Increasing Laser Fluence Research Articles

The influence of laser wavelength and fluence on palladium nanoparticles produced by pulsed laser ablation in deionized water

Homogeneous spherical palladium (Pd) nanoparticles were synthesized by pulsed laser ablation of a solid Pd foil target submerged in deionized water, without the addition of any external chemical surfactant. The influence of laser wavelength (355, 532, and 1064 nm) and fluence (8.92, 12.74, and 19.90 J/cm2) on nucleation, growth, and aggregation of Pd nanoparticles were systematically studied. Microstructural and optical properties of the obtained nanoparticles were studied by field emission transmission electron microscopy (FETEM), energy dispersive X-ray spectroscopy, and UV–vis spectroscopy. FETEM micrographs indicate that the average nanocrystallite sizes are relatively low (3–6 nm) and homogeneous for the particles synthesized at the laser wavelengths of 355 and 532 nm. However, at a laser wavelength of 1064 nm, the average nanocrystallite size is relatively large and inhomogeneous in nature. Moreover, we observe that the mean diameter and production rate of particles increases with an increase in laser fluence. The selected area electron diffraction patterns obtained from isolated Pd nanoparticles show the characteristic diffused electron diffraction rings of polycrystalline materials with a face-centered cubic structure. Absorbance spectrum of the synthesized nanoparticle solution shows a broad absorption band, which corresponds to a typical inter-band transition of a metallic system, indicating the production of pure palladium nanoparticles. The present work provides new insights into the effect of laser wavelength and fluence on the control of size and aggregation of palladium nanoparticles in the liquid medium.

Alginate gelation-induced cell death during laser-assisted cell printing

Modified laser-induced forward transfer has emerged as a promising bioprinting technique. Depending on the operating conditions and cell properties, laser cell printing may cause cell injury and even death, which should be carefully elucidated for it to be a viable technology. This study has investigated the effects of alginate gelation, gelation time, alginate concentration, and laser fluence on the post-transfer cell viability of NIH 3T3 fibroblasts. Sodium alginate and calcium chloride are used as the gel precursor and gel reactant solution to form cell-laden alginate microspheres. It is found that the effects of gelation depend on the duration of gelation. Two-minute gelation is observed to increase the cell viability after 24 h incubation, mainly due to the protective cushion effect of the forming gel membrane during droplet landing. Despite the cushion effect from 10 min gelation, it is observed that the cell viability decreases after 24 h incubation because of the forming thick gel membrane that reduces nutrient and oxygen diffusion from the culture medium. In addition, the cell viability after 24 h incubation decreases as the laser fluence or alginate concentration increases.

Synthesis of ultra-thin oxide layer in laser-treated 3× (Al/Fe)/Si multilayer structure

The conditions of ultra-thin oxide films synthesized as the Al/Fe multilayer structures after the surface treatment with femtosecond laser radiation were studied. Thin films composed of three (Al/Fe) bilayers were deposited by DC ion sputtering on (100) Si wafers to the total thickness of 410 nm. Laser irradiations were performed by 40-fs pulses of a Ti:Sapphire laser operating at 800 nm, at fluences slightly higher than the ablation threshold. The samples were characterized by Auger electron spectroscopy, X-ray photoelectron spectroscopy, and scanning electron microscopy. Progressed intermixing of components was achieved for the irradiation with fluence of 0.43 J cm−2, when intermetallic phase was formed and all components were almost uniformly distributed through the entire multilayer structure. Exception is the subsurface region, to the thickness of 20 nm, where the distribution of constructive components and their oxide is changing with the distance from the surface, and this distribution depends on the applied laser fluences. An ultra-thin oxide layer was formed at the surface, with the specific combination of the oxide phases, Al2O3 and Fe2O3, depending on the applied fluences. Laser-induced modification was accompanied with the ablation of the material and the creation of parallel periodic structures. These effects of laser-induced morphological features are increased with increasing laser fluences, caused by the pronounced mobility of the materials. The special morphology of the laser-assisted synthesis of mixture of Al2O3 and Fe2O3 in the form of an ultra-thin oxide layer can improve their characteristics for functional applications, as catalysts and sensors.

Modelling of laser ablation and reactive oxygen plasmas for pulsed laser deposition of zinc oxide

Pulsed laser deposition (PLD) in a low-pressure oxygen atmosphere is commonly used for the production of high-quality, stoichiometric zinc oxide thin films. An alternative approach that has the potential benefit of increased process control is plasma-enhanced PLD, i.e. the use of a low-temperature oxygen plasma instead of a neutral gas. So far, the development of PE-PLD, and PLD in general, has been hampered by a lack of detailed understanding of the underpinning physics and chemistry. In this paper, we present modelling investigations aimed at further developing such understanding. Two-dimensional modelling of an inductively-coupled radio-frequency oxygen plasma showed that densities of 1014–1015cm−3 of reactive oxygen species O and O2* can be produced for operating pressures between 3 and 100Pa. Together with the absolute densities of species, also the ratio between different reactive species, e.g. O and O2*, can be controlled by changing the operating pressure. Both can be used to find the optimum conditions for stoichiometric zinc oxide thin film deposition. Additionally, we investigated laser ablation of zinc using a different two-dimensional hydrodynamic code (POLLUX). This showed that the amount of material that is ablated increases from 2.9 to 4.7μg per pulse for laser fluences from 2 to 10J/cm2. However, the increased laser fluence also results in an increased average ionisation of the plasma plume, from 3.4 to 5.6 over the same fluence range, which is likely to influence the chemistry near the deposition substrate and consequently the film quality.

Laser generation of hypersound by a terahertz photo-Dember electric field in a piezoelectric GaAs semiconductor

We experimentally demonstrate the optical generation of hypersound in a piezoelectric GaAs semiconductor through laser excitation of the THz photo-Dember electric field. Such an ultrashort transient Dember electric field is linked to the spatial separation of photoexcited electrons and holes right after above-band-gap femtosecond laser excitation. Through time-domain coherent Brillouin scattering we demonstrate that photoinduced piezoelectric generation of hypersound can dominate even in the absence of preexisting built-in fields and we observe, with increasing laser fluence, a nonlinear optoacoustic excitation process. These results reveal the onset of the THz photo-Dember electric field and highlight the transition from non-ambipolar flow to ambipolar diffusion of the photoexcited electron-hole plasma.

Optical properties of zinc phthalocyanine thin films prepared by pulsed laser deposition

ZnPc thin films were prepared by pulsed laser deposition (KrF laser, λ = 248 nm, τ = 5 ns, f = 50 Hz) on suprasil substrates in vacuum. Optical properties in UV–Vis spectral region were analyzed as functions of laser fluence from 40 to 100 mJ/cm2 by spectrophotometric and spectral ellipsometry measurements. The spectral ellipsometry data were treated using a three-layer model (substrate, film, roughness). The best results of data fitting were obtained when Q band was characterized by two Lorentz oscillators, while two Gaussian oscillators were used for B and C band fitting. We derived the band gap using Tauc plot considering ZnPc a direct band gap semiconductor. The band gap values were found decreasing from 3.13 to 3.09 eV with increasing laser fluence, which might be related with formation of trapping sites at higher fluence.

Aluminum doping of 4H-SiC by irradiation of excimer laser in aluminum chloride solution

We have found that aluminum doping into 4H-SiC is performed by irradiating excimer laser light to 4H-SiC immersed in aluminum chloride solution. Aluminum is introduced in SiC at the concentration of over 1 × 1020 cm−3 near the surface while, chlorine hardly diffuses into 4H-SiC. After the laser irradiation in aluminum chloride solution, the resistance of the laser-irradiated region decreases with increasing laser fluence. Hall effect measurement shows that the laser irradiation produces a p-type layer and that its sheet carrier concentration is 2.14 × 1011 cm−2. In addition, we produce a pn junction by doping the surface of n-type 4H-SiC and by aluminum doping. The pn junction shows rectifying characteristics whose on/off ratio is about 7 decades and ideality factor is 1.15. This technique is one of the strong candidate local doping techniques for SiC.

Thermal-dynamical analysis of blister formation in chirped mirror irradiated by single femtosecond lasers.

The laser-induced damage behaviors of chirped mirrors (CMs) are studied by single 800 nm, 38 fs lasers. The CMs provide group delay dispersion of around -60 fs² and average reflectivity of about 99.4% with bandwidth range of 200-300 nm at a central wavelength of 800 nm. Interestingly, a circular blister feature appears in the CMs at a wide range of laser fluence. An optical microscope, atomic force microscope, scanning electron microscope, and surface profiler are applied to describe the blister characteristics. An adiabatic expansion model of ideal gas is adopted to illustrate the formation dynamics of blisters. The evolution of blisters can be explained by partial evaporation of the film and a subsequent gas expansion, driving the bulging of the film stack up to the stress limit, where the blister fractures. According to this model, the energy absorption ratio of blisters increases monotonously with increasing laser fluence before the occurrence of the focal spot confinement effect.

Investigation of laser-induced plasma evolution in flexible pad laser shock forming with high speed camera

Abstract This study investigated the effect of plasma evolution, which dominates the forming load, on the fabrication of microcraters in flexible pad laser shock forming (FPLSF) using a high speed camera. It has been found that the plasma lifetime starting from plasma formation, expansion, decaying to vanishing was less than 13.3 μs at single pulse ablation, 350 times longer than the pulse duration. When 45 pulses were applied as 5 cycles with 9 pulse train in each, the plasma size increased gradually to its maximum at the fifth or sixth pulse. There was no interference between the plasma generated from each pulse. The first pulse was sufficient for the fabrication of a crater. The crater depth and diameter increased only by 10% and 25% respectively at ablation with 45 pulses. At 45 pulses ablation for fluence from 7.3 J/cm 2 to 20.9 J/cm 2 in water confinement, the change factor appeared in descending sequence from laser fluence, maximum plasma diameter, maximum plasma pressure, to crater depth by the order of 2.86, 2.18, 1.69 and 1.47 respectively. In glass, the plasma diameter increased by 3.28 times at increasing laser fluence. The confined plasma in glass resulted in deeper craters. The smaller craters in water were attributed to the forming load diminution due to the plasma expansion, shockwave attenuation in ablative overlay, and the laser energy reduction.

Fabrication of luminescent a-Si:SiO2 structures by direct irradiation of high power laser on silicon surface

• Nanostructured a-Si:SiO 2 fabricated by direct laser irradiation onto Si. • Compositional and structural properties with increasing laser fluence are reported. • a-Si:SiO 2 structures shows an asymmetric wide band PL around red region. • The origin of PL is NBOHC defects in SiO 2 matrix as revealed by PL and EDX studies. In this paper, the structural and compositional modification of polished silicon (Si) wafers by irradiation of second harmonic of Q switched high power Nd: YAG laser in air is reported. The surface morphology, recorded by scanning electron microscope (SEM), shows micro cluster formation. Raman spectra reveal the presence of amorphous silicon embedded in silicon dioxide (SiO 2 ) matrix in these structures which is further confirmed by energy dispersive X-ray (EDX) and Fourier transform infrared (FTIR) spectroscopic studies. These nanostructures of amorphous Si embedded in SiO 2 matrix (a-Si:SiO 2 ) showed luminescence in far red region. The effect of laser fluence on the photoluminescence properties and its possible origin were discussed.

Efficient Intracellular Delivery of Molecules with High Cell Viability Using Nanosecond-Pulsed Laser-Activated Carbon Nanoparticles

Conventional physical and chemical methods that efficiently deliver molecules into cells are often associated with low cell viability. In this study, we evaluated the cellular effects of carbon nanoparticles believed to emit photoacoustic waves due to nanosecond-pulse laser activation to test the hypothesis that this method could achieve efficient intracellular delivery while maintaining high cell viability. Suspensions of DU145 human prostate carcinoma cells, carbon black (CB) nanoparticles, and calcein were exposed to 5–9 ns long laser pulses of near-infrared (1064 nm wavelength) light and then analyzed by flow cytometry for intracellular uptake of calcein and cell viability by propidium iodide staining. We found that intracellular uptake increased and in some cases saturated at high levels with only small losses in cell viability as a result of increasing laser fluence, laser exposure time, and as a unifying parameter, the total laser energy. Changing interpulse spacing between 0.1 and 10 s intervals showed no significant change in bioeffects, suggesting that the effects of each pulse were independent when spaced by at least 0.1 s intervals. Pretreatment of CB nanoparticles to intense laser exposure followed by mixing with cells also had no significant effect on uptake or viability. Similar uptake and viability were seen when CB nanoparticles were substituted with India ink, when DU145 cells were substituted with H9c2 rat cardiomyoblast cells, and when calcein was substituted with FITC-dextran. The best laser exposure conditions tested led to 88% of cells with intracellular uptake and close to 100% viability, indicating that nanosecond-pulse laser-activated carbon nanoparticles can achieve efficient intracellular delivery while maintaining high cell viability.

Effect of UV Laser Conditioning on the Structure of KDP Crystal

Multiparametric raster scanning experiments for KDP crystals are carried out to study the laser conditioning efficiency as a function of laser fluence, fluence step, and pulse sequence by using ultraviolet (UV) laser irradiation with pulse duration of approximately 7 ns. It indicates that damage resistance of KDP can be enhanced after conditioning process. And laser conditioning efficiency depends on the maximal fluence which is below the damage threshold. Raman spectra and photothermal absorption have also been studied on KDP crystals before and after multiparametric laser conditioning. Photothermal absorption data reveal that absorbance of conditioned KDP crystal decreases with the increase of laser fluence and the damage threshold of low absorption area is higher. Raman analysis reveals that the effectiveness of laser conditioning relies mainly on the individual mode of PO4molecule.

A Roughness Study of Ytterbium-Doped Potassium Yttrium Tungstate (YB: KYW) Thin-Disk Femtosecond Ablated Dentin.

The aim of this study was to evaluate the morphological changes and quantitatively assess the roughness of dentin after the ablation with a Ytterbium-Doped Potassium Yttrium Tungstate (YB: KYW) thin-disk femtosecond pulsed laser of different fluences, scanning speeds and scanning distances. Twelve extracted human premolars were sectioned into crowns and roots along the cementum-enamel junction, and then the crowns were cut longitudinally into sheets about 1.5 mm thick with a cutting machine. The dentin samples were fixed on a stage at focus plane. The laser beam was irradiated onto the samples through a galvanometric scanning system, so rectangular movement could be achieved. After ablation, the samples were examined with a scanning electron microscope and laser three-dimensional profile measurement microscope for morphology and roughness study.With increasing laser fluence, dentin samples exhibited more melting and resolidification of dentin as well as debris-like structure and occluded parts of dentinal tubules. When at the scanning speed of 2400mm/s and scanning distance of 24μm, the surface roughness of dentin ablated with femtosecond pulsed laser decreased significantly and varied between values of dentin surface roughness grinded with two kinds of diamond burs with different grits. When at the scanning speed of 1200mm/s and scanning distance of 12μm, the surface roughness decreased slightly, and the surface roughness of dentin ablated with femtosecond pulsed laser was almost equal to that grinded with a low grit diamond bur. This study showed that increased laser influence may lead to more collateral damage and lower dentin surface roughness, while scanning speed and scanning distance were also negatively correlated with surface roughness. Adequate parameters should be chosen to achieve therapeutic benefits, and different parameters can result in diverse ablation results.

Pyramid-like spikes in a single crystal superalloy produced by picosecond laser irradiation

Pyramid-like spikes in a single crystal superalloy were investigated upon irradiation with picosecond (ps) laser pulses (200 ps, 800 nm, 1 kHz) under different laser fluences and pulse numbers. Both sides and grooves of pyramid-like spikes were covered with ripples, which had a period of ∼760 nm. The pyramid-like spike separation increased obviously with increasing laser fluence. Microstructural investigations indicate that the pyramid-like spikes were initiated with subsequent pulses from a smooth surface with corrugations and ripples. The coexistence of capillary waves for spikes and capillary waves for ripples in the melted material can be used to explain the formation of the pyramid-like spikes.

Laser irradiation effects on the surface, structural and mechanical properties of Al–Cu alloy 2024

Laser irradiation effects on surface, structural and mechanical properties of Al–Cu–Mg alloy (Al–Cu alloy 2024) have been investigated. The specimens were irradiated for various fluences ranging from 3.8 to 5.5 J/cm2 using an Excimer (KrF) laser (248 nm, 18 ns, 30 Hz) under vacuum environment. The surface and structural modifications of the irradiated targets have been investigated by scanning electron microscope (SEM) and X-ray diffractometer (XRD), respectively. SEM analysis reveals the formation of micro-sized craters along the growth of periodic surface structures (ripples) at their peripheries. The size of the craters initially increases and then decreases by increasing the laser fluence. XRD analysis shows an anomalous trend in the peak intensity and crystallite size of the specimen irradiated for various fluences. A universal tensile testing machine and Vickers microhardness tester were employed in order to investigate the mechanical properties of the irradiated targets. The changes in yield strength, ultimate tensile strength and microhardness were found to be anomalous with increasing laser fluences. The changes in the surface and structural properties of Al–Cu alloy 2024 after laser irradiation have been associated with the changes in mechanical properties.

Effect of Laser Fluence on the Growth of Mo Films Prepared by a Pulsed Laser Deposition Method

Molybdenum films with flat and smooth surfaces were prepared on Si (100) substrates using a pulsed laser deposition method. results indicate that increased laser fluence results in narrower diffraction peak, higher surface roughness, and transition from amorphous state into polycrystalline state with gradually increased grain size. The growth mechanism of the films and the effects of particle energy on the growth of the films are discussed based on an analysis of the changes in the crystallization process with laser fluence.

Investigation of the ablation of fluorine-doped tin oxide thin films by square top-hat ultraviolet laser beams

This paper presents a square top-hat beam shaper in an ultraviolet (UV) laser processing system for electrode patterning and investigates the interaction between square laser beams and fluorine-doped tin oxide (FTO) thin films deposited on glass substrates. The various patterning parameters to ablate the FTO films used in this experiment are included laser fluences, pulse repetition frequencies, and feeding rates of a motorized platform. The laser pulse repetition frequency and the feeding rate of the motorized platform were used to calculate the overlapping rate of laser spot and to determine the patterning quality. The line width and depth, edge quality, three-dimensional topography, and electrical conductivity properties of the patterned FTO films were measured and analyzed using a confocal laser scanning microscope and a four-point probe instrument. Experimental results indicated that the ablated line width and depth increased with increasing laser fluence. After electrode patterning with laser fluence of 2.07J/cm2 and 80% overlapping rate, no damages were observed in the patterned lines on the glass substrate. Moreover, the patterned line paths were narrow, smooth, clear, and flat in response to uniform laser energy distribution interacted in FTO films. The measured values of sheet resistance ranged from 109.7±3.8Ω/□ to 139.3±4.9Ω/□ at 2.07J/cm2 to 9.37J/cm2 laser fluences, respectively. The proposed process can reduce the fabrication steps and improve the removal efficiency of transparent conductive oxide materials and no waste etching chemical solution is produced.

Probing timescales during back side ablation of Molybdenum thin films with optical and electrical measurement techniques

In this study we present a new measurement technique to investigate the timescales of back side ablation of conductive films, using Molybdenum as an application example from photovoltaics. With ultrashort laser pulses at fluences below 0.6 J/cm(2), we ablate the Mo film in the shape of a fully intact Mo 'disc' from a transparent substrate. By monitoring the time-dependent current flow across a specifically developed test structure, we determine the time required for the lift-off of the disc. This value decreases with increasing laser fluence down to a minimum of 21 ± 2 ns. Furthermore, we record trajectories of the discs using a shadowgraphic setup. Ablated discs escape with a maximum velocity of 150 ± 5 m/s whereas droplets of Mo forming at the center of the disc can reach velocities up to 710 ± 11 m/s.

Laser-induced circular nanostructures in fused silica assisted by a self-assembling chromium layer

Nanostructures have a widespread field of applications and are of growing industrial importance. However, the economic fabrication of nanostructures poses a critical challenge. In this work, a fundamental research of a laser-induced surface nanostructuring of fused silica using the dynamic self-assembling structure formation in metal layers is presented. This method may offer promising opportunities for nanostructuring of dielectrics. This new approach is demonstrated by the formation of randomly distributed concentric nanostructures into fused silica. The irradiation of chromium-covered fused silica samples with a KrF excimer laser results in melting, partial ablation, restructuring, and resolidification of both the metal layer and the dielectric surface. In this way, concentric circular structures into the dielectric were formed with dimensions that can be controlled by the laser fluence Φ and by the pulse number N. The distance of the concentric rings increases with increasing laser fluence. The experimental results were compared with simulated structure dimensions taking into account the heat equation and the Navier–Stokes equation. Despite the currently applied decoupled approach for the simulations, i.e. separating the heat equation and the fluid flow, a good agreement of simulation results with experimental data was achieved.

A comparative study on reflection of nanosecond Nd-YAG laser pulses in ablation of metals in air and in vacuum

A comparative study on reflection of nanosecond Nd-YAG laser pulses in ablation of aluminum in air and in vacuum under the same other experimental conditions is performed. We find that, hemispherical total reflectivity of aluminum undergoes a sharp drop at the plasma formation threshold both in the air and in vacuum. The initial large value (0.8) of aluminum reflectivity decreases to a level of about 0.14 and 0.24 for ablation in the air and in vacuum, respectively. These decreased reflectivity values remain virtually unchanged with further increasing laser fluence. The reflectivity drop in the air is observed to be sharper than in vacuum. Our study indicates that the reflectivity drop is predominantly caused by absorption of the laser light in plasma. Nano/micro-structural defects present on practical sample surfaces play the important role in the plasma formation, especially for the ablation in the air, where the plasma formation threshold is found to be by a factor of 3 smaller than in vacuum.