Femtosecond laser ablation of metals:a molecular dynamics simulation study

Using molecular dynamics (MD) methods combining with two-step radiation heating model, the mechanisms of ablation and the thermodynamic states at Ni surface under femtosecond laser irradiation are investigated. Simulation results show that the main mechanisms of ablation are evaporation and tensile stresses generated inside the target. The velocity of stress wave is predicted to be nearly equal to sound velocity. The rates of ablation at different fluences obtained from simulations are in good agreement with experimental data.     

γ-Fe2O3 nanoparticles prepared by laser ablation of a tiny wire

A new method of preparing nanoparticles by pulsed laser ablation of a tiny wire is reported. A Nd:YAG pulsed laser with a wavelength of 1064 nm was used to ablate a 0.5-mm-diameter iron wire in a sealed chamber in a flowing mixed gas of N₂, O₂, and air to generate -Fe₂O₃ nanoparticles. In the meantime, a bulk Fe sample was ablated in the same chamber with the same laser processing parameters in order to compare the effect of the bulk sizes on the production rates and the sizes of the nanoparticles. The experimental results demonstrated that the production rate of nanoparticles prepared by laser ablation of tiny wires was about eight times that of laser ablation of bulk targets with the same composition, while the sizes of the nanoparticles were basically the same. With a higher power density and/or smaller diameters of the metal wires, it is possible to obtain smaller sizes of the nanoparticles with higher production rates. PACS 81.07.Wx     

Deposition of nanoparticles during laser ablation of nanoparticle-containing targets

We analyze the morphology of ablated nanoparticles after their laser-induced deposition on various substrates. We show that, at moderate laser intensity of the 210 ps pulses on the surface of nanoparticle-containing materials (<5×10⁹ W?cm^?2), the deposited material remains approximately the same as the initial nanoparticles. We compare these deposited nanoparticles with the debris obtained by the laser ablation of bulk material of the same origin as nanoparticles at different intensities of laser radiation. The presence of nanoparticles in laser plumes allowed for analyzing high-order nonlinear optical properties of nanoparticles. The efficient high-order harmonic generation was achieved during propagation of femtosecond pulses through such plasmas.     

VUV 157 nm laser ablation of composite structures

We report on the laser ablation of composite prismatic structures using a vacuum ultraviolet (VUV) 157 nm F₂ laser. Polycarbonate and CR-39 substrates have been intentionally seeded with silver wires and silicon carbide whiskers respectively. The seed particles remain attached to the underlying substrate after laser ablation, forming composite silver-polycarbonate and silicon carbide-CR-39 interfaces. Strong optical absorption at 157 nm in the polymeric substrates allows precise control over the depth between the base of the substrate and composite interface. The surface roughness of the as-received seed particles has a significant effect on the final surface quality of the ablated structures. The textured surface on the silicon carbide whiskers is resolved on the walls of the ablated structures. This is in contrast to the composite structures formed using silver wires, which have a comparatively smoother surface.     

Femtosecond laser ablation of Au film around single pulse threshold

Ablation process of 1-kHz femtosecond lasers (pulse duration of 148 fs, wavelength of 775 nm) of Au film on silica substrates is studied. The thresholds for single and multi pulses can be obtained directly from the relation between the squared diameter D2 of the ablated craters and the laser fluence φ0. From the plot of the accumulated laser fluence Nφth(N) and the number of laser pulses N, incubation coefficient of Au film is obtained to be 0.765. Some experimental data obtained around the single pulse threshold axe in good agreement with the theoretical calculation.     

Fabrication of two types of one-dimensional Si–C nanostructures by laser ablation

Two types of one-dimensional (1D) nanostructures—amorphous silicon carbide (SiC) nanowires, 5–30 nm thick and 0.5–2 μm long, and carbon nanotubes (CNTs) filled completely with crystalline SiC nanowires, 10–60 nm thick and 2–20 μm long—were synthesized by the laser ablation of carbon-silicon targets in the presence of high-pressure Ar gas up to 0.9 MPa. All the CNTs checked by transmission electron microscopy contained SiC, and no unfilled CNTs were produced. We discuss the growth of the two nanostructures based on the formation of molten Si–C composite particles and their instabilities leading to the precipitation of Si and C.     

The study of nanojoule femtosecond laser ablation on organic glass

The Ti:sapphire oscillator is used to realize structural change in an organic glass (polymethyl -methacrylate (PMMA)). Single pulse fluence threshold of PMMA and the relation of the breakdown threshold with different numerical aperture objectives are determined using a formula deduced from an existent equation. Three-dimensional dots in the organic glass is performed at the same time.     

Wavelength dependence of soft tissue ablation by using pulsed lasers

Pulsed laser ablation of soft biological tissue was studied at 10.6-,2.94-,and 2.08-μm wavelengths.The ablation effects were assessed by means of optical microscope,the ablation crater depths were measured with reading microscope.It was shown that Er:YAG laser produced the highest quality ablation with clear, sharp cuts following closely the spatial contour of the incident beam and the lowest fluence threshold.The pulsed CO_2 laser presented the moderate quality ablation with the highest ablation efficiency.The craters drilled with Ho:YAG laser were generally larger than the incident laser beam spot,irregular in shape,and clearly dependent on the local morphology of biotissue.The ablation characteristics,including fluence threshold and ablation efficiency,varied substantially with wavelength.It is not evident that water is the only dominant chromophore in tissue.     

Model predictive inverse method for recovering boundary conditions of two-dimensional ablation

A model predictive inverse method (MPIM) is presented to estimate the time- and space-dependent heat flux onthe ablated boundary and the ablation velocity of the two-dimensional ablation system. For the method, first of all, therelationship between the heat flux and the temperatures of the measurement points inside the ablation material is establishedby the predictive model based on an influence relationship matrix. Meanwhile, the estimation task is formulated as aninverse heat transfer problem (IHTP) with consideration of ablation, which is described by an objective function of thetemperatures at the measurement point. Then, the rolling optimization is used to solve the IHTP to online estimate theunknown heat flux on the ablated boundary. Furthermore, the movement law of the ablated boundary is reconstructedaccording to the estimation of the boundary heat flux. The effects of the temperature measurement errors, the numberof future time steps, and the arrangement of the measurement points on the estimation results are analyzed in numericalexperiments. On the basis of the numerical results, the effectiveness of the presented method is clarified.     

Nanoparticle preparation of quinacridone and β-carotene using near-infrared laser ablation of their crystals

Quinacridone nanoparticles with a mean size of about 200 nm are successfully prepared using nanosecond near-infrared (NIR) laser ablation of its microcrystalline powders in heavy water. The absorption spectra of the formed colloidal solutions depend on the excitation wavelengths, which is eventually ascribed to number and energy of absorbed photons. β-carotene has low photostability and is easily decomposed upon UV/VIS laser ablation of its solid, while its nanoparticles are prepared utilizing this NIR laser ablation technique. The advantage of nanoparticle preparation by NIR laser ablation is discussed.     

Measurement of crater geometries after laser ablation of bone tissue with optical coherence tomography

The feasibility of measuring crater geometries by use of optical coherence tomography(OCT)is examined. Bovine shank bone on a motorized translation stage with a motion velocity of 3 mm/s is ablated with a pulsed CO_2 laser in vitro.The laser pulse repetition rate is 60 Hz and the spot size on the tissue surface is 0.5 mm.Crater geometries are evaluated immediately by both OCT and histology methods after laser irradiation.The results reveal that OCT is capable of measuring crater geometries rapidly and noninvasively as compared to histology.There are good correlation and agreement between crater depth estimates obtained by two techniques,whereas there exists distinct difference between crater width estimates when the carbonization at the sides of craters is not removed.     

Mechanisms of femtosecond laser ablation of dielectrics revealed by double pump–probe experiment

We study experimentally the electronic excitation mechanisms involved in the breakdown and ablation of wide band gap dielectrics. A femtosecond pump–probe interferometry technique, with 100 fs temporal resolution, allows measuring the modification of refractive index induced by ultra-short intense laser pulses. To get more information in the complex process of excitation and relaxation mechanisms involved during and after the interaction, we use a sequence of two excitation pulses: a first short pulse at 400 nm excites a controlled density of carriers, and a second one at 800 nm with variable pulse duration, from 50 fs to 10 ps, reaches an excited solid. In Al₂O₃, we show that the total density of carriers never exceeds the sum of the densities excited by the two pulses sent independently. This means that the second pulse deposits further energy in the material by heating the previously excited carriers, and that no electronic multiplication occurs. On the other hand, in SiO₂, it is possible, under specific conditions, to observe an increase of carrier density due to impact ionization. All these results demonstrate that the avalanche process, which is often invoked in the laser breakdown literature, does not play a dominant role in optical breakdown induced by short pulses.     

Nanoparticle formation during laser ablation of metals at different pressures of surrounding noble gases

We demonstrate that the nanoparticle formation during laser ablation of metals by short (of a few tens of ps) laser pulses strongly depends on the concentration of surrounding gas. While, at vacuum conditions, nanoparticle formation shows very “sharp” atomic force microscope images of aggregated clusters, following with clear appearance of plasmon resonance on the absorption spectra of deposited films, an addition of gas particles starts to decrease the probability of cluster formation. This process shows a threshold for both helium (33 torr) and xenon (12 torr) above which no surface plasmon resonance and correspondingly no observable nanoparticles on the deposited surfaces were detected. The destruction of nanoparticle formation was attributed to the negative influence of surrounding gas particles on ablated particles aggregation.     

Generation of CdS clusters using laser ablation: the role of wavelength and fluence

The formation of cationic clusters in the laser ablation of CdS targets has been investigated as a function of wavelength and fluence by mass spectrometric analysis of the plume. Ablation was carried out at the laser wavelengths of 1064, 532, 355, and 266 nm in order to scan the interaction regimes below and above the energy band gap of the material. In all cases, the mass spectra showed stoichiometric Cd _n S _n ⁺ and nonstoichiometric Cd _n S _n−1⁺, Cd _n S _n+1⁺, and Cd _n S _n+2⁺ clusters up to 4900 amu. Cluster size distributions were well represented by a log-normal function, although larger relative abundance for clusters with n=13, 16, 19, 34 was observed (magic numbers). The laser threshold fluence for cluster observation was strongly dependent on wavelength, ranging from around 16 mJ/cm² at 266 nm to more than 300 mJ/cm² at 532 and 1064 nm. According to the behavior of the detected species as a function of fluence, two distinct families were identified: the “light” family containing S₂⁺ and Cd⁺ and the “heavy” clusterized family grouping Cd₂⁺ and Cd _n S _m ⁺. In terms of fluence, it has been determined that the best ratio for clusterization is achieved close to the threshold of appearance of clusters at all wavelengths. At 1064, 532, and 355 nm, the production of “heavy” cations as a function of fluence showed a maximum, indicating the participation of competitive effects, whereas saturation is observed at 266 nm. In terms of relative production, the contribution of the “heavy” family to the total cation signal was significantly lower for 266 nm than for the longer wavelengths. Irradiation at 355 nm in the fluence region of 200 mJ/cm² has been identified as the optimum for the generation of large clusters in CdS.     

Au–Cu nanoparticles produced by laser ablation of mixtures of Au and Cu microparticles

In this study we investigate the possibility of producing alloy nanoparticles (NP) from mixtures of elemental microparticle (MP) powders using the laser ablation of microparticle (LAM) process. Mixtures of Au and Cu particles with a diameter of 1.5–2.0 m were fed in aerosol form into a laser ablation cell and ablated using a pulsed laser. The resulting NP were collected electrostatically and characterized using TEM. The NP were spherical and crystalline with a size that depended on the collection location but ranged from 2 to 15 nm. Using TEM/SAD, it was determined that the NP had a face-centered cubic (fcc) crystal structure and, with EDS, it was found that individual NP consisted of both Au and Cu. These experimental results confirm previous numerical models that suggested that it might be possible to form alloy NP from mixtures of elemental MP using the LAM process.     

Fabrication of compositionally graded thin films by gravity-assisted pulsed laser ablation

A compositionally graded thin film of FeSi₂ was fabricated by a gravity-assisted pulsed laser ablation (GAPLA) system. By this method, a compositionally graded structure was successfully produced under a gravity field of 5400 G. We demonstrate that the atomic fraction of Fe, the heavier component of the thin film measured by scanning electron microscope/energy dispersive X-ray (SEM-EDX), showed increasing spatial distribution with the direction of gravity. We found that optimal laser fluence exists to give a thin film having the largest possible spatial compositional gradient. We found that surface energy density on the substrate surface is the key parameter to control the composition distribution. Furthermore, the ratio of Fe/Si of the film did not match that of the target. This result shows that the Si component is selectively etched during the film-forming process. Relatively high laser fluence as well as a very narrow space between the target and the substrate are essential to etch the film once it is deposited, in order to re-ionize and etch Si selectively while gravity accelerates both Fe and Si particles to the direction of gravity. We hypothesize that this process accounts for both the change in the stoichiometry and the formation of composition distribution.     

Formation of absorbing heterogeneous plasma layer by femtosecond laser-induced melting and ablation of silicon

The experimental study of absorption in silicon in infrared and visible spectral ranges, where the photon energy is less or more than the bandgap width, is performed by means of the ultrafast interferometry technique. The exactly solvable model in the electromagnetic of heterogeneous lossy plasma layer was developed. The density of carriers, their frequency of collisions, absorbing depth of the probing waves, real and imaginary parts of dielectric function of nonuniform layer and their spatial gradients are determined from the reflectance data by means of this model subject to the pump fluence. The heterogeneity-induced effects are visualized due to comparison of obtained plasma parameters with those calculated in the framework of homogeneous plasma model It is shown that in the intensity range near thresholds of melting and ablation the absorption, occurring in both cases mainly within a thin (∼10 nm) absorbing layer (similarly to metals), is due to free carrier intraband absorption.     

The effects of ambient oxygen pressure on the 1.54 μm photoluminescence of Er-doped silicon-rich silicon oxide films grown by laser ablation of a Si+Er target

We fabricated Er-doped silicon-rich silicon oxide (SRSO:Er) films by pulsed laser deposition. A Si+Er target consisting of an Er metallic strip and a silicon disk was adopted with a goal to achieve a convenient control of the Er and oxygen density in the film. We found that the photoluminescence (PL) at 1.54 m is highly dependent on the ambient oxygen pressure, which determines the relative ratio of Si-Si, SiO_x, and SiO₂ phase in the film. The PL intensity increased drastically with increase in the annealing temperature and reached the maximum intensity at 500 °C. PACS 81.15.Fg; 81.07.-b     

Study of the dynamics of the absorption spectra of human tooth enamel and dentine under heating and ablation by submillisecond pulse radiation of an erbium laser with a generation wavelength of 2.79 μm

We studied the absorption spectrum of intact human tooth enamel and dentine in the range of 0.26–10 μm. We present the infrared absorption spectra of destruction products of human tooth enamel and dentine by submillisecond laser pulses on a crystal of yttrium-scandium-gallium garnet, activated by chrome and erbium ions with a wavelength of 2.79 μm. We discuss the effect of water spraying on the mechanism of laser ablation and the infrared absorption spectra. We report for the first time transformations observed in the absorption spectra of human tooth enamel in the wavelength range of 2.5–3.5 μm under its heating to +700°C.     

Formation of cavitation-induced pits on target surface in liquid-phase laser ablation

It is well known that a crater is formed on the target surface by the irradiation of intense laser pulses in laser ablation. In this work, we report that additional pits are formed on the bottom surface of the ablation crater due to the collapse of a cavitation bubble in liquid-phase laser ablation. We observed the formation of several cavitation-induced pits when the fluence of the laser pulse used for ablation was approximately 5 J/cm². The number of cavitation-induced pits decreased with the laser fluence, and we observed one or two cavitation-induced pits when the laser fluence was higher than 10 J/cm². In addition, we examined the influence of the liquid temperature on the formation of cavitation-induced pits. The collapse of the cavitation bubble was not observed when the liquid temperature was close to the boiling temperature, and in this case, we found no cavitation-induced pits on the bottom surface of the ablation crater. This experimental result was discussed by considering the cavitation parameter.