Laser Ablation Parameters Research Articles

Sensing signatures mediated by chemical structure of molecular solids in laser-induced plasmas.

Laser ablation of organic compounds has been investigated for almost 30 years now, either in the framework of pulse laser deposition for the assembling of new materials or in the context of chemical sensing. Various monitoring techniques such as atomic and molecular fluorescence, time-of-flight mass spectrometry, and optical emission spectroscopy have been used for plasma diagnostics in an attempt to understand the spectral signature and potential origin of gas-phase ions and fragments from organic plasmas. Photochemical and photophysical processes occurring within these systems are generally much more complex than those suggested by observation of optical emission features. Together with laser ablation parameters, the structural and chemical-physical properties of molecules seem to be closely tied to the observed phenomena. The present manuscript, for the first time, discusses the role of molecular structure in the optical emission of organic plasmas. Factors altering the electronic distribution within the organic molecule have been found to have a direct impact on its ensuing optical emissions. The electron structure of an organic molecule, resulting from the presence, nature, and position of its atoms, governs the breakage of the molecule and, as a result, determines the extent of atomization and fragmentation that has proved to directly impact the emissions of CN radicals and C2 dimers. Particular properties of the molecule respond more positively depending on the laser irradiation wavelength, thereby redirecting the ablation process through photochemical or photothermal decomposition pathways. It is of paramount significance for chemical identification purposes how, despite the large energy stored and dissipated by the plasma and the considerable number of transient species formed, the emissions observed never lose sight of the original molecule.

A limitation of Hartmann-Shack system in measuring wavefront aberrations for patients received laser refractive surgery.

PurposeTo explore the relationship between ablation parameters of myopic laser surgery and measurement area of wavefront aberration (WA) with Hartmann-Shack wavefront sensor.Methods58 subjects undergone myopic laser surgeries and 74 uncorrected myopic subjects were enrolled in this experiment. The laser ablation parameters were obtained from surgical records, which included spherical error (Rx), depth, and optical zone (OZ) of ablation. The measured area of WA was tested by the WASCA, and the real pupil size was tested by Pentacam. The corneal eccentricity (E value) and curvature was also measured with the Pentacam. All the measurements were performed under mydriatic condition.ResultsFor uncorrected myopic eyes, the measured area of WA was similar with the real pupil size. But for the corrected eyes, the measured area of WA was smaller than the real pupil size with a mean difference of 0.66 ± 0.54 mm for moderate myopia (t = 6.45, p < 0.0001) and 1.76 ± 0.55 mm for high myopia (t = 18.92, p < 0.0001), but not for mild myopia. The Rx (t = -3.20, p = 0.0017), OZ (t = 64.4, p < 0.0001) and postoperative corneal E value (t = 2.52, p = 0.017) were the independent factors of measured area of WA. Measured area of WA = -0.81*Rx + 1.13*OZ + 0.49*postoperative corneal E value (r2 = 0.997).ConclusionsThe WASCA has a limitation in measuring wavefront aberration over the whole pupil area when it’s used for patients received myopic laser surgery. The measured area is smaller than the real pupil size and depends linearly on ablation depth, optical zone and corneal eccentricity.

Synthesis and characterization of PVK/AgNPs nanocomposites prepared by laser ablation

Nanocomposites of Poly (n-vinylcarbazole) PVK/Ag nanoparticles were prepared by laser ablation of a silver plate in aqueous solution of chlorobenzene. The influences of laser parameters such as; time of irradiation, source power and wavelength (photon energy) on structural, morphological and optical properties have been investigated using X-ray diffraction (XRD), Transmission electron microscopy (TEM), Ultraviolet–visible (UV–Vis) and Photoluminescence (PL). A correlation between the investigated properties has been discussed. XRD, TEM and PL indicated that the complexation between AgNPs and PVK in the composite system is possible. Only the reflection peak at 2θ=38° of AgNPs appeared in the composite nanoparticles while the other reflection peaks were destroyed. The nanoparticles shape and size distribution were evaluated from TEM images. TEM analysis revealed a lower average particle size at long laser irradiation time 40min and short laser wavelength 532nm together with high laser power 570mW. From UV–Visible spectra the values of absorption coefficient, absorption edge and energy tail were calculated. The reduction of band tail value with increasing the laser ablation parameters confirms the decrease of the disorder in such composite system. The PL and UV–Vis. spectra confirm that nanocomposite samples showed quantum confinement effect.

ArF Excimer Laser Micromachining of MEMS Materials: Characterization and Applications

Excimer laser ablation is a versatile technique that can be used for a variety of different materials. Excimer laser ablation overcomes limitations of conventional two-dimensional (2D) microfabrication techniques and facilitates three-dimensional (3D) micromanufacturing. Previously, we have reported a characterization study on 248 nm KrF excimer laser micromachining. This paper extends the study to 193 nm ArF excimer laser micromachining on five representative micro-electro-mechanical systems (MEMS) materials (Si, soda-lime glass, SU-8, polydimethylsiloxane (PDMS), and polyimide). Relations between laser parameters (fluence, frequency and number of laser pulses) and etch performances (etch rates, aspect ratio, and surface quality) were investigated. Etch rate per shot was proportional to laser fluence but inversely proportional to number of laser pulses. Laser frequency did not show a notable impact on etch rates. Aspect ratio was also proportional to laser fluence and number of laser pulses but was not affected by laser frequency. Materials absorbance spectrum was found to have important influence on etch rates. Thermal modeling was conducted as well using numerical simulation to investigate how the photothermal ablation mechanism affects the etching results. Thermal properties of material, primarily thermal conductivity, were proved to have significant influence on etching results. Physical deformation in laser machined sites was also investigated using scanning electron microscopy (SEM) imaging. Element composition of redeposited materials around ablation site was analyzed using energy dispersive x-ray spectroscopy (EDXS) analysis. Combined with our previous report on KrF excimer laser micromachining, this comprehensive characterization study provides guidelines to identify optimized laser ablation parameters for desired microscale structures on MEMS materials. In order to demonstrate the 3D microfabrication capability of ArF excimer laser, cutting and local removal of insulation for a novel floating braided neural probe made of polyimide and nichrome was conducted successfully using the optimized laser ablation parameters obtained in the current study.

Laser ablation of polished and nanostructured titanium surfaces by nanosecond laser pulses

A comparison of the IR nanosecond laser ablation parameters for polished and nanostructured titanium samples has been performed. The titanium foil was mechanically polished and pres-structured by multiple 744-nm femtosecond laser pulses producing large surface spots covered by ripples with periods in range of 400–500nm. In order to evaluate the influence of such nanoripples, the nanosecond laser ablation and laser plasma properties were compared for polished surface, surface with nanoripples parallel and orthogonal to the laser beam polarization. A substantial decrease of the nanosecond ablation threshold was observed for the nanostructured in contrast to polished surface was detected while no influence of the ripple orientation vs. beam polarization was revealed. The comparison of plasma spectra for the ablation cases demonstrated that intensity of basic atomic lines and plasma emission duration were 2–5 times larger for the polished sample while spectra evolution was faster for the nanostructured sample. Plasma temperature and electron density were slightly lower for nanostructured sample while laser beam polarization has no effect on plasma properties.

Al+ -doping of Si by laser ablation of Al2 O3 /SiN passivation

We show that one-pulse laser-ablation of Al2O3/SiNx layer stacks from silicon wafers can produce substantial p+-type Al-doped regions in the substrate. We present doping profile measurements which show that the Al-doping profiles of the laser-treated region strongly depend on the laser ablation parameters and thickness of the Al2O3 layer. We find for non-passivated laser Al-doping profiles with sheet resistance Rsheet = 180 Ω □−1 a saturation current densities J0,surf = 4 pA cm−2, which is approximately the same recombination activity as that of a boron furnace diffused layer of the same sheet resistance.

Ultraviolet laser ablation of fluorine-doped tin oxide thin films for dye-sensitized back-contact solar cells

In this study, laser ablation of a fluorine-doped tin oxide (FTO) thin film on a glass substrate was conducted using a 355nm Nd:YVO4 ultraviolet (UV) laser to obtain a 4×4mm microstructure. The microstructure contains a symmetric set of interdigitated FTO finger electrodes of a monolithic back-contact dye-sensitized solar cell (BC-DSC) on a common substrate. The effects of UV laser ablation parameters (such as laser fluence, repetition frequency, and scanning speed) on the size precision and quality of the microstructure were investigated using a 4×4 orthogonal design and an assistant experimental design. The incident photon-to-electron conversion efficiency and the current–voltage characteristics of the BC-DSC base of the interdigitated FTO finger electrodes were also determined. The experimental results show that an FTO film microstructure with high precision and good quality can be produced on a glass substrate via laser ablation with high scanning speed, high repetition frequency, and appropriate laser fluence.

Copper Light-induced Plating Contacts with Ni Seed-layer for mc-Si Solar Cells after Laser Ablation of SiNx:H

This study focuses on the electrochemical Ni/Cu metallization of multi-crystalline silicon (mc-Si) without alignment steps. Selective ablation of the silicon nitride (SiNx:H) anti-reflection and passivation coating was performed prior to metallization with a frequency tripled Nd:YAG laser. Electroless nickel-phosphorous layers of different thicknesses were deposited as a seed-layer at 95°C on two batches of samples before electrolytic copper thickening. The thickening of the Ni contact by copper was done by light-induced plating (LIP). The influence of laser ablation parameters as well as chemical etching prior to metal deposition was investigated. The morphology of electrochemically deposited Ni and Cu metal layers was investigated by SEM and optical microscope. Laboratory scale solar cells were fabricated to evaluate the electrical properties of the front contacts. A copper thickness of between 9 and 16μm was necessary in order to optimize the fill factor. The best efficiency measured on 200μm thick p-type mc-Si solar cell with an area of 4.4cm2 was 15.5%. An average efficiency of 15% over 18 samples has been demonstrated. Such results were obtained without any additional thermal annealing treatment of the Ni seed-layer. The limiting factors as well as possible improvements are discussed.

Microstructural Investigations of Ca and La Doped CoO Thin Films Prepared by Pulsed Laser Deposition Technique

Two types of different oxides multifunctional thin films were deposited by PLD technique on the surface of Si and MgO substrates. First of them was CoO doped with Ca content characterized by fcc (halite) structure and second one was perovskite-type LaCoO3 compound. Their microstructure, chemical/phase composition and morphology were examined by means of diverse techniques (SEM, TEM, EDS). For the surface topography observations of thin films the Atomic Force Microscopy (AFM) was applied. Nanohardness and scratch tests (adhesion measurements) were also performed for estimation of (Co,Ca)O and LaCoO3 films mechanical properties and quality. Obtained results confirm that using PLD technique it is possible to carry stoichiometric composition of different compounds from the target to single crystal substrate however the microstructure and properties of obtained thin films are highly influenced by the substrate’s material and laser ablation parameters (laser wavelength, energy density, time and target-substrate distance).

KrF excimer laser micromachining of MEMS materials: characterization and applications

Conventional photolithography-based microfabrication techniques are widely used to create microscale structures and devices nowadays. However, these techniques are limited to two-dimensional fabrication and also to particular materials. Excimer laser micromachining enables us to overcome those limitations and facilitates three-dimensional (3D) microfabrication. This paper presents a comprehensive characterization study for excimer laser micromachining of micro-electro-mechanical system (MEMS) materials. By using a 248 nm KrF excimer laser and four representative MEMS materials (Si, soda-lime glass, SU-8 photoresist and poly-dimethysiloxane (PDMS)), the relations between laser ablation parameters (fluence, frequency and number of laser pulses) and etch performance such as the etch rates in the vertical and lateral directions, aspect ratio, and surface quality were obtained. The etch rate increased almost linearly for all four materials as the fluence increased but no significant variation in etch rate was observed as the frequency of laser pulses was changed. The etch rate was also inversely proportional to the number of laser pulses. Physical deformation in the laser-machined sites on PDMS and SU-8 was investigated using SEM imaging. In order to demonstrate the 3D microfabrication capability of an excimer laser and the utility of this characterization study, two novel implantable biomedical microscale devices made of SU-8 and PDMS were successfully fabricated using the optimized laser ablation parameters obtained in this study.

Effect of Laser Fluence on Siliver Nanoparticles Colloid

In order to study the effects of laser fluence on silver nanoparticles colloid, the silver nanoparticles colloid was prepared by pulsed laser ablation of silver target for 10min in distilled water at different laser fluence. The particles size，morphologies and absorption spectroscopy of the obtained nanoparticles colloid were characterized by ultraviolet to visible (UV-Vis) spectrometer and transmission electron microscopy (TEM), the average diameter and its distribution were analyzed by Image-ProPlus software. The results shown that the average diameter of the silver nanoparticles prepared at the laser fluence of 4.2J/cm2 was the smallest (D=17.54nm), also, the distribution of particle size was narrowest (=36.86nm) and the morphologies were more homogeneous. It was confirmed that the nanoparticles size and shape could be controlled by pulsed laser ablation parameters.

Laser Ablation Imparts Controlled Micro-Scale Pores in Electrospun Scaffolds for Tissue Engineering Applications

Electrospun scaffolds have been used extensively for tissue engineering applications due to the simple processing scheme and versatility. However, many additional benefits can be imparted to these materials via post-processing techniques. Specifically the addition of structured pores on the micro-scale can offer a method to enable patterned cell adhesion, enhanced diffusional properties, and/or guide vascular infiltration upon implantation in vivo. In this study, we laser ablated electrospun poly(L: -lactic acid) (PLA) scaffolds and assessed the ablation process and cellular interaction by examining human adipose-derived stem cell (hASC) viability and proliferation on laser micro-machined scaffolds. Laser ablated pores of 150, 300, and 600 μm diameter were micro-machined through electrospun PLA scaffolds. Laser ablation parameters were varied and it was determined that the aperture and z-travel direction of the laser linearly correlated with the ablated pore diameter. To assess cytocompatibility of the micro-machined scaffolds, hASCs were seeded on each scaffold and cell viability was assessed on day 7. Human ASCs were able to adhere around the micro-machined features. DNA content was quantified on all scaffolds and it was determined that hASCs were able to proliferate on all scaffolds. The process of laser ablation could impart many beneficial features to electrospun scaffolds by increasing mass transport and mimicking micro-scale features and assisting in patterning of cells around micro-machined features.

Biomonitoring of lead in Antarctic lichens using laser ablation inductively coupled plasma mass spectrometry

A method for direct determination of Pb by LA-ICP-MS in lichen samples collected in a heavily anthropogenically impacted area of Maritime Antarctica was developed. The lichen samples were cryogenically ground and digested with nitric acid and hydrogen peroxide. Lead determination by solution-based ICP-MS was validated by the use of 2 certified reference materials. Once the Pb concentrations were measured with adequate accuracy and precision, a method for its direct determination by LA-ICP-MS was developed. The laser ablation parameters were optimized by the use of a Doehlert design matrix. The final optimized conditions were: laser energy (60%), spot size (150 μm) and repetition rate (10 Hz). Lead concentration of the analyzed lichen samples were plotted against the intensity of the lead isotopes (208Pb, 207Pb and 206Pb) determined by LA-ICP-MS. Satisfactory linear correlation factors were obtained when 13C was used as internal standard. Lead distribution profiles in lichen thalli cross-sections were also obtained. By these results, LA-ICP-MS emerges as a potential analytical tool for Pb concentration estimation in lichen samples. Minimum amount of sample required, bioimaging capability, high analytical throughput, and minimization of waste generation are the major analytical features of this approach. The significant differences between the Pb concentration in the lichen samples from the control site (1.12 ± 0.05 mg kg−1) and from the impacted points (mean = 5.03 ± 0.57 mg kg−1) permitted qualify these organisms as good biomonitors. Elemental bioimaging of lichens demonstrated that the medulla region of lichen thalli is consistently the main atmospheric lead bioaccumulating lichen tissue.

Experimental femtosecond laser photodisruption of rabbit sclera for minimally invasive laser sclerostomy: An in vitro study

Femtosecond laser technology, used as a minimally invasive tool in intrastromal refractive surgery, may also have potential as a useful instrument for glaucoma filtration surgery. The purpose of the present study was to evaluate the feasibility of minimally invasive laser sclerostomy by femtosecond laser photodisruption and seek the appropriate patterns of laser ablation and relevant laser parameters. A femtosecond laser (800 nm/50 fs/1 kHz), focused by a 0.1 numerical aperture (NA) objective lens, with different pulse energies and exposure times was applied to ablate hydrated rabbit sclera in vitro . The irradiated samples were examined by scanning electron microscopy (SEM). By moving a three-dimensional, computer-controlled translation stage to which the sample was attached, the femtosecond laser could produce three types of ablation patterns, including linear ablation, cylindrical aperture and rectangular cavity. With pulse energies ranging from 37.5 to 150 μJ, the linear lesions were consistently observed at the inner surface of sclera, whereas it failed to make any photodisruption if pulse energy was below the threshold value of 31.25 μJ, with the corresponding threshold intensity of 4.06×10 14 W/cm 2 . The depths of the linear lesions increased linearly with both pulse energy (37.5–150 μJ) and exposure time (0.1–0.4 s). Histological examination showed the incisions produced by femtosecond laser photodisruption had precise geometry and the edges were sharp and smooth, with no evidence of collateral damage to the surrounding tissue. Our results predict the potential application of femtosecond laser pulses in minimally invasive laser sclerostomy for glaucoma treatment.

Investigation of plume expansion dynamics and estimation of ablation parameters of laser ablated Fe plasma

A systematic investigation of expansion dynamics of plasma plume, generated by a Q-switched Nd–YAG laser at 532 nm with a fluence of ∼30 J cm−2 on Fe target, was done for three different ambient pressures of 2 × 10−4, 2 and 20 mbar of Ar using fast gated intensified charge coupled device imaging. The experimental observations are then fitted with the snow plow and shock wave models to estimate various laser ablation parameters such as laser energy deposited to the ablated material, mass ablated per laser shot, laser spot size and the background gas density. The estimated laser energy deposited to the ablated material is about 70% of the input energy. The density of ambient gas atoms, having interaction with the ablated species, is found to vary from 12% to 50% at 2 mbar and 24% to 80% at 20 mbar. Scanning electron microscope images confirm the effect of ambient gas pressure on the topography of the deposited thin film and size of the nanoparticles which is qualitatively explained on the basis of the changing kinetic energy of impinging plasma plume species due to change in ambient gas pressure.

Structure and Phase Behavior of Gold Nanocondensates: Effects of Laser Ablation Parameters and Carbon Catalysis

Au nanocondensates in the form of amorphous clusters, multiply twinned particles (MTPs), and face-centered cubic (fcc) structures with progressively larger particle size, as characterized by transmission electron microscopy (TEM), were fabricated by pulsed laser ablation on Au in vacuum with or without argon gas flow at 1.5 × 108 versus 1.4 × 1012 W/cm2 power density for a rapid heating/cooling effect. The observed critical size, ca. 2 nm, for the amorphous to MTP transformation implies a minimum {111} surface area of ca. 4 nm2 for MTP. Coarsening and {111}-specific coalescence of the MTPs and fcc to form defects are competitive under the influence of laser parameters and Ar gas flow. In situ TEM observations further indicated that the MTP → fcc transformation occurred for a surprisingly small particle size of 5 nm due to the catalytic effect of graphite-like materials (G) with varied extent of rolling following the habit plane (0001)G/(111)fcc. The ultimate relationship would be (2110)G//(211)fcc; [0...

Femtosecond laser micromachining of dielectric materials for biomedical applications

Techniques for microfluidic channel fabrication in soda-lime glass and fused quartz using femtosecond laser ablation and ablation in conjunction with polymer coating for surface roughness improvement were tested. Systematic experiments were done to characterize how process variables (laser fluence, scanning speed and focus spot overlap, and material properties) affect the machining feature size and quality. Laser fluence and focus spot overlap showed the strongest influence on channel depth and roughness. At high fluence, the surface roughness was measured to be between 395 nm and 731 nm RMS. At low fluence, roughness decreased to 100 nm–350 nm RMS and showed a greater dependence on overlap. The surface roughness of laser ablation was also dependent on the material properties. For the same laser ablation parameters, soda-lime glass surfaces were smoother than fused quartz. For some applications, especially those using quartz, smoother channels are desired. A hydroxyethyl methacrylate (HEMA) polymer coating was applied and the roughness of the coated channels was improved to 10–50 nm RMS.

Detection of counterfeit tablets of an antiviral drug using δ34S measurements by MC-ICP-MS and confirmation by LA-MC-ICP-MS and HPLC-MC-ICP-MS

A new approach for pharmaceutical counterfeit detection using laser ablation multicollector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) and high performance liquid chromatography multicollector inductively coupled plasma mass spectrometry (HPLC-MC-ICP-MS) has been developed. Laser ablation and chromatographic separation parameters have been previously optimised and helium has been used as the carrier gas for better sensitivity. A homogeneity study of 288 pharmaceutical tablets from different batches of the genuine drug has been performed by MC-ICP-MS and characteristic sulfur isotopic signature has been obtained for the genuine product. δ34S measurements by MC-ICP-MS using silicon internal standardisation for the correction of instrumental mass bias effects led to a δ34S of 3.6‰ and an associated combined expanded uncertainty of 1‰ (k = 2). The active pharmaceutical ingredient (API) of the tablets, a S-containing compound, has been separated by HPLC and sulfur isotope amount ratios have been measured by MC-ICP-MS. δ34S values obtained by HPLC-MC-ICP-MS for the genuine tablets agreed with those obtained by laser ablation MC-ICP-MS, thus confirming that the sulfur isotopic signature is inherent to the S-containing API. Following the initial development work, a blind exercise was performed for four hundred tablets. The discriminating power of the technique was assessed and uncertainties associated to δ34S values for counterfeit and genuine tablets varied depending on the sample introduction technique utilised. The three approaches were able to differentiate genuine from counterfeit tablets. During the exercise, four distinct groups of counterfeit tablets were detected. The MC-ICP-MS method therefore has potential as a rapid screening tool for pharmaceutical counterfeit detection and classification.

Femtosecond laser production of metal surfaces having unique surface structures that are broadband absorbers

Femtosecond laser induced modification of surface structure and the optical properties of metals are studied. A Ti:sapphire femtosecond laser is used for ablating bulk aluminum 2024 T3 alloy and highly pure neutron activation quality gold metal samples at different laser irradiation parameters. The ablation parameters are optimized for the two metals to form a low reflection surface in the wavelength range of 0.3–50 μm. “Black aluminum” and “black gold” metal surfaces are formed upon laser ablation, having a unique surface structure with micro- and nanosized features. Sample analysis is performed by several characterization techniques like scanning electron microscope, energy dispersive x ray, profilometry, and integrating sphere reflectance measurements. Correlation is drawn between the laser ablation parameters and the formed surface structure and its properties. The reflection of light is found to depend primarily on the surface structure (roughness) of the highly modified metal surfaces.

Percutaneous laser ablation in patients with isolated unresectable liver metastases from colorectal cancer: Results of a phase II study

We prospectively evaluated the safety, local tumor control, and impact on survival parameters of percutaneous laser ablation (PLA) in patients with colorectal liver metastases not amenable to surgical resection. The study included 44 individuals with 75 unresectable liver metastases and no known extrahepatic disease. The median number of metastases treated for each patient was one, with a range of 1–4. Metastases had a median diameter of 3.4 cm (range 0.5–9 cm), and a median volume of 16.8 cm3 (range 0.4–176.4 cm3). All patients also received systemic chemotherapy with modalities that differed according to the type of response to PLA. After treatment, 61% (46/75) of the tumors were ablated completely. The likelihood of achieving a complete ablation was significantly higher when metastases had a diameter <3.0 cm (p = 0.004). Overall survival was 30.0±12.7 months in patients with a complete ablation, and 20.2±10.2 months in those with a partial ablation (p = 0.002). There were no major complications during or after PLA, the most frequent side effect being abdominal pain that required analgesics. These findings indicate that PLA can be safely used as an adjunct to chemotherapy in unresectable colorectal liver metastases, and may have a positive impact on survival.