Laser Spot Size Research Articles

Adhesive Bonding of Copper Prepared by Laser-Interference near the Interference Structuring Limits.

Adhesive bonding requires adequate surface preparation for ensuring an appropriate joint quality. The interest in adhesive joining has recently expanded to thermal systems having a large number of joints employed for manufacturing and assembly. This study presents surface topology of copper 110 produced by a laser-interference setup that would theoretically yield a periodicity of 1.7 μm, which is near the 1.6–2 μm structuring limit that was estimated based on thermal diffusion length scale for an 8 ns laser pulse. The results show that although the expected periodic interference structuring was not attained, the melt-induced texturing was affected by the laser-interference profile. Single-lap shear tests were performed with specimen surfaces prepared by traditional abrasion and laser-interference structuring methods. Several laser processing parameters, such as the laser spot size, density, number of pulses, and raster speed, were studied. Scanning electron microscope and profilometry measurements were used to characterize the processed surface microstructures. Web-like structures, which indicate widespread melting, were shown to be formed at different processing conditions. Based on the surface topologies investigated, two laser raster speeds were selected to make single-lap-joint specimens. Baseline joints were prepared by abrading joining specimens. The shear-lap strength and displacement at maximum load were shown to be higher by 16.8% and 43.8% for the laser-structured specimens than those of the baseline specimens, respectively. Moreover, the load-displacement curves indicated that the laser-structured joints were more ductile than those without laser-structuring. The increased ductility for the laser-structured joints was found to yield an increase in the energy absorbed during shear-lap testing of approximately of 80–90% over those measured for baseline joints. It is another indicator that laser-interference structuring enhanced the bonding performance of single-lap shear joints.

Large-Area Photoreceptor Degeneration Model in Rabbits by Photocoagulation and Oxidative Stress in the Retina.

Photocoagulation is used for the treatment of retinal ischemic disease. However, due to the invasive nature of photocoagulation and variety of melanin concentrations between individuals, it is challenging to avoid damaging the adjacent photoreceptors and inducing several side effects. Previous studies indicate the role of laser power, duration, and spot size on retinal lesions, but the effect of interspot distance of the laser pulses needs to be considered in panretinal photocoagulation. In this study, we examine different parameters of photocoagulation on lesions of the retina in rabbit, finding that the lesion level of the outer nuclear layer of the retina depended on the pulse duration and laser spot size, and decreasing interspot distance could completely abolish the photoreceptor layer. The degeneration of the photoreceptor by photocoagulation occurred in 24 h and was not restored afterward. We then conducted panretinal photocoagulation in rabbit and found that oxidative stress was decreased in the inner nuclear layer of the retina, and pupillary light reflex and ERG signals were impaired. Our study could provide a rabbit model to explore the mechanism of photoreceptor degeneration and therapies for the side effects after photocoagulation.

Delamination in titanium-based carbon-fibre/epoxy laminates under laser shock peening

Surface modification of titanium-based carbon-fibre/epoxy laminates (Ti-CF FMLs) is a challenging topic due to its multilayer structure. This paper investigates the delamination behavior of the Ti-CF FMLs under laser shock peening (LSP) based on experimental and numerical methods. The results show that the onset of the delamination of the Ti-CF FMLs depends on LSP treated area and the delamination width along the thickness direction increases with LSP area. The direct effect of LSP is to degrade the scalar stiffness of the interface between metal and CFRP layers in the Ti-CF FMLs instead of inducing delamination. As massive LSP are performed, the compressive residual stress produces reverse bending, thus delamination occurs at the damaged interface. The damage of the adhesive layer can be effectively prevented by reducing the laser spot size.

Frequency domain measurements of melt pool recoil force using modal analysis

Recoil pressure is a critical factor affecting the melt pool dynamics during Laser Powder Bed Fusion (LPBF) processes. Recoil pressure depresses the melt pool. When the recoil pressure is low, thermal conduction and capillary forces may be inadequate to provide proper fusion between layers. However, excessive recoil pressure can produce a keyhole inside the melt pool, which is associated with gas porosity. Direct recoil pressure measurements are challenging because it is localized over an area proportionate to the laser spot size producing a force in the mN range. This paper reports a vibration-based approach to quantify the recoil force exerted on a part in a commercial LPBF machine. The measured recoil force is consistent with estimates from high speed synchrotron imaging of entrained particles, and the results show that the recoil force scales with applied laser power and is inversely related to the laser scan speed. These results facilitate further studies of melt pool dynamics and have the potential to aid process development for new materials.

Effect of spherical gold nanoparticles size on nanoparticle enhanced Laser Induced Breakdown Spectroscopy

Recently the use of nanoparticles (NPs) for enhancing Laser ablation based analytical techniques, is getting a growing interest in analytical atomic spectroscopy. This technique is based on the deposition of NPs on the sample surface before the laser irradiation. The effect of the coupling of NPs plasmon with the laser pulse allows a notable improvement of the analytical signal. In this study, different sizes of spherical gold nanoparticles (AuNPs), 10 nm, 40 nm, 60 nm and 100 nm, have been employed to clarify the role of the NPs size on the signal enhancement during Laser Induced Breakdown Spectroscopy (LIBS). Therefore, a systematic investigation of the Nanoparticle Enhanced LIBS (NELIBS) have been done by varying the key experimental parameters as NPs surface concentration, laser fluence and laser spot size. The experimental results show that improvement of the emission signal during NELIBS occurs at specific NPs surface concentration and that this critical concentration depends on the NP size. When the concentration has been optimized for each NP size, the plasmon-enhanced ablation is moderately affected by the NPs size.

Real-time characterization of particles produced by laser ablation for analysis by inductively coupled plasma mass spectrometry

Laser ablation (LA) inductively coupled plasma mass spectrometry (ICPMS) is a powerful technique used for trace element analysis of solid samples. While quantitative analysis can be achieved for many samples with careful use of matrix-matched standards, the complex relationship between the properties of the LA-generated particles and their dependence on the operating conditions, the vagaries of particle transport in the cell and transfer system, and the preferential or incomplete particle evaporation, atomization and ionization in the ICP limits this technique from being fully quantitative over a wide range of sample types and or analytical conditions. The characterization of LA-generated particles represents a particular challenge because of their complex shapes, morphologies, and large and size-dependent void fractions. Improved understanding of how laser settings affect the detailed properties of the LA-generated particles and how particle properties relate to ICPMS signal can potentially lead to ways to effectively tailor or modify inherent sample particles or to add engineered particles as standards, tracers, modifiers, carriers, or concentrators. This work investigates changes in physicochemical properties of LA-generated particles in response to altered laser ablation conditions, including laser power, repetition rate, flow rate, scan rate, and spot size. This is achieved using a multidimensional particle characterization approach that measures in real-time number concentrations, mobility and vacuum aerodynamic diameter distributions, mass, chemical composition, and effective density of individual particles as a function of LA conditions, using a glass standard and a pitchblende ore as samples. These measurements yield fractal dimension, average diameter of primary spherules, number of spherules, void fraction, and dynamic shape factors as a function of particle mass or size. This approach can be extended to other samples and laser wavelengths to comprehensively characterize LA-generated particles and their effects on corresponding ICPMS signals.

Experimental and Computational Investigation of Layer-Dependent Thermal Conductivities and Interfacial Thermal Conductance of One- to Three-Layer WSe2.

Two-dimensional materials such as graphene and transition metal dichalcogenides (TMDCs) have received extensive research interest and investigations in the past decade. In this research, we used a refined opto-thermal Raman technique to explore the thermal transport properties of one popular TMDC material WSe2, in the single-layer (1L), bilayer (2L), and trilayer (3L) forms. This measurement technique is direct without additional processing to the material, and the absorption coefficient of WSe2 is discovered during the measurement process to further increase this technique's precision. By comparing the sample's Raman spectroscopy spectra through two different laser spot sizes, we are able to obtain two parameters-lateral thermal conductivities of 1L-3L WSe2 and the interfacial thermal conductance between 1L-3L WSe2 and the substrate. We also implemented full-atom nonequilibrium molecular dynamics simulations (NEMD) to computationally investigate the thermal conductivities of 1L-3L WSe2 to provide comprehensive evidence and confirm the experimental results. The trend of the layer-dependent lateral thermal conductivities and interfacial thermal conductance of 1L-3L WSe2 is discovered. The room-temperature thermal conductivities for 1L-3L WSe2 are 37 ± 12, 24 ± 12, and 20 ± 6 W/(m·K), respectively. The suspended 1L WSe2 possesses a thermal conductivity of 49 ± 14 W/(m·K). Crucially, the interfacial thermal conductance values between 1L-3L WSe2 and the substrate are found to be 2.95 ± 0.46, 3.45 ± 0.50, and 3.46 ± 0.45 MW/(m2·K), respectively, with a flattened trend starting the 2L, a finding that provides the key information for thermal management and thermoelectric designs.

Post-acceleration of electron bunches from laser-irradiated nanoclusters

In this paper the energy gain of attosecond electron bunches emitted during the interaction of intense, few-cycle linearly polarized lasers with nanoscale spherical clusters is determined. In this case electron bunches are emitted from the rear side of the cluster and are then further accelerated while co-propagating with the laser. A previous study has shown how this two-stage process readily occurs for clusters whose radii lie between the relativistic skin depth, δ r = γ 1/2 c/ω p , and the laser spot size σ L (Di Lucchio & Gibbon, Phys. Rev. STAB 18, 2015). An analytical model for focused light waves interacting with compact, overdense electron bunches in vacuum is derived heuristically from world-line equations of motion of an electron. The functional integral approach is followed under the mathematical point of view of integration with respect to a stochastic variable. The resulting picture of the laser wave crossing the electron’s trajectory leads to a finite energy gain of the electron in light–matter interaction in vacuum. The analytical theory is compared with three-dimensional PIC simulations from which trajectories of the electron bunches can be extracted. The effective increase in bunch energy is determined under realistic conditions both for the peak (mode) and the cutoff energy of the emitted bunch, in order to make quantitative comparisons with theory and the experimental findings of Cardenas et al , Nature Sci. Reports 9 (2019).

Laser‐Assisted Thermal Exposure Lithography: Arbitrary Feature Sizes

In traditional optical exposure lithography, the feature size is restricted by the Abbe limit, and it is difficult to obtain patterns with a feature size smaller or larger than the optical spot itself. Herein, a laser‐assisted thermal exposure lithography technique is reported where the feature size can be arbitrarily tuned and changed, and is not limited to the spot size. The minimum feature size of the obtained patterns is 90 nm, which is ≈1/7 of the laser spot size (0.62 μm). The corresponding aspect ratio is ≈1:1. The maximum feature size is 2.7 μm, which is ≈30 times the minimum feature size. A variety of multiscale and multifunctional structures with different feature sizes are obtained successfully through a high‐speed laser‐assisted thermal exposure system, where the writing speed is more than 10 m s−1. This method offers a pathway for direct laser writing with arbitrary feature sizes, high throughput, and low cost.

Mesoscale modeling of metal-based selective laser melting: evolution of the vapor capillary

In this work we apply a multiphysics approach to selective laser melting to model the evolution of the vapor capillary, taking into account the effects of variable laser intensity, scan speed, and spot size. Restricting the work to a single line scan with a continuous wave laser, we focused on the application of Ti-6Al-4V alloy. In addition to heat, momentum and mass transfer, the solid/liquid/vapor interface was simulated using a front-tracking, level-set method, and an imposed recoil pressure was used to model the evolving vapor capillary. Initial benchmark applications focusing on an evolving melting front served to validate the heat transfer/phase change numerical framework.

Adjusted Process Conditions in Laser Powder Bed Fusion to Obtain a Single-Crystal-Like Microstructure in IN718

The nickel base alloy IN718 was manufactured by Laser Powder Bed Fusion (LPBF) using a laser spot size of 1024 μm and a laser power of 1.3 kW at a layer thickness of 160μm. The resulting porosity, microstructure and mechanical properties are presented. Very coarse and in build direction elongated grains are stacked to form a polycrystalline material with sharp single-crystal-like texture. The appearance of ∑5 grain boundaries between coincidence side lattices is recognized. Tensile testing shows highly anisotropic mechanical properties according to the revealed texture. Increasing the hatch distance reduces the severity of the texture.

Laser-impact-induced splashing: an analysis of the splash crown evolution after Nd:YAG ns-pulse laser impact on a liquid tin pool

The splash created by intense laser pulse impact onto a liquid tin layer is studied experimentally using time-delayed stroboscopic shadowgraphy. An 8-ns infrared (1064 nm) laser pulse is focused onto a deep liquid tin pool. Various laser spot sizes (70, 120, and 130 upmum in diameter) and various laser pulse energies (ranging 2.5–30 mJ) are used, resulting in laser fluences of sim 10–1000 J/cm^2 inducing pronounced splashing. Specifically, we study the time evolution of the splash crown-width. The crown width expansion velocity is found to be linearly dependent on the laser energy, and independent of the focal spot size. A collapse of all crown width evolution data onto a single master curve confirms that the hydrodynamic evolution of our laser-impact-induced splash is equivalent to droplet-impact-induced splashing. Laser-impact splashing is particularly relevant, e.g. for high-brightness laser-assisted discharge-produced plasma and laser-produced plasma sources of extreme ultraviolet light for nanolithography.

Long lifetime of bialkali photocathodes operating in high gradient superconducting radio frequency gun

High brightness, high charge electron beams are critical for a number of advanced accelerator applications. The initial emittance of the electron beam, which is determined by the mean transverse energy (MTE) and laser spot size, is one of the most important parameters determining the beam quality. The bialkali photocathodes illuminated by a visible laser have the advantages of high quantum efficiency (QE) and low MTE. Furthermore, Superconducting Radio Frequency (SRF) guns can operate in the continuous wave (CW) mode at high accelerating gradients, e.g. with significant reduction of the laser spot size at the photocathode. Combining the bialkali photocathode with the SRF gun enables generation of high charge, high brightness, and possibly high average current electron beams. However, integrating the high QE semiconductor photocathode into the SRF guns has been challenging. In this article, we report on the development of bialkali photocathodes for successful operation in the SRF gun with months-long lifetime while delivering CW beams with nano-coulomb charge per bunch. This achievement opens a new era for high charge, high brightness CW electron beams.

Probing thermal conductivity of subsurface, amorphous layers in irradiated diamond

In this study, we report on the thermal conductivity of amorphous carbon generated in diamond via nitrogen ion implantation (N3+ at 16.5 MeV). Transmission electron microscopy techniques demonstrate amorphous band formation about the longitudinal projected range, localized approximately 7 μm beneath the sample surface. While high-frequency time-domain thermoreflectance measurements provide insight into the thermal properties of the near-surface preceding the longitudinal projected range depth, a complimentary technique, steady-state thermoreflectance, is used to probe the thermal conductivity at depths which could not otherwise be resolved. Through measurements with a series of focusing objective lenses for the laser spot size, we find the thermal conductivity of the amorphous region to be approximately 1.4 W m−1 K−1, which is comparable to that measured for amorphous carbon films fabricated through other techniques.

The measurement of anisotropic thermal transport using time-resolved magneto-optical Kerr effect

Owing to the increasing demand for the improved management of thermal transport in devices, it is necessary to develop an optimal method for the measurement of anisotropic heat conduction. Although several techniques have been used to measure the heat transport in anisotropic materials, the accurate determination of anisotropic thermal conductivity remains a major challenge. In this study, we present time-resolved magneto-optical Kerr effect measurement using a pump–probe technique to investigate the thermal characterization of various materials. A substrate/ferromagnetic metal (FM) structure is prepared, where FM acts as a transducer to detect the thermal transport in substrates of silicon, sapphire, and highly ordered pyrolytic graphite. By performing experiments under various modulation frequencies and laser spot sizes, we determine both in-plane and through-plane thermal conductivities. We also discuss the limitations of measurement and provide useful guidelines to determine the thermal properties of different materials with high accuracy.

3D printing of silica glass through a multiphoton polymerization process.

We introduce a laser-based process relying on multiphoton-induced polymerization to produce complex three-dimensional (3D) glass parts. A focused, intense laser beam is used to polymerize a transparent resin, loaded with additives and silica nanoparticles, at the wavelength of the laser beam through nonlinear absorption processes. The object is created directly in the volume, overcoming the limitation of the layer-by-layer process. The process enables the production of silica parts with consecutive debinding and sintering processes. With bulk silica density and a resolution that depends on the laser spot size, 3D objects of centimetric dimensions are obtained.

Parameter optimization of self-reflecting all-laser-driven Thomson scattering based on laser wakefield acceleration

All-laser-driven Thomson scattering based on laser wakefield acceleration can provide high quality X-ray and greatly reduce the source size. Compared with two-pulse setting, the self-reflecting setting can reduce the requirement for temporal and spatial synchronization in experiment. However, it is difficult to optimize X-ray because Thomson scattering is coupled with laser wakefield acceleration in this process. In this paper, we correct theory formula through numerical simulation, and analyze the parameters quantitatively in laser wakefield acceleration and Thomson scattering, such as spot size, duration and energy of laser and electron beam, and reflectivity of plasma mirror. Then we can trace the parameters by using the modified formula rather than the numerical simulation with similar accuracy and less time. The modified formula is also used to optimize the self-reflecting all-laser-driven Thomson scattering X-ray under the given laser conditions. The optimal X-ray luminance and photon number can be obtained by changing the plasma density and the position of the plasma mirror.

A predictive model for in situ distortion correction in laser powder bed fusion using laser shock peen forming

Described is a hybrid metal additive manufacturing (AM) method that integrates in situ laser shock peen (LSP) forming with laser powder bed fusion (PBF) to mitigate vertical distortions during part builds. LSP has recently been proposed to reduce tensile residual stresses during selective laser melting (SLM). The effects of LSP on part distortion, however, have not been rigorously examined. It is proposed here that SLM can be integrated with in situ LSP forming to reduce distortion of the upper surface of parts during or after printing. To study the distortion correction capability, a 2-stage computational framework is created, which includes physics-based models of the SLM process and LSP treatment. Stage 1 includes thermomechanical SLM simulation to predict surface geometry and is applied to model four 50-μm layers of a 316L part having a 4 mm × 4 mm footprint. Stage 2 of the framework includes an elastic-plastic thermomechanical shock-wave simulation to predict LSP surface treatment forming effects. Surface distortion is examined for varying laser spot size, overlap, and part temperatures from 300 to 500 K, using a nanosecond-pulsed infrared laser. For the 316L SLM sample, the upper surface is predicted to have $\sim $ 9-μm vertical distortion on the 200-μm 4-layer build. With a 2-μm allowable distortion, only 44.13% of the surface initially conforms. After one LSP forming treatment at 300 K, conformance improves to 84.75%. After a third LSP forming, with 50% laser power-density increase, surface conformance increases to 91%, demonstrating potential of the hybrid AM-LSP process in reducing finish-machining.

Development of thermal model for the determination of SLM process parameters

In this study, a mathematical model is developed for determining the process parameters used in the manufacturing process of powder materials with selective laser melting method. Although the studies carried out to date cover detailed modeling studies including three-dimensional and time-dependent situations, obtaining quite different approaches from experimental results has led to the idea that a change should be made in the construction of the mathematical problem. Therefore, different from other studies and for the first time, the Eigen-function expansion method was used in the analytical solution of the selective laser melting thermal model. The developed mathematical model includes the steady-state solution with the appropriate boundary conditions of the 2-D non-homogeneous heat equation. The mathematical model was first solved analytically with the Eigen-function expansion method, and the function obtained as a result of the solution was introduced into the MATLAB software. The parametric study was performed numerically over laser power, laser spot size and powder bed thickness. With the developed model, the solution was converged in 30 s and 11.5% more accurate with respect to experimental results were obtained in width, 29% in depth and 3% in temperature as compared to other analytical models exist in the literature.

Temperature evolution, phase ratio and corrosion resistance of duplex stainless steels treated by laser surface heat treatment

• Laser surface heat treatment (LSHT) was performed on duplex stainless steels 2304, 2205, and 2507. • The effect of laser spot size and cooling time t 12/8 was investigated by numerical simulation and microstructure analysis. • LSHT obtained the peak temperature, the t 12/8 and the effective depth as 1350 °C, 8.9 s and 1.5 mm respectively. • Austenite percentage increased from 2%, 5%, and 38% to 27%, 36% and 55 % respectively for 2304, 2205, and 2507. • LSHT increased the corrosion pitting temperature from 13, 42 and 66 °C to 19, 52 and 76 °C respectively for 2304, 2205, and 2507. Duplex stainless steels (DSS) are promising materials due to excellent corrosion resistance and good mechanical properties. However, the corrosion resistance of DSS welded joints deteriorates unavoidably due to the unbalanced phase ratio, namely a low austenite percentage resulted from the welding thermal cycle. Laser surface heat treatment (LSHT) exhibits tremendous advantages to restore the corrosion resistance especially regarding efficiency and flexibility. By adjusting laser energy input, including laser power and spot size, different LSHT conditions were applied to DSS 2304, 2205 and 2507, whose austenite content were deliberately decreased to a very low percentage. The temperature evolution during the LSHT was calculated out by finite element analysis. Critical pitting temperature (CPT) was measured to evaluate the corrosion resistance. A long length of laser spot and a slow scanning speed were beneficial for increasing the austenite percentage during the LSHT. With the length of laser spot as 100 mm and the scan speed as 2 mm/s, the laser power was adjusted to obtain a target peak temperature of 1350 ℃, which provided a cooling time t 12/8 as 8.9 s and an effective action depth as 1.5 mm. For DSS 2304 and 2205, the austenite percentage increased from 2% and 5% in deteriorated samples to 28 % and 56 % after the LSHT; the CPT increased from 12.5 and 42.3 ℃ to 19.2 and 51.8 ℃ respectively. DSS 2507 remained the highest austenite percentage after the same deterioration and exhibited the best corrosion resistance under all the used conditions. The influence of LSHT processing parameters and material compositions on the phase transformation and the corrosion resistance was discussed accordingly.