Laser Ablation Craters Research Articles

Laser speckle interferometry approach for measuring the first wall surface micro-morphology in fusion devices with only single interferogram based on Hilbert transform

During the operation of the EAST device, the surface morphology of the first wall in will change due to the erosion and redeposition from plasma-wall interaction (PWI). In this paper, a novel phase extraction approach using only single interferogram based on twice Hilbert transform (THT) has been proposed and implemented. The current approach has a significant advance for eliminating the phase-shifting errors in principle, but the problems of phase flipping and loss due to ignoring non-physical negative frequency spectrum must to be overcome. To address these problems a judgement method is proposed in current investigation to distinguish the regions of the negative spatial frequency. Then a binary matrix was developed to modify the wrapped phase of the measured interferogram. The validation of the proposed THT approach has been demonstrated by the micro-morphology reconstruction of laser ablation craters on molybdenum (Mo) and tungsten (W) tiles, which are commonly used as the first wall in EAST device. Results indicate that the THT approach has a reasonable measurement accuracy with relative errors around 10%. Also, the current THT approach presents an enhanced robust under the EAST-like vibrational environments.

Analysis of an Al-Sn-Cu alloy by calibration-free laser-induced breakdown spectroscopy under varying sample temperature conditions

This study investigates the impact of sample temperature Ts on Laser-Induced Breakdown Spectroscopy (LIBS) emission characteristics and Calibration-Free LIBS (CF-LIBS) quantitative analysis in vacuum and air. Using an aluminum‑tin‑copper alloy, we analyze spectral lines within a 20 °C to 170 °C temperature range. Evaluation encompasses emission line intensities, plasma parameters, expansion imagery, and laser ablation crater morphologies. CF-LIBS analysis underlines that elevated Ts maintains quantitative accuracy even for minor elements with weak lines. In atmospheric conditions, tin segregation occurs, yet CF-LIBS remains robust. These results highlight CF-LIBS's potential for varied applications by mitigating Ts-induced effects.

Laser-produced craters in minerals of a palladium ore sample

Laser-induced breakdown spectroscopy (LIBS) is an emerging technique in geochemistry that allows rapid in-situ analysis of the elemental composition and concentration of minerals by laser ablation of the material surface and measurement of the light emitted by the resulting plasma. However, this type of application is still under development for geochemical analyses. Indeed, it is still difficult to know how minerals are ablated under laser pulses in the context of LIBS geochemical analysis using a high-power Q-switched Nd:YAG laser operating at 1064 nm with pulse durations on the order of nanoseconds. Important questions remain unanswered regarding the volume sampled by the laser beam on the minerals to be analyzed, as well as the plasmas induced by the laser on the minerals in air at atmospheric pressure. The objective of this work is to provide insight into laser-mineral interactions within the framework of LIBS geochemical analysis of ore samples with emphasis on the characterization of plasmas and laser ablation craters under ambient air at atmospheric pressure. We study the crater morphology in the three main phases of a palladium ore fragment (Lac des Iles mine, Canada), namely plagioclase feldspar, amphibole and sulfides [Mohamed et al., Geostand Geoanal Res 45:539, (2021)] We performed four series of laser shots (50, 250, 500 and 1000 shots) in the three mineral phases and characterized the morphology of the craters obtained by scanning electron microscopy and optical coherence tomography. It turns out that laser ablation is most effective in plagioclase, presumably due to its lower thermal conductivity. In addition, the temperature and electron density of the plasma were determined for each phase from the iron and nickel lines of LIBS spectra taken 4 µs after the laser shots. They are between 6300 and 8600 °C and about 2 × 1017 cm−3, respectively.

Ex-situ quantification of impurity deposition depth on HL-2A divertor graphite tile by laser-induced breakdown spectroscopy

During tokamak discharges, the Plasma Wall Interaction (PWI) would lead to impurity deposition and fuel retention on the surface of the Plasma Facing Materials (PFMs). Laser-induced breakdown spectroscopy (LIBS) has emerged as a promising technique to facilitate in-situ, real-time, online characterization of the impurity deposition and fuel retention on the wall of tokamaks for PWI studies. The purpose of this investigation is to quantify the impurity deposition layer on the graphite tile removed from the dome region of the HL-2A tokamak lower divertor by using the ex-situ LIBS approach. The investigations indicate that the primarily deposited impurity elements are Fe, Cr, Ni, Mn, Ca, Al, Cu, Si, C, which may originate from the stainless steel first wall, divertor, limiter or the siliconized wall conditioning due to the PWI process in HL-2A tokamak. This has been further supported by observing a significantly positive correlation between the Cr, Ni, Mn, Al and Fe elements by spectral cumulative intensity linear regression analysis. The 3D morphology of the laser-ablated craters is reconstructed using a confocal microscope, revealing the laser ablation rates for the deposition layer and the substrate graphite tile to be 348 nm/pulse and 220 nm/pulse, respectively, under the laser fluence of 10 J/cm2. Then the thickness of the deposition layer is determined by using the correlation coefficient method. Along the poloidal direction of HL-2A tokamak, the deposited impurity shows an inhomogeneous pattern, with the thickness of the deposition layer varying from 1.74 μm to 5.92 μm. Finally, the absolute depth distribution of each element is also quantitatively measured.

Quantitative laser-matter interaction: a 3D study of UV-fs-laser ablation on single crystalline Ru(0001).

Laser ablation is nowadays an extensively applied technology to probe the chemical composition of solid materials. It allows for precise targeting of micrometer objects on and in samples, and enables chemical depth profiling with nanometer resolution. An in-depth understanding of the 3D geometry of the ablation craters is crucial for precise calibration of the depth scale in chemical depth profiles. Herein we present a comprehensive study on laser ablation processes using a Gaussian-shaped UV-femtosecond irradiation source and present how the combination of three different imaging methods (scanning electron microscopy, interferometric microscopy, and X-ray computed tomography) can provide accurate information on the crater's shapes. Crater analysis by applying X-ray computed tomography is of considerable interest because it allows the imaging of an array of craters in one step with sub-µm accuracy and is not limited to the aspect ratio of the crater. X-ray computed tomography thereby complements the analysis of laser ablation craters. The study investigates the effect of laser pulse energy and laser burst count on a single crystal Ru(0001) sample. Single crystals ensure that there is no dependence on the grain orientations during the laser ablation process. An array of 156 craters of different dimensions ranging from <20 nm to ∼40 µm in depth were created. For each individually applied laser pulse, we measured the number of ions generated in the ablation plume with our laser ablation ionization mass spectrometer. We show to which extent the combination of these four techniques reveals valuable information on the ablation threshold, the ablation rate, and the limiting ablation depth. The latter is expected to be a consequence of decreasing irradiance upon increasing crater surface area. The ion signal generated was found to be proportional to the volume ablated up to the certain depth, which enables in-situ depth calibration during the measurement.

Influence of UO2 crystal orientation on laser ablation performance

Crystallographic orientation dependence deteriorates the performance of surface analysis methods such as secondary ion mass spectrometry (SIMS) and focused ion beam (FIB). This study explores the corresponding potential challenges of laser ablation (LA) as a powerful sampling tool for inductively coupled plasma-mass spectrometry (ICP-MS). To this end, three UO2 single crystals of different orientation as well as polycrystalline UO2 were produced and characterized. Subsequently, a ns-laser ablation system was employed to study laser-matter interaction in detail. Firing the laser continuously at 1Hz with various single shot fluence (2, 4, 6, 8, 12Jcm-2) for diverse periods created LA craters impacted by cumulative fluence between 50 and 650Jcm-2. Repeated LA experiments on the (100) plane of a UO2 single crystal at the beginning and end of the entire study revealed highly reproducible (<3%) LA rates, only limited by the fluctuation of the laser energy output of the ns-LA system. After thorough cleaning of the ablated samples, surface roughness and average depth of LA craters were determined using confocal laser scanning profilometry. Both LA rate and average depth of craters decreased exponentially with increasing single shot fluence independently of the crystal orientation. Surface roughness increased linearly with increasing cumulative fluence having largest intensification for lowest single shot fluence. Scanning electron microscope (SEM) images not only revealed the conical silhouette of LA craters, but also identified a convex meniscus at its bottom. This particular shape of the crater bottom with a deeper ring surrounding the central region is a result of melted and re-solidified UO2 generated during the LA process and the main limiting factor for the achievable depth resolution. The rapid re-solidification of the liquid phase after each single laser shot created tiles of different shape and orientation, depending on UO2 crystal orientation. Three different types of ejected particles radially distributed around the LA craters were identified by SEM, providing profound insights into laser-UO2 interaction.

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.

LIBS repeatability study based on the pulsed laser ablation volume measuring by the extended depth of field microscopic three-dimensional reconstruction imaging

To efficiently monitor the laser ablation volume, the extended depth of field imaging microscope three-dimensional measurement technology and Laser-Induced Breakdown Spectroscopy (LIBS) technology are combined into one system. By means of the sequence images acquisition, images matching, filtering, focusing evaluation, multi-image fusion and 3D reconstruction, a measurement can be realized shortly after the laser ablation and LIBS signal acquisition, and some primacy experiments show that this method has enough performance to discriminate the volume of laser ablation craters. The combined system was used to evaluate LIBS signals and ablative crater volumes, and it was shown that there is a positive correlation between the volume of laser ablation craters and LIBS line intensity, and the repeatability of LIBS signal can be improved by correcting LIBS signal with ablation volume.

The interplay between laser focusing conditions, expansion dynamics, ablation mechanisms, and emission intensity in ultrafast laser-produced plasmas

The interplay between ultrafast laser focusing conditions, emission intensity, expansion dynamics, and ablation mechanisms is critical to the detection of light isotopes relevant to nuclear energy, forensics, and geochemistry applications. Here, we study deuterium (2Hα) emission in plasmas generated from femtosecond laser ablation of a Zircaloy-4 target with a deuterium concentration of ≈37 at. %. Changes in emission intensity, plume morphology, crater dimensions, and surface modifications were investigated for varying focusing lens positions, where the laser was focused behind, at, and in front of the target. Spatially resolved optical emission spectroscopy and spectrally integrated plasma imaging were performed to investigate emission spectral features and plume morphology. Laser ablation crater dimensions and morphology were analyzed via optical profilometry and scanning electron microscopy. The 2Hα emission intensity showed significant reduction at the geometrical focal point or when the focal point is in front of the target. For all laser spot sizes, a two-component plume was observed but with different temporal histories. At the best focal point, the plume was spherical. When the laser was focused behind the target, the plume was elongated and propagated to farther distances than for the best focal position. In contrast, when the laser was focused in front of the target, filaments were generated in the beam path, and filament-plasma coupling occurred. By focusing the laser behind the target, the amount of material removal in the laser ablation process can be significantly reduced while still generating a plasma with a sufficient 2Hα emission signal for analysis.

Optical fibers for endoscopic high-power Er:YAG laserosteotomy

.Significance: The highest absorption peaks of the main components of bone are in the mid-infrared region, making Er:YAG and lasers the most efficient lasers for cutting bone. Yet, studies of deep bone ablation in minimally invasive settings are very limited, as finding suitable materials for coupling high-power laser light with low attenuation beyond is not trivial.Aim: The first aim of this study was to compare the performance of different optical fibers in terms of transmitting Er:YAG laser light with a wavelength at high pulse energy close to 1 J. The second aim was to achieve deep bone ablation using the best-performing fiber, as determined by our experiments.Approach: In our study, various optical fibers with low attenuation () were used to couple the Er:YAG laser. The fibers were made of germanium oxide, sapphire, zirconium fluoride, and hollow-core silica, respectively. We compared the fibers in terms of transmission efficiency, resistance to high Er:YAG laser energy, and bending flexibility. The best-performing fiber was used to achieve deep bone ablation in a minimally invasive setting. To do this, we adapted the optimal settings for free-space deep bone ablation with an Er:YAG laser found in a previous study.Results: Three of the fibers endured energy per pulse as high as 820 mJ at a repetition rate of 10 Hz. The best-performing fiber, made of germanium oxide, provided higher transmission efficiency and greater bending flexibility than the other fibers. With an output energy of 370 mJ per pulse at 10 Hz repetition rate, we reached a cutting depth of in sheep bone. Histology image analysis was performed on the bone tissue adjacent to the laser ablation crater; the images did not show any structural damage.Conclusions: The findings suggest that our prototype could be used in future generations of endoscopic devices for minimally invasive laserosteotomy.

Microanalysis of a ductile iron by microchip laser-induced breakdown spectroscopy

Laser beams with ns pulse width are generally employed as an excitation source in the process of detecting inclusions and elemental segregation on a workpiece surface by microanalysis of the laser-induced breakdown spectroscopy. In addition, the ablation crater interval of laser sampling on the sample surface is generally 20 μm or more. It is difficult to detect the morphology of inclusions smaller than 50 μm in diameter and the micro-segregation of elements. However, in this work, when the laser ablation crater is 10 μm and the sampling resolution of the laser on the sample surface is 5 μm, the morphology and distribution of spherical inclusions (20–60 μm) in ductile iron can be detected according to the difference of the Fe spectrum on the Fe matrix and the spheroidal inclusions. Moreover, the distribution of micro-segregation of Mg and Ti elements in ductile iron was also studied.

Characterization of femtosecond laser ablation processes on as-deposited SnAg solder alloy using laser ablation ionization mass spectrometry

Laser ablation ionization mass spectrometry studies on polished surface structures of hard, high melting point multicomponent interconnect structures using our femtosecond-laser time-of-flight mass spectrometer have previously been proven capable of providing spatially resolved chemical depth profile data at the micro- and nanometer level. Transfer of experimental protocols tailored to suit such type of samples to low melting point materials with high surface roughness, however, is highly challenging, as thermal effects caused by interactions of the laser light with the targeted surface play a severe role in the latter case and may significantly deteriorate the analysis outcome. In this contribution we present a dedicated and detailed comparison of the distinct effects of near infrared and ultraviolet irradiation on the ablation process in femtosecond laser ablation ionization mass spectrometry studies on low melting point materials. As model substrate we use high surface roughness, as-deposited SnAg solder alloy, a material of major industrial relevance in microchip fabrication. We will demonstrate herein that polydimethylsiloxane replica casting is possible also from granularly structured, high surface roughness samples. Characterization of the laser ablation craters as well as of their polydimethylsiloxane replicas is carried out by mass spectrometric analysis, Scanning Electron Microscopy imaging, and furthermore, as a novel approach for this purpose, by white light interferometry. Integration of all obtained results significantly adds to the detailed understanding of laser ablation processes on low melting point, high surface roughness materials and allows identifying optimal depth profiling conditions for these demanding samples.

Experimental study of the effect of pressure and laser fluency on laser ablation craters and LIBS emission features of brass sample

We performed measurements on an ablated volume of a brass sample using optical microscopy and spectroscopic analysis of the plasma generated by Nd:YAG laser irradiation. A parametric study was investigated under different gas pressures and laser energy conditions for several time delays in order to determine the laser ablation plume with "ideal" properties. The ablation volume of the brass sample was determined by measuring the volume of the crater after laser ablation. As a result, we have observed that the surrounding gas pressure influences the amount of the ablated material and the expansion dynamics of the plasma plume. The mass fractions of copper, zinc and lead were measured by using CF-LIBS technique. Slight elemental concentration variations, not exceeding 4%, were found when compared with certified values.

Effect of initial γ-irradiation on infrared laser ablation of poly(vinyl alcohol) studied by infrared spectroscopy

The effect of initial γ-irradiation of poly(vinyl alcohol) (PVA) on the kinetics of polymer ablation under CO2 laser irradiation was studied using infra-red spectroscopy. The rate of laser ablation rapidly decreases with increasing time of laser ablation up to 10 s. Initial γ-irradiation of a dose up to 100 kGy reduces the rate of the laser ablation, just like the effect of laser exposure. The number of OH and CH groups decrease in PVA both with laser irradiation and with γ-irradiation. The number of OH groups decreases more under laser irradiation. Unsaturated bonds are formed in γ–irradiated PVA on the surface of the crater formed by laser ablation of the polymer that has undergone radiolysis. Increasing the dose of initial γ-irradiation from 100 to 2300 kGy leads to an increase in the intensity of the IR bands of the unsaturated bonds. The number of IR bands in the spectra of the laser ablation crater of PVA does not change with the dose of initial γ-irradiation. A dose of radiolysis of 3500 kGy significantly changes the IR spectra of both the ablation crater and the coating formed from the ablation products. Many bands characteristic for PVA are no longer present and the intensities of bands corresponding to unsaturated bonds are increased. A computational model for loss of water and ketone formation was developed for PVA and PVA including a diol using G3(MP2). The highest activation energy barrier step in the decomposition reaction was formation of the enol species and water was shown to catalyze all decomposition steps.

Promises and pitfalls of ns-laser ablation for depth profiling of UO2 single crystals

A total number of 960 laser ablation (LA) craters produced by a ns-LA system were characterized systematically for their depth profiling capability of UO2 single crystals. For gaining improved understanding of laser-UO2 interaction, a variable number of circular laser pulses, having a nominal diameter of 25 µm, were shot repeatedly (N = 5) on the surface of a UO2 single crystal. To this end, performance at both diverse fluence settings and different number of individual laser pulses was evaluated at laser repetition rates of 1 Hz and 2 Hz. As such, the impact of cumulative fluence of 50, 150, 250, 350, 450, 550, and 650 J cm−2 on the integrity of a UO2 single crystal was investigated. Confocal laser scanning profilometry and a dual beam focused ion beam microscope were employed to assess essential parameters such as LA rate, surface roughness, silhouette and depth of LA craters. While the LA rate decreased with increasing single shot fluence, surface roughness continuously intensified with expanded cumulative fluence. The average depth of LA craters was higher, the higher was the cumulative fluence. The conical geometry and the variability of the bumpy bottom of LA craters limited the experimentally achievable depth resolution to ~ 1 µm. Identical LA experiments carried out weeks apart, produced very similar data, proving high reproducibility of the laser-UO2 interaction.

The role of ambient gas confinement, plasma chemistry, and focusing conditions on emission features of femtosecond laser-produced plasmas

Image of the filament ablation with femtosecond laser and filament ablation craters.

The effect of sample surface roughness on the microanalysis of microchip laser-induced breakdown spectroscopy

In the microanalysis of laser-induced breakdown spectroscopy, the influence of surface roughness on spectral stability and quantitative analysis capability was studied for the first time when the laser ablation crater diameter was approximately 10 μm.

Efficient generation of nitrogen vacancy centers by laser writing close to the diamond surface with a layer of silicon nanoballs

We proposed a method to effectively fabricate negatively charged nitrogen vacancy (NV−) centers close to the diamond surface by applying femtosecond laser writing technique. With a thick layer of silicon (Si) nanoballs coated, diamond surface was irradiated by high-fluence femtosecond laser pulses. A large number of NV− centers were created around the laser ablation crater area without thermal annealing. The distribution of the NV− centers was expanded to about 50 μm away from the crater center. To demonstrate the function of Si nanoballs, we performed the exactly same laser illumination process on the bare region of the sample surface. In this case, only a few NV− centers were generated around ablation crater. At distance of 32 μm away from crater centers, the NV− density for the case with nanoballs was up to 15.5 times higher compared to the case without nanoballs. Furthermore, we also investigated the influence of laser fluence and pulse number on the NV− density for the case with Si-nanoball layer. Finally, the formation mechanism of NV− centers and the role of Si nanoballs were explained via Coulomb explosion model. The method is demonstrated to be a promising approach to efficiently and rapidly fabricate NV− centers close to the surface of the diamond, which are significant in quantum sensing. Furthermore, the results provide deep insights into complex light-matter interactions.

Ultraviolet Laser Patterning of Fluorine-Doped Tin Oxide with Different Radiation Directions

Selective laser patterning of thin films in a multilayered structure is an emerging technology for fabricating optoelectronics and microelectronics devices. As the application of ultraviolet (UV) lasers is more and more popular in electronics manufacturing, in this study, we compare UV laser patterning of fluorine-doped tin oxide (FTO) films from the front and back sides. We summarize a formula for calculating the focus offset in back-side ablation (BSA) and analyze systematically the relationships between the laser ablation crater size, the damage on glass substrate, and the laser ablation parameters. In addition, the laser ablation process and mechanisms of the BSA and the front-side ablation (FSA) are also elaborated in this paper.

Information depths of analytical techniques assessing whitefish otolith chemistry

The elemental composition of otoliths provides historical information on migration and provenance of fish. Due to the complex structure of the otoliths, the depth from which the elemental information originates has to be considered. In this work, three analytical methods often used to assess otolith chemistry are compared. The information depths are calculated for µ-XRF and PIXE and measured for LA-ICP-MS. The information depth in PIXE depends on the energy of the incident particles and on the element to be analysed while in XRF it depends mostly on the element to be analysed as the energy of the incident X-rays usually is high enough to excite atoms at depths of several hundreds of micrometres. If we assume that the otolith is exposed to X-rays from a Rh-tube (20.16 keV) about 50% of the detectable Sr(Kα) X-rays will be emitted from a depth ranging from 0 to 114 µm. In the case of PIXE with 3 MeV protons the corresponding range is 0–15 µm. The information depths in LA-ICP-MS were determined by measuring the depth of the laser-ablated craters or trenches that remained on the otolith surface. The depths measured with a scanning white light interferometer (SWLI) were found to be about 40 µm (spots) and 11 µm (trenches).The correlation between strontium concentrations obtained by spot analysis of whitefish otoliths with PIXE and LA-ICP-MS was excellent (92%), although different spot sizes were used. A comparison of strontium concentration profiles measured with µ-XRF and LA-ICP-MS showed that the higher information depth of µ-XRF in combination with the 52 degree angle of the incident X-rays smoothens out the sharp edges seen in the LA-ICP-MS profiles. Otoliths from whitefish captured in the Baltic Sea (n = 30) were analysed in this comparison of methods. Among these whitefish there are sea spawners, river spawners and stocked fish with different life history and different otolith chemistry.