Ablation Regime Research Articles

Formation of copper tin sulfide films by pulsed laser deposition at 248 and 355 nm

The influence of the laser wavelength on the deposition of copper tin sulfide (CTS) and SnS-rich CTS with a 248-nm KrF excimer laser (pulse length τ = 20 ns) and a 355-nm frequency-tripled Nd:YAG laser (τ = 6 ns) was investigated. A comparative study of the two UV wavelengths shows that the CTS film growth rate per pulse was three to four times lower with the 248-nm laser than the 355-nm laser. SnS-rich CTS is more efficiently ablated than pure CTS. Films deposited at high fluence have submicron and micrometer size droplets, and the size and area density of the droplets do not vary significantly from 248 to 355 nm deposition. Irradiation at low fluence resulted in a non-stoichiometric material transfer with significant Cu deficiency in the as-deposited films. We discuss the transition from a non-stoichiometric material transfer at low fluence to a nearly stoichiometric ablation at high fluence based on a transition from a dominant evaporation regime to an ablation regime.

Dependence of silicon ablation regimes on fluence during ultrafast laser irradiation

AbstractModels and experiments were developed to study femtosecond laser ablation of silicon using 800 nm, 40 fs pulses with fluences ranging from 0.5 to 35 J/cm2. At low fluences, ablation was found to occur due to bubble formation and splashing within the melt layer. At higher fluences, it was found that the ablation depth exceeded the melt layer depth due to shockwave ablation. The variation in ion flux and ion velocity was also studied both experimentally and theoretically. It was found that the variation in ion flux is mainly dependent on the variation in the average charge state, with only a small variation in the total number of ions above $\sim \!\!1.5\; \,{\rm J/c}{{\rm m}^2}$. Comparisons between the theoretical and experimental ion flux showed that higher charge state ions received greater portion of the laser energy compared with lower charge state ions.

Molecular signatures in femtosecond laser-induced organic plasmas: comparison with nanosecond laser ablation.

During the last few years, laser-induced breakdown spectroscopy (LIBS) has evolved significantly in the molecular sensing area through the optical monitoring of emissions from organic plasmas. Large efforts have been made to study the formation pathways of diatomic radicals as well as their connections with the bonding framework of molecular solids. Together with the structural and chemical-physical properties of molecules, laser ablation parameters seem to be closely tied to the observed spectral signatures. This research focuses on evaluating the impact of laser pulse duration on the production of diatomic species that populate plasmas of organic materials. Differences in relative intensities of spectral signatures from the plasmas of several organic molecules induced in femtosecond (fs) and nanosecond (ns) ablation regimes have been studied. Beyond the abundance and origin of diatomic radicals that seed the plasma, findings reveal the crucial role of the ablation regime in the breakage pattern of the molecule. The laser pulse duration dictates the fragments and atoms resulting from the vaporized molecules, promoting some formation routes at the expense of other paths. The larger amount of fragments formed by fs pulses advocates a direct release of native bonds and a subsequent seeding of the plasma with diatomic species. In contrast, in the ns ablation regime, the atomic recombinations and single displacement processes dominate the contribution to diatomic radicals, as long as atomization of molecules prevails over their progressive decomposition. Consequently, fs-LIBS better reflects correlations between strengths of emissions from diatomic species and molecular structure as compared to ns-LIBS. These new results entail a further step towards the specificity in the analysis of molecular solids by fs-LIBS.

Investigation on the Ablation of thin Metal Films with Femtosecond to Picosecond-pulsed Laser Radiation

Ultrafast-pulsed laser radiation enables defined removing of different materials with very high processing accuracy due to the short interaction time of the radiation with the material, and the reduced heat affected zone compared to the removal with nanosecond laser radiation. For further processing improvements, a deep understanding of the involved processes of the laser radiation material interaction, especially the complex ablation phenomena, is necessary. Therefore, the ablation of metal films (d = 0.3–2μm) of gold, platinum, aluminum, nickel and molybdenum was investigated using single-pulsed focused laser radiation (λ = 1030nm) in dependence on the pulse duration (τH = 200 fs–15ps). The metals were chosen due to their different thermophysical properties, especially their electron-phonon-relaxation time. Two different ablation regimes, so called strong- and gentle-ablation, are observed for all investigated materials. To explain the experimental results, theoretical investigations based on the thermophysical properties as well as on the Two-Temperature-Model, including a temperature dependent reflectance, were performed.

Train wheel diagnostics by laser ultrasonics

The integrity of the wheel is very important for the safety of railway. In this paper a laser-ultrasonic diagnostic measurement procedure has been designed for the inspection of the train wheels with the aid of a FE-model simulating the ultrasound propagation within the wheel itself.The laser-ultrasonic method exploits an air-coupled ultrasonic probe that detects the ultrasonic waves generated by a high-energy pulsed laser. As a result, the measurement chain is completely non-contact, from generation to detection, this making it possible to considerably speed up the testing set-up time and make it more flexible. This is an important advantage with respect to the conventional NDT technologies currently applied to train component diagnostics, as contact phase array methods. Laser ultrasonics is a complete remote technique since both the laser source and the receiving probe are installed in the proximity of the wheel directly on the bogie and therefore it is a more flexible technology with respect to standard techniques applied for wheel train diagnostics, as phased array.The applications of laser-ultrasonic technique available in the state-of-the-art work with high energy ultrasonic waves to guarantee good signal to noise ratio. Therefore, conventional laser-ultrasonic systems operates under ablative regime that assures high energy ultrasonic waves generation. On the other hand, the ablation produces damages on the surface of the component inspected. In this paper, it has been demonstrated that it is possible to work with lower energy waves, i.e. in the limit between ablative and thermo-elastic regime, if the experiment is properly designed on the basis of a numerical model. This operation regime allowed to guarantee a material removal below the threshold admitted in rail wheel application. The diagnostic procedure developed has been applied for the inspection of train wheels provided by the Italian railway company Trenitalia, on which dominant wheel failure cracks have been expressly created.

Determination of laser beam intensity to maximize amplitude of ultrasound generated in ablation regime via monitoring plasma-induced air-borne sound

The laser ultrasonic technique in the ablation regime was studied for the effective excitation of ultrasound. First, the optimal laser beam intensity to maximize the amplitude of ultrasound was obtained. This is useful because the amplitude of ultrasound does not always increase as the laser beam intensity increases due to the plasma shielding effect. When the laser energy is fixed, for a steel specimen, the maximum ultrasonic amplitude is obtained at the laser beam intensity in the range of 10–20 GW/cm2. In addition, an inline method to obtain the optimal laser beam intensity was proposed based on the phenomena that the amplitude of the air-borne sound induced by plasma is inversely proportional to the amplitude of the generated ultrasound. Experimental results verified the usefulness of the proposed method by showing that the amplitude of ultrasound reached its maximum when the plasma-induced airborne sound disappeared.

Thin film interface stresses produced by high amplitude laser generated surface acoustic waves

Surface acoustic waves (SAWs) have been explored for nondestructive metrology of thin film elastic properties and thickness due to confinement of their energy within a shallow depth from a material surface. In this paper, we study the dynamic interfacial stresses produced by high amplitude SAWs generated by a strongly ablative source in a thin film-substrate system, with the goal of investigating the possibility of inducing thin film delamination at high loading rates. For modeling purposes, we represent the mechanical loading resulting from the pulsed laser-sample interaction in the ablative regime by an equivalent compressive surface load, and the resulting stresses and particle velocities induced by the generated SAWs are calculated using a linear finite element model. We explore the numerical model to study the dependence of the film-substrate interface tractions on the ratio of the film thickness and SAW wavelength for a soft film on a stiff substrate. Furthermore, by matching the numerical results obtained from the finite element model with experimental results, we are able to predict the dynamic interfacial stresses for a copper film on a fused silica substrate produced by SAWs excited by a high power pulsed laser line source. This study has implications for exploring SAWs in the characterization of interfacial failure in thin-film substrate systems.

Visualization of surface transformations during laser ablation of solids by femtosecond pump–probe time-resolved microscopy

A femtosecond time-resolved microscope (fs-TRM) based on pump–probe excitation has been used to follow the dynamic of the processes occurring during laser–matter interaction, from initial surface alterations to final solidification through transient melting. The time-resolved microscope described in the manuscript has been designed to allow a precise control of the excitation beam to cover ranges below and above the plasma formation energy, and a large temporal variation in the pump–probe delay to include the different timescales of the different processes occurring up to the plasma formation. The microscope has been demonstrated to be robust and allows the subpicosecond monitoring of laser ablation single-shot events, of importance in the analysis of ultra thin layers, or biological tissues.The fs-TRM excites (pump) the sample with 35-fs laser pulses at 800nm and follows the processes by a second (probe) beam at 400nm. The relative delay between both beams allows the acquisition of pictures with a temporal resolution of 200fs up to 3ns after the reaching of the pump pulse. In the ablative regime near the ablation threshold, transient surface reflectivity patterns (dynamic Newton fringes) are observed from a ps to ns time-scale. The timescale and number of such rings are affected by the fluence value. Significant differences between metals (Al, Cu and Sn), semiconductors (Si) and polymers (polytetrafluoroethylene and polyurethane) have been also observed in the transformation patterns.

Femtosecond laser ablation and nanoparticle formation in intermetallic NiAl

The ablation behavior of a stoichiometric intermetallic compound β-NiAl subjected to femtosecond laser pulsing in air has been investigated. The single-pulse ablation threshold for NiAl was determined to be 83±4mJ/cm2 and the transition to the high-fluence ablation regime occurred at 2.8±0.3J/cm2. Two sizes of nanoparticles consisting of Al, NiAl, Ni3Al and NiO were formed and ejected from the target during high-fluence ablation. Chemical analysis revealed that smaller nanoparticles (1–30nm) tended to be rich in Al while larger nanoparticles (>100nm) were lean in Al. Ablation in the low-fluence regime maintained this trend. Redeposited material and nanoparticles remaining on the surface after a single 3.7J/cm2 pulse, one hundred 1.7J/cm2 pulses, or one thousand 250mJ/cm2 pulses were enriched in Al relative to the bulk target composition. Further, the surface of the irradiated high-fluence region was depleted in Al indicating that the fs laser ablation removal rate of the intermetallic constituents in this regime does not scale with the individual pure element ablation thresholds.

Light-curing polymers for laser plasma generation

Solid rather than liquid media are used in pulsed laser plasma generators despite sophisticated transportation and dosing system need for a long-term operation. Liquid media could be more preferable due to transfer and dosing (down to 10-14 L) being well developed, but plasma generation of those results in intense droplet formation and kinetic energy losses. Combination of liquids transportation advantages and solids plasma generation efficiency might resolve this trade-off. Liquid-to-solid transition can be induced by cooling down to sublimation temperature, thermo-, photo- or electron induced polymerization (curing). Light cured polymers seem to be very useful as active media for plasma generators, since they can be solidified very fast (ca. 30 ms) just before impact. We considered experimentally several UV- curing polymer and mixtures ablation regimes and supply schemes for laser plasma generation. The best results were obtained for liquid polymer at high-power pulsed irradiation matching curing optimum wavelength.

Non-contact measurement of elastic modulus by using laser ultrasound

Non-contact measurement of elastic modulus is necessary for the in-line assessment of material in harsh environments such as high temperature. In this paper, a fully non-contact method to measure elastic modulus is proposed based on the laser ultrasonic technique (LUT) that uses a short-pulsed laser to generate ultrasound and the other laser coupled to an interferometer using a photorefractive crystal to detect the ultrasonic wave displacement. Basically, this method measures velocities of shear wave and longitudinal wave to obtain the elastic modulus. The uniqueness is that the velocity of shear wave is measured in the thermo-elastic regime first and then the velocity of longitudinal wave is measured in the ablation regime. This is because the strong mode of generated ultrasound is the shear wave in the thermo-elastic regime while the longitudinal wave in the ablation regime. Regime change can be achieved simply by switching the laser power, with no change in the measurement setup. In order to demonstrate the usefulness of the proposed method, the elastic modulus of aluminum casting alloy has been measured and the results were compared with a conventional contact method and a destructive tensile test. They showed good agreement with each other, which verified the usefulness of the proposed noncontact elastic modulus measurement method.

Optical characterization of SERS substrates based on porous au films prepared by pulsed laser deposition

The SERS (surface enhanced Raman spectroscopy) substrates based on nanocomposite porous films with gold nanoparticles (Au NPs) arrays were formed using the method of the pulsed laser deposition from the back low-energy flux of erosion torch particles on the glass substrate fixed at the target plain. The dependencies of porosity, and morphology of the surface of the film regions located near and far from the torch axis on the laser ablation regime, laser pulses energy density, their number, and argon pressure in the vacuum chamber, were ascertained. The Au NPs arrays with the controllable extinction spectra caused by the local surface plasmon resonance were prepared. The possibility of the formation of SERS substrates for the detection of the Rhodamine 6G molecules with the concentration 10-10 Mol/L with the enhancement factor 4ċ107 was shown.

Microstructure and gas-surface interaction studies of a low-density carbon-bonded carbon fiber composite in atmospheric entry plasmas

Carbon-bonded carbon fiber (CBCF) composites are a cost-effective solution for the production of low-density carbon-phenolic Thermal Protection Systems (TPS). This new TPS for spacecraft requires new experimental data for model development and validation. Ablation experiments of a CBCF composite were carried out in an inductively-coupled plasma generator to assess the performance in high-enthalpy flows. Surface temperatures up to 2900K led to strong surface ablation and test samples of hemispherical shape responded with constant surface temperatures and recession rates. Cylindrical samples experienced a continuous surface temperature increase. Emission spectra of the cyano radical CN were indicative of a 4–5mm reactive boundary layer. Deviation from thermal equilibrium was found by comparison to simulated spectra. Micrographs revealed an oxidation zone in the order of 0.2mm at the surface, suggesting a gas phase diffusion controlled ablation regime. Strong corrosion of the fibers in nitrogen plasma is attributed to wall nitridation.

Laser cutting of carbon fiber reinforced plastic using a 30 kW fiber laser

Today, industrial usage of carbon fiber reinforced plastic (CFRP) is steadily increasing, with an amount of 67 000 ton/yr [“Carbon fibers and carbon fiber reinforced plastics (CFRP)—A global market overview,” Report, Research and Markets Ltd., Dublin, Ireland, 2013]. Products such as the Boeing 787 and Airbus A350 in the aerospace sector, as well as the BMW i3 from the automotive industry consist of more than 50% of CFRP in their structural weight. At the same time, these products also have comparatively high production volumes, e.g., >10 000 cars/yr in the case of the BMW i3 [“BMW undertakes global launch of i3 EV,” see http://www.ihs.com/products/global-insight/industry-economic-report.aspx?id=1065981621, May 20, 2014]. Therefore, a higher degree in automation and cost-efficiency is needed in production. Due to the highly abrasive carbon fibers, conventional machining processes result in short tool life and high costs. Therefore, laser cutting of CFRP as a wear-free alternative has lately become the focus of several research groups. Two different approaches are commonly chosen: cutting by short- and ultra-short pulsed laser systems to reach a process regime of cold ablation and cutting with continuous wave (cw) lasers at high cutting speeds. For the latter approach, it has already been shown that by increasing power and cutting speed, the heat affected zone (HAZ) can be reduced due to less time allowed for heat conduction [Bluemel et al., “Laser machining of CFRP using a high power laser—Investigation on the heat affected zone,” in Proceedings of 15th European Conference on Composite Materials, Venice, Italy (2012)]. Graf and Weber introduced a perpendicular heat flow model, calculating that the required intensity to cut 2 mm of CFRP with a HAZ of 10 μm using a cw laser is 109 W/cm2. The required cutting speed is 8.3 m/s [T. Graf and R. Weber, “Laser applications from production to machining of composite materials,” in Proceedings of EALA, Bad Nauheim, Germany (2012), pp. 289–299]. In this paper, experiments using an ultra-high power fiber laser system of 30 kW to cut CFRP laminates are presented. Although it is not possible to fully achieve the intensities proposed by Graf and Weber, the intensities of approx. 108 W/cm2 of the setup still allow for a practical validation of the CFRP cutting at very high laser power. Due to the high intensities, high cutting speeds per laser pass are necessary. A special experimental setup is chosen with a rotational movement of the specimen, reaching a feed rate of 85 m/s. The heat affected zone was considerably reduced to 78 μm with the 30 kW system, 10% lower than with a 5 kW system under comparable conditions. Although today no scanner systems are available that could handle these high intensities at such high cutting speeds, the experiments still show that processing of CFRP with cw laser systems at highest power has a potential in order to reduce the heat damage to the material.

The Experimental Substantiation of Radiofrequency Vein Ablation Regimes

The Experimental Substantiation of Radiofrequency Vein Ablation Regimes

Optical Characterization of SERS Substrates Based on Porous Au Films Prepared by Pulsed Laser Deposition

The SERS (surface enhanced Raman spectroscopy) substrates based on nanocomposite porous films with gold nanoparticles (Au NPs) arrays were formed using the method of the pulsed laser deposition from the back low‐energy flux of erosion torch particles on the glass substrate fixed at the target plain. The dependencies of porosity, and morphology of the surface of the film regions located near and far from the torch axis on the laser ablation regime, laser pulses energy density, their number, and argon pressure in the vacuum chamber, were ascertained. The Au NPs arrays with the controllable extinction spectra caused by the local surface plasmon resonance were prepared. The possibility of the formation of SERS substrates for the detection of the Rhodamine 6G molecules with the concentration 10−10 Mol/L with the enhancement factor 4·107 was shown.

Comparison on Morphological and Optical Properties of TiO<SUB>2</SUB> Thin Films Grown by Single-Pulse and Multi-Pulse Laser Ablation

TiO2 thin films were prepared on glass substrates using the PLD (Pulsed Laser Deposition) technique. In order to carry out the ablation process, a Nd:YAG laser was used emitting in 1064 nm wavelength at 10 Hz repetition rate, set up for operating in both single-pulse and multi-pulse regimes. A comparison of the deposition rate, the optical and morphological properties of the layers obtained from both ablation regimes was made, which showed that the multi-pulsed ablation produced layers with a higher surface quality and better optical properties.

The thermo-mechanical behavior of thin metal films under nanosecond laser pulse excitation above the thermoelastic regime

This article presents experimental results supported by advanced three-dimensional modeling for the dynamics emerging from the interaction of nanosecond laser pulses with thin metal films on dielectric substrates, especially at the melting and ablation regimes. Matter dynamics, such as the generation and propagation of surface acoustic waves and permanent deformations, are imaged with the use of a very high spatial and temporal resolution interferometric method accompanied by white-light interferometry. A three-dimensional finite element model is developed aiming to fully describe the spatiotemporal dynamics and predict with high accuracy the thermo-mechanical phenomena around melting and ablation regimes where phase changes take place. The ability of very high spatial and temporal resolution, the whole-field three-dimensional imaging as well as the simultaneous study of the laser pulse–thin film interaction regimes, makes this study valuable for applications where detailed knowledge of the thermo-mechanical behavior of matter under pulsed laser excitation is critical.

Effect of geometry measurements on characteristics of femtosecond laser ablation of HR4 nickel alloy

This paper presents an investigation on ablation characteristics of HR4 nickel cast alloy with a Ti:Sapphire femtosecond laser (100fs pulse width, 800nm wavelength and 1kHz repetition rate). The ablation characteristics including laser material interaction parameters, i.e. ablation threshold, optical penetration depth and incubation factor are estimated based on ablated geometries from two ablation methods, i.e. spot ablation and line ablation. The work shows that the single pulse ablation threshold is dependent on the laser fluence and the number of pulses (NOP). The ablation threshold decreases with the NOP, where it reaches an asymptotical value due to incubation effect. By varying the laser fluence, two ablation regimes are identified which are the low fluence ablation regime and the high fluence ablation regime. The former is governed by optical penetration depth; and the latter is governed by the energy penetration depth. It is also observed that the laser material interaction parameters obtained based on ablation diameters through the line ablation agree with the values obtained through the spot ablation but not with those obtained based on ablated depth through the line ablation. It is found that the evaluation of the laser material interaction parameters based on ablated diameters is more reliable compared to those based on ablated depth. The ablation threshold (low fluence ablation region) for the HR4 nickel alloy is estimated to be Fth1=0.39J/cm2 with the incubation factor, ξ=0.8 and optical penetration depth of 47.3nm.

Dynamics of femtosecond laser absorption of fused silica in the ablation regime

We investigate the ultrafast absorption dynamics of fused silica irradiated by a single 500 fs laser pulse in the context of micromachining applications. A 60-fs-resolution pump-probe experiment that measures the reflectivity and transmissivity of the target under excitation is developed to reveal the evolution of plasma absorption. Above the ablation threshold, an overcritical plasma with highly non-equilibrium conditions is evidenced in a thin layer at the surface. The maximum electron density is reached at a delay of 0.5 ps after the peak of the pump pulse, which is a strong indication of the occurrence of electronic avalanche. The results are further analyzed to determine the actual feedback of the evolution of the optical properties of the material on the pump pulse. We introduce an important new quantity, namely, the duration of absorption of the laser by the created plasma, corresponding to the actual timespan of laser absorption by inverse Bremsstrahlung. Our results indicate an increasing contribution of plasma absorption to the total material absorption upon raising the excitation fluence above the ablation threshold. The role of transient optical properties during the energy deposition stage is characterized and our results emphasize the necessity to take it into account for better understanding and control of femtosecond laser-dielectrics interaction.