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In the present work, nanoparticles of copper and silver synthesized via pulsed laser ablation of the respective targets in distilled water are applied to cellulose filter paper to check their effectiveness in the annihilation of bacteria from contaminated water. The treatment of the filter paper with the nanoparticles is found to be an excellent way to get rid of two common bacteria, Staphylococcus aureus and Escherichia coli, from contaminated water. The spread plate method on agar, employed to test the antibacterial efficacy of the nanoparticle-treated papers, clearly shows the absence of bacterial growth upon coming into contact with the nanoparticles in the filter paper. These results were further substantiated by the growth kinetic study of the bacteria that exhibited slow growth of the bacteria that were exposed to the nanoparticles. The morphology of the bacteria that came into contact with the nanoparticles is found to be adversely affected by the nanoparticles. Both copper and silver nanoparticles show a similar extent of antibacterial activity. [ABSTRACT FROM AUTHOR]

A novel method was developed for studying the ink-paper interface and the structural variations of a deposited layer of ink. Combining high-resolution x-ray tomography with laser ablation, the depth profile of ink (toner), i.e., its varying thickness, could be determined in a paper substrate. X-ray tomography was used to produce the 3D structure of paper with about 1 μm spatial resolution. Laser ablation combined with optical imaging was used to produce the 3D structure of the printed layer of ink on top of that paper with about 70 nm depth resolution. Ablation depth was calibrated with an optical profilometer. It can be concluded that a toner layer on a light-weight-coated paper substrate was strongly perturbed by protruding fibers of the base paper. Such fibers together with the surface topography of the base paper seem to be the major factors that control the leveling of toner and its penetration into a thinly coated paper substrate. [ABSTRACT FROM AUTHOR]

Understanding peculiar dynamics in transient laser-produced plasmas is of great importance for a number of applications such as energetic ion production, ion implantation, laser micro-machining, etc. Continuing a series of papers on the electrical and optical characterization of laser ablation plasma plumes, we report here on the observation of oscillations in the ionic and electronic currents at a long distance (17.5 cm) from the ablation target. An electrostatic energy analyzer was used to finely probe the dynamics of charge carriers in a carbon plasma generated in vacuum by KrF excimer laser ablation of a graphite target. A short-time Fourier transform data processing approach was applied to extract valuable information from the recorded currents. This method not only reveals different frequencies for two distinct expanding plasma structures, but also unveils different trends in their temporal evolution. These new experimental results corroborate previous studies supporting a scenario based on the plasma structuring in multiple double layers. A discussion with respect to an alternative scenario based on low-frequency sheath instabilities is also presented. [ABSTRACT FROM AUTHOR]

An experimental study of the cavitation flow around the National Advisory Committee for Aeronautics 0012 hydrofoil with different surface morphology was carried out in this work. The surface morphology was set by modern laser ablation technology. The average values and intensity of vapor–gas cavities were determined. It has been revealed that laser texturing delays the emerging cavitation and somewhat decreases its intensity at higher cavitation numbers. A decrease in the cavitation number leads to an increase in its intensity for a smooth hydrofoil in comparison with a rough one, which is also expressed in an increase in the frequency of cavities. The paper presents a comparison of the flow regime with equal cavitation numbers, which clearly describes the features of the development of a vapor–gas cavity on the suction side of the foil with different surface morphologies. The paper provides an explanation of the reasons for the influence of surface morphology on the development of cavities. [ABSTRACT FROM AUTHOR]

Superhydrophobic surfaces with a slip effect have been proven to be effective to achieve surface drag reduction. In this paper, we fabricated superhydrophobic disks via laser ablation and chemical modification, and their drag-reduction performance is tested by a rheometer. However, the superhydrophobic disk achieves a limited drag-reduction effect (33.5% in maximum) with poor consistency (decay rapidly as rotating speed increases). To enhance the drag-reduction performance of the superhydrophobic surfaces, surfaces with tunable patterns consisting of superhydrophobic and hydrophobic surfaces are proposed. The difference in wettability and slip length between different regions is employed to control the flow direction. The effect of different patterns is investigated both experimentally and numerically, and the results indicate that the pattern distribution performs a great impact on the drag-reduction effect. The pattern consistent with the primary flow can not only enhance the effect (60.3% in maximum) but also the consistency (maintained at a higher rotating speed) of drag-reduction. On the contrary, the pattern perpendicular to the flow direction is harmful to drag reduction and can even increase the drag. [ABSTRACT FROM AUTHOR]

For a nanosecond laser ablation of metals, the key physical phenomena involved are thermal evaporation, melt ejection, instability of the molten metal, etc., which depend on the initial temperature evolution in the metal. Understanding the evolution of temperature of the metal needs an effective simulation. In the present paper, we report on the finite element method-based simulation of nanosecond laser ablation of copper in the absence and presence of the magnetic field. Our studies showed that the effective thermal conductivity of the melted layer on the copper surface in the presence of the magnetic field affects the viscosity of the layer, mass ablation rate, instability, and then particle formation. The calculations showed that the condensed nuclei of large critical size are produced in the magnetic field. It is attributed to an increase in the collision rate of plasma particles in the magnetically confined plasma. The simulations are in good agreement with the experimentally measured values. [ABSTRACT FROM AUTHOR]

Laser-induced particle ejection on the exit surface of fused silica serves as an important contaminant source in a high-power laser system. The transport process of molten silica particles in a gas environment or vacuum is important in understanding the change in size and temperature of silica particles, which influence the ultra-clean manufacturing of optical components. In this paper, the ejection process of fused silica is investigated using molecular dynamics simulation. The results show that the geometry of a surface scratch influences the mass of the microjet. With shallower groove depth and a smaller vortex angle, the mass of the microjet is less under shock loading. The size of ejected particles tends to decrease gradually and does not change any more eventually. Besides, these particles become dispersed during the transport process in a vacuum. On the other hand, background gas suppresses the particle flow and slows down the particle flow. As the ejected particles compress gas, vapor and small clusters (N < 50) are stripped from the microjet continuously. Eventually, the number of nanoparticles that exceed the free surface decreases to zero. The stripped small clusters behind the head of the microjet recombine with other clusters, which change the volume density of ejected particles near the free surface. The higher velocity of ejected particles induces a stronger gas stripping effect, which makes an increase in the number of small clusters (N < 50). The results can help understand the behavior of particle ejection and the transport process of silica particles in a gas environment or vacuum, especially in the field of laser-induced particle ejection on the exit surface or the laser ablation of fused silica producing aerosol. [ABSTRACT FROM AUTHOR]

An accepted approach to computing laser-induced peak surface temperature is to employ the enthalpy formulation of the transient heat conduction equation [Grigoropoulos et al., Adv. Heat Transfer 28, 75–144 (1996); Sawyer et al., J. Laser Appl. 29, 022212 (2017)]. This approach is generally implemented using an explicit numerical scheme to solve the thermal transport equation. While it offers the advantage of modeling the solid-melt phase transition automatically, the approach results in instability-like behavior in the computed surface temperature. When laser-induced ablation becomes significant, the heating rate in the surface cell becomes unrealistically large. This results in spikes in the computed peak surface temperature due to large errors in calculating the heating rate. In this paper, we present a new approach, which we refer to as the Moving Frame Solver, that employs a moving-coordinate frame of reference, located at the receding evaporating surface. We also use an analytical representation for the phase transition region of the enthalpy-temperature relationship. The Moving Frame Solver combined with an implicit scheme leads to a stable solution without surface temperature, pressure, or velocity spikes. In other words, any instability in these computed parameters due to use of an explicit scheme (such as Dufort–Frankel) has been eliminated. Details of the new thermal solver and example calculations are presented. Numerical experiments suggest that the surface cell size needs to be small, ∼0.1 μm, to obtain a highly accurate solution with a typical metal such as aluminum. Using the Moving Frame Solver with a refined grid near the surface, but coarse elsewhere, enables accurate and stable surface temperature computation. [ABSTRACT FROM AUTHOR]

Hydrogen-powered polymer electrolyte membrane fuel cells (PEMFCs) show promising potential to power a wide range of mobile and stationary applications and to reduce greenhouse gas emissions significantly. In PEMFCs, the oxygen transport and the water transport are essential for a long lifetime and high-performance characteristics. The diffusion media (DM), located between the bipolar plate and the catalyst-coated membrane, is a crucial component of the fuel cell that significantly affects the cell-internal processes. Usually, the DM is a two-layer material system consisting of a microporous layer based on carbon black particles coated onto a porous gas diffusion layer (e.g., carbon paper). The properties of the microporous layer regarding the water transport at high current densities and, consequently, the fuel cell's performance and lifetime can be improved by laser structuring. Within this work, different microporous layers with varying binder content and porosities were structured by locally ablating the material using ultrashort-pulsed laser radiation in the infrared wavelength range. The effect of varying process parameters was additionally investigated. Furthermore, the ablation efficiencies were calculated for increasing pulse repetition rates to qualify a process window for an industrial structuring process. The size of the micro-drillings and the heat-affected zone surrounding the hole were evaluated through topographic and microstructure analyses using a laser scanning microscope and a scanning electron microscope with energy-dispersive x-ray spectroscopy. The results showed a rather small influence of the porosity and composition of the microporous layer on the ablation behavior. In contrast, the laser structuring parameters influenced the micro-drilling geometry significantly. [ABSTRACT FROM AUTHOR]

It is believed that isolated defects within the capsule (e.g., void, high-density inclusion) can be one of the essential factors for implosion performance degradation by seeding hydrodynamic instabilities in implosions. Nonetheless, a systematic study on how the isolated defects evolve and why they are not stabilized by ablation given the length scale comparable with the typical cutoff wavelength is still lacking. This paper addresses the above concerns by looking into a simplified model where a planar shell (without convergent geometry) is driven by laser direct-drive, with a single defect (low/high density) of micrometer or sub-micrometer scale residing at different locations inside. The underlying dynamics of two key physical processes are analyzed, i.e., the shock–bubble interactions as well as the subsequent nonlinear evolution of ablative hydrodynamic instabilities initiated by the direct interaction of the deformed defect and ablation front, revealing that compressibility and baroclinic effects drive vorticity production during the interactions between the shock wave and the isolated defect. In the "light-bubble" case, the vortex pair generated in the first process is further strengthened by the laser ablation. Hence, a directed flow is formed in companion with the persistent flow entering the bubble of the surrounding ablator. The bubble exhibits a remarkable growth both laterally and deeply, seriously threatening the shell's integrity. The positive feedback mechanism of the vortex pair is absent in the "heavy-bubble" counterpart, and the ablation stabilization manifested itself in the reduction of spike amplitude. A systematic study of localized perturbation growth as a function of defect placement, size, and preheating intensity is presented. [ABSTRACT FROM AUTHOR]

Fiber lasers are characterized by high efficiency, good beam quality, and high reliability and are widely used in industries. In addition, the high repetition frequency of fiber lasers can increase the analytical efficiency greatly. In this paper, a nanosecond fiber laser was introduced to be the ablation source in laser-induced breakdown spectroscopy (LIBS) for determining copper, magnesium, and manganese in aluminum alloys. Because of the high repetition frequency of the fiber laser, the conventional delay-acquisition strategy was unavailable. Discrete wavelet transform (DWT) was used to mitigate the interference of the Bremsstrahlung background. Standard calibration and internal calibration methods were also compared for quantitative analyses. The results demonstrated that the combination of DWT and internal calibration effectively improved the analytical accuracy. After optimization, the R2s of the FL-LIBS system for Cu, Mg, and Mn were 0.9897, 0.9964, and 0.9979, respectively; the root mean squared error of cross validation was 0.0414, 0.0462, and 0.031 wt. %, respectively. This study provides a convenient method for the rapid elemental analysis of aluminum alloys. [ABSTRACT FROM AUTHOR]

Skin-pass rolls are used for setting the final sheet thickness and surface texture. For sheet metal that is produced for forming, textured skin-pass rolls featuring a high-low structure are used in order to improve the formability and paint adhesion of the sheet. In this paper, new textures for skin-pass rolls generated by high-speed laser melt injection (HSLMI) are presented and characterized. Furthermore, it is studied how the texture of the roll is transferred to steel and aluminum sheets. With HSLMI, metal matrix composite (MMC) layers featuring spherical fused tungsten carbide (SFTC) particles with a high hardness could be produced on skin-pass rolls. For generating an increased high-low structure, laser ablation and electrolytic etching were carried out after HSLMI and grinding of the rolls. An analysis of the topography showed that different protruding heights between SFTC particles and matrix can be set. The textures generated by laser ablation showed a topography featuring two homogeneous height levels, whereas a texture with spherically shaped particles could be generated by electrolytic etching. Furthermore, it was found that all textures were transferred from the roll to both steel and aluminum sheets. The transfer of the textures mainly depended on the protruding height of the SFTC particles and the SFTC particle content of the roll. [ABSTRACT FROM AUTHOR]

A novel high-speed multi-frame photographic system is presented in this paper. The system demonstrates exceptional compactness and flexibility, requiring only the introduction of a cavity comprising multiple beam-splitters in the optical path to enable multi-frame imaging of sub-nanosecond events. The number and temporal delay of frames can be easily adjusted by adjusting the distance and angle between beam-splitters. These capabilities are demonstrated by observing the laser ablation process, highlighting the great potential for application in capturing ultrafast time-evolving events such as optical breakdown, the evolution of laser-produced plasmas, and the propagation of shock waves. [ABSTRACT FROM AUTHOR]

A close-packed micro-lens array (MLA) is widely used not only in novel optical systems but also in various engineering fields, such as semiconductors and display devices. In this paper, we present a simple and efficient method for fabricating MLAs on a glass substrate via laser ablation with a single femtosecond laser pulse and chemical wet etching in a hydrofluoric acid solution. The shapes of concave micro-lenses were optimized for laser pulse energy and etching time, and ∼70 000 micro-lenses with 15 µm diameter were formed. The shape of the micro-lens varies significantly with respect to the etching time in accordance with an initial ablated area (or laser energy), which is a key feature in fabricating micro-lenses of several tens of μm. Using the glass MLA as a mold, a polydimethylsiloxane convex-plano lens array was replicated, and the performance of the optical imaging and beam integrator was then examined. [ABSTRACT FROM AUTHOR]

We report the results of the design, simulation, fabrication, and cold-test measurements of millimeter-band 2D planar microstrip slow-wave structures (SWSs) on dielectric substrates. Such structures have a high slow-wave factor, which allows for low-voltage operation and reduction in the size and weight of the device. A low-cost and flexible fabrication technology based on magnetron sputtering and subsequent laser ablation has been developed and is reported in the paper. Microstrip meander-line SWS circuits at V-, W-, and D-bands have been fabricated and characterized. The fabrication of ring-bar planar SWSs by the photolithographic method is also discussed. Experimental measurement of S-parameters of the fabricated structures reveals good transmission properties. Return loss (S11) does not exceed −10 dB and attenuation is about 2 dB/cm in the V-band, 10 dB/cm in the W-band, and 8.5 dB/cm in the D-band. [ABSTRACT FROM AUTHOR]

Nanosecond (ns) laser ablation of silicon has important applications in electronics, photovoltaics, and many other areas. However, most of the previous work is mainly focused on ultraviolet (UV) or visible ns lasers, and the investigation is very limited for infrared lasers (e.g., at 1064 nm), which have the advantage of much lower cost and external energy consumption. In this paper, a thermal model has been developed and experimentally verified for ablation of silicon by 200 ns and 1064 nm laser pulses. This kind of work has been rarely reported in literature. The model predictions agree reasonably well with experimental measurements. The investigation indicates that surface evaporation should be the dominant physical mechanism of material removal for laser silicon ablation under the studied conditions. It has also been found that the physical processes of multipulse laser-silicon interaction at 1064 nm may strongly depend on laser pulse number and repetition rate (the pulse-to-pulse temporal distance). [ABSTRACT FROM AUTHOR]

This is the second in a series of three papers examining nanomaterial formation in laser ablation in liquids (LAL). Here we study the effect of the laser wavelength and fluence on the mass yield and size distribution of nanoparticles prepared by laser ablation of a platinum target immersed in water. For all wavelengths tested, laser fluences in the range of 10–70 J/cm2 resulted in spheroidal, nonagglomerated platinum nanoparticles with sizes ranging from 1 to 30 nm. Nanoparticle size distributions are found to be composed of two modes that are attributed to thermal vaporization and explosive boiling mechanisms. The peak of the smaller size mode remains nearly constant at 3 nm for all laser conditions, which is suggested to be due to the strong confinement of the vapor plume by the liquid. The larger size mode peaks in the range of 5–15 nm with a population that is strongly dependent on the laser parameters. It is concluded that changes in the mean size reported in many earlier studies on LAL of metal targets are a result of the relative quantity of nanoparticles from each mechanism rather than direct control over the ablation process. Additionally, it was observed that the yield of platinum nanoparticles was significantly larger for 1064 nm wavelength at fluences greater than 10 J/cm2. The maximum ablation rate was approximately 4.4 mg/h, with an estimated ablation and collection efficiency of 0.9 μg/J. Dependence of the mass yield on wavelength and fluence is seen to be dependent primarily on the extent of the explosive mechanism. [ABSTRACT FROM AUTHOR]

This is the third paper in our series studying the laser-target-liquid interactions occurring in laser ablation in liquids (LAL). Here, laser ablation of a platinum target in pure water at 355 nm wavelength is studied as a function of laser energy. We describe three distinct reaction regimes between the ablated target species and water at different laser focusing conditions. At low laser fluence (<10 J/cm2), material removal is caused by laser heating of the platinum surface and the primary products are small clusters with a large percentage of platinum atoms in a nonzero oxidation state. At intermediate fluences (10–70 J/cm2), platinum nanoparticles are the primary products. Our previous studies demonstrated that in this fluence regime ablation occurs through both thermal vaporization and explosive ejection of molten droplets. In both cases reactivity is small due to the low reactivity of platinum with water. At high fluences (>70 J/cm2), we find large, faceted particles that are attributed to the drying of PtOx gels formed by reactive plasma etching of the target. Taken together these results demonstrate that significant tunability in the target-liquid interaction is possible during nanomaterial synthesis by LAL. [ABSTRACT FROM AUTHOR]

This is the first in a series of three papers aimed at better understanding the processes that lead to nanomaterial formation during laser ablation of solid targets in liquids. Here we study the variation of the target surface morphology versus laser fluence and wavelength in order to suggest an ablation mechanism. A key finding is that an explosive ablation mechanism is prominent for a wide range of laser fluences for all wavelengths tested. Interestingly, however, ultraviolet (355 nm) and infrared (1064 nm) wavelengths show characteristically different explosive behaviors. In the infrared case, numerous large craters with diameters around 20 μm form at localized points within the laser irradiated area. In contrast, ultraviolet ablation results in a striking transition to nanoscale surface roughness across the entire irradiated area. This texture is attributed to spinodal decomposition at the molten target surface. We propose that the wavelength and fluence dependence of the ablation craters can be explained by the amount of energy absorbed in the target. The consequences of the ablation mechanism for nanomaterial synthesis are discussed. [ABSTRACT FROM AUTHOR]

A cone-shaped texture is observed when a transverse cross section of human dentin cut perpendicularly to the tooth axis is processed with KrF excimer laser radiation using appropriate processing parameters. In the present paper, a mathematical model describing the kinetics of cone growth as a function of the number of laser pulses is presented. It is shown that the fractional area occupied by cones is adequately described by a modified Johnson-Mehl-Avrami-Kolmogorov equation and that the boundary lines between cones define a Voronoi tessellation generated from the corresponding initial tubule location. This model allows the surface texture evolution in dentin to be predicted from the initial tubule distribution. [ABSTRACT FROM AUTHOR]

Micropores fabricated on organic polymer films have a wide range of applications in fields such as microfiltration, new energy, and biomedical separation. The use of laser processing technology can complete the processing of micropores on the surface of ultrathin films with high precision, but there is still some difficulty in the processing of ultrathick films. In this paper, a picosecond ultraviolet (UV) laser was used to explore the high-precision manufacturing process of micropores on the surface of ultrathick polyimide (PI) films. The effects of laser power, laser frequency, and scanning speed on the cone angle and spatter deposition area of micropores' fabrication on ultrathick PI were studied based on orthogonal experiments. The mechanism of processing micropores on ultrathick PI was analyzed by studying the deposition area and morphology of the spatter generated during the laser ablation process. It was found that high-quality micropores can be fabricated at low laser frequency and high power. [ABSTRACT FROM AUTHOR]

In this paper, the influences of surface plasmons and thermal effects on the formation of subwavelength periodic ripples on Au films irradiated by 800nm femtosecond laser pulses were studied by collinear pump-probe imaging. The spatial and temporal resolutions of the experiment were 300 nm and 1 ps, respectively. No periodic ripples were observed on the Au film during ablation by the first pump pulse, but during ablation by the second pump pulse the appearance of transient ripples was evident from a delay time of hundreds of picoseconds to several nanoseconds. These ripples, however, were not retained after solidification. When the sample was immersed in water during ablation, however, the ripples were retained. It is proposed that, during the second laser pulse irradiation, the surface defects produced by the first pulse induced surface plasmon polaritons on the Au film, which caused a modulated energy deposition and the formation of transient ripples. The weak electron-phonon coupling and significant residual heat erase these ripples after the molten surface was solidified. [ABSTRACT FROM AUTHOR]

The laser induced fluorescence (LIF) spectrum of the à ²Δ - X ²Π transition was obtained for SiCN generated by laser ablation under supersonic free jet expansion. The vibrational structures of the dispersed fluorescence (DF) spectra from single vibronic levels (SVL's) were analyzed with consideration of the Renner-Teller (R-T) interaction. Analysis of the pure bending (ν2) structure by a perturbation approach including R-T, anharmonicity, spin-orbit (SO), and Herzberg-Teller (H-T) interactions indicated considerably different spin splitting for the μ and ? levels of the X ²Π state of SiCN, in contrast to identical spin splitting for general species derived from the perturbation approach, where μ and ? specify the lower and upper levels, respectively, separated by R-T. Further analysis of the vibrational structure including R-T, anharmonicity, SO, H-T, Fermi, and Sears interactions was carried out via a direct diagonalization procedure, where Sears resonance is a second-order interaction combined from SO and H-T interactions with ΔK = ±1, Δ∑ = ±1, and ΔP = 0, and where P is a quantum number, P = K + ∑. The later numerical analysis reproduced the observed structure, not only the pure ν2 structure but also the combination structure of the ν2 and the Si-CN stretching (ν3) modes. As an example, the analysis demonstrates Sears resonance between vibronic levels, (01¹0) ? ∑(+) and (0200) μ Π½, with ΔK = ±1 and ΔP = 0. On the basis of coefficients of their eigen vectors derived from the numerical analysis, it is interpreted as an almost one-to-one mixing between the two levels. The mixing coefficients of the two vibronic levels agree with those obtained from computational studies. The numerical analysis also indicates that some of the vibronic levels show chaotic characteristics in view of the two-dimensional harmonic oscillator (2D-HO) basis which is used as the basis function in the present numerical analysis; i.e., the eigen vectors for some of the observed levels have several components of the basis, and we have not been able to give precise vibronic assignments for the levels, but just vibronically labeled, referring the largest component in their vectors. (To emphasize this situation, we do not use the word "assignment," but prefer to use "label" as the meaning of just "label," but not "assign," throughout this paper.) The latter shows that the vibronic labels of the levels are meaningless, and the P quantum number and the order of their eigen states in the P matrix block derived in the numerical analysis only characterize the vibronic levels. Comparing the constants obtained for all of the interactions with those of species showing Sears resonance and studied previously, it is found that none of them are strong, but are moderate. It is thus concluded that the chaotic appearance is not derived by any strong interaction, but is induced by complex and accidental proximities of the vibronic levels caused by the moderate interactions. [ABSTRACT FROM AUTHOR]