpulse delays of the order of the characteristic time of electron-lattice thermalization. More detailed analyses show that the plume changes occur on two different characteristic times, indicating two different mechanisms at its origin. Here, we discuss the involved processes and we propose a simple and efficient technique for the measurement of electron-lattice thermalization times based on plume observations during double pulse laser ablation.

Ultra-short pulse laser ablation of metals: threshold fluence, incubation coefficient and ablation rates

In this paper we present femtosecond laser ablation studies of the metals copper, silver and tungsten. Measurements of the threshold fluence determined from the hole diameters versus fluence provide incubation coefficients of the three materials, which are found to be equal within one standard deviation. Furthermore, we have determined the single-shot threshold fluences to be 1.7±0.3 J/cm2, 1.5±0.4 J/cm2 and 0.44±0.02 J/cm2 for copper, silver and tungsten, respectively. These are in good agreement with theoretical values calculated neglecting heat diffusion.

Atomistic Modeling of Short Pulse Laser Ablation of Metals: Connections between Melting, Spallation, and Phase Explosion

The mechanisms of short pulse laser interactions with a metal target are investigated in simulations performed with a model combining the molecular dynamics method with a continuum description of laser excitation, electron−phonon equilibration, and electron heat conduction. Three regimes of material response to laser irradiation are identified in simulations performed with a 1 ps laser pulse, which corresponds to the condition of stress confinement: melting and resolidification of a surface region of the target, photomechanical spallation of a single or multiple layers or droplets, and an explosive disintegration of an overheated surface layer (phase explosion). The processes of laser melting, spallation, and phase explosion are taking place on the same time scale and are closely intertwined with each other. The transition to the spallation regime results in a reduction of the melting zone and a sharp drop in the duration of the melting and resolidification cycle. The transition from spallation to phase e...

Fundamental processes in nanosecond pulsed laser ablation of metals in vacuum

Velocity and kinetic energy distributions (VDs, KEDs) of metal ions generated by nanosecond (ns) pulsed laser ablation under high vacuum have been determined using an electrostatic analyzer plus time-of-flight coupled system. A number of metal and alloy targets, principally involving Fe and Ni, have been studied at different laser fluences. At low fluence, the ion distributions have been shown to fit single Maxwell-Boltzmann-Coulomb (MBC) distributions; for medium and higher fluences, each ion distribution is found to comprise that of the surviving ``precursor'' ion, itself, overlapped with sidebands which arise from ion-electron recombination and/or ionization. The so-called surviving ``precursor'' ion of a distribution is that which underwent no change of charge. The Coulomb velocities of the surviving ``precursor'' ions and those of the ion products resulting from ion-electron collisions have been compared. Ion velocities are correlated with the local electric field resulting from ejection of the photoelectrons following laser ablation. Under identical conditions of laser fluence, the ions are seen to experience an electrical field nearly independent of their charge. The transit times of ions in the plasma have been estimated to be of the order of $1\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$. An overall quantitative mechanism for metal ablation on this basis is presented, including the ejection time for photoelectrons and differences in ion distributions resulting from employing laser pulses in the nanosecond (ns) and femtosecond (fs) regimes.

Generation of titanium-oxide nanoparticles in liquid using a high-power, high-brightness continuous-wave fiber laser

Previous studies on laser-assisted nanomaterial formation in liquids have focused on using pulsed laser ablation of metals. We report, for the first time to our knowledge, the fabrication of nanoparticles via high-power high-brightness continuous-wave fiber laser ablation of titanium in liquids. Analysis revealed the generation of spherical nanoparticles of titanium-oxide ranging mainly between 5 nm and 60 nm in diameter. A mechanism of formation for crystallized nanoparticles, based on the self-organized pulsations of the evaporated metal, is proposed. This may account for the observed substantial efficiency gain owing to the high average power and brightness of the source.

Simulation of the thermionic emission during ultrashort pulse laser ablation of metals

The characteristics of thermionic emission of metal films during ultrashort pulse laser ablation are investigated by numerical simulation. The two-temperature model is used to calculate the electron and lattice temperatures while thermionic emission is incorporated into the model as a surface phenomenon. The Richardson-Dushman equation is employed to estimate the rate of thermionic electron emission. The influence of laser irradiance and the film thickness on the emission rate is examined. The change of quantum efficiency is predicted and compared with reported experimental data. The evolution of electron and lattice temperatures and the variation of relaxation time are also investigated in accordance with thermionic emission.

Direct simulation monte carlo of pulsed laser ablation of metals with clusterization processes in vapor

A mathematical model is proposed to describe laser ablation of metals in vacuum under the action of nanosecond laser pulses of moderate intensity taking into account the processes of cluster formation and decay in the vapor cloud. To describe the laser radiation absorption and metal heating, the thermal model based on the unsteady one-dimensional heat equation with a volume heat source is used, and the method of statistical modelling is employed for modelling vapor expansion and the processes of cluster formation. The efficiency of the proposed complex model is considered by the example of pulsed laser ablation of a niobium target.

Identification of the (H‐)(H2)12 Hydride Anion Cluster in Solid Hydrogen.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Identification of the (H-)(H2)12 Hydride Anion Cluster in Solid Hydrogen

Pulsed-laser ablation of metals with normal hydrogen [deuterium] condensing at 3.5 K produces a strong 3972.0 [2869.8] cm-1 absorption the same as high energy irradiation of solid hydrogen [deuterium]. New experimental observations and quantum chemical calculations show that these charge centers are (H-)(H2)n [(D-)(D2)n] anion clusters, which have not been observed in the gas phase and are unique to solid hydrogen. First, doping our H2 [D2] samples with CCl4, a very efficient electron scavenger, replaces the above absorptions with new 4067.0 [2925.6] cm-1 bands, which are due to the (Cl-)(H2)n [(Cl-)(D2)n] anion clusters. Second, (H-)(H2)n is destroyed by λ > 290 nm but restored with 193 nm radiation through photoionization of metal atoms to produce electrons for capture by H atoms. Third, model MP2 calculations for (H-)(H2)6 with the D6h structure, which is a contracted portion of the solid hydrogen lattice, strongly supports these new assignments and suggests that the complex has the same coordination number 12 as the hydrogen lattice. Thus, (H-)(H2)12 cluster anions are characterized in the solid hydrogen microcrystal.

Preface

This twelfth volume of Annual Review of Heat Transfer marks the beginning of an expanded editorial team that is committed to maintaining the fine tradition of this series by providing improved and timely communications on new and important developments in the field of heat transfer. The theory and practice of heat transfer have been significantly broadened in recent years, particularly in biomedical engineering and biotechnology, micro- and nano-technologies, energy-efficient processes and systems, and in materials technologies such as laser and plasma processing. While continuing to feature advancements in the fundamentals of heat transfer, the editors will give special attention to presenting state-of-the-art reviews on topics of interest in such rapidly evolving areas of technology. The present volume starts with a review of the advances and challenges in micro and miniaturized heat pipes, which are being used for heat dissipation in many modern communication and computing systems. The second article deals with heat transfer in low energy plasmas used for microelectronic manufacturing, while the third article reviews heat transfer and phase-change during high-power, pulsed laser ablation of metals, as in thin film deposition, micro-machining and the synthesis of nanomaterials. Processing of thermoplastic, matrix composite materials, which involves complex, multi-scale heat and mass transfer, is considered in the fourth article, the next two articles deal with heat transfer in crystal growth, a precursor for electronic and opto-electronic device fabrication. The first of these articles treats convective instabilities under various growth conditions, while the second article considers the effects of an applied magnetic field on growth phenomena, associated hydrodynamics and the control of flow instability. The last article of this volume reviews the fundamentals of interferometric tomography and application of the technique to three-dimensional temperature measurement. The editors would like to thank the participating authors for their valuable contributions, to Begell House for editorial assistance, to Nancy Hughes of UC Berkeley and Alina Stroia of SUNY Stony Brook for their secretarial help, and to the thermal science and processing community for their continued support and input.