Influence Of Laser Power Density Research Articles

Two-photon polymerization for fabrication of metalenses for diffraction-limited focusing and high-resolution imaging

Metalenses consisting of nanoscale unit cells provide new opportunities for miniaturization, light-weight, and diversification of conventional optical lenses. Fabrication of metalenses is conventionally by photolithography technique, which is complex involving multi-steps. In this study, two-photon polymerization (TPP) was used to directly write single-layer metalenses. The metalenses was designed according to Pancharatnam Berry (P-B) phase distribution model at the wavelengths of 515, 628, and 1030 nm, respectively. The focusing performance of the designed metalenses was simulated using a Finite Difference Time Domain (FDTD) software. Near optical diffraction limit focusing of the incident light was achieved on a low refractive index photoresist. The geometrical dimensions of the fabricated unit cells were compared with the designed parameters. The influence of laser power density on dimensions of the unit and the resultant the focusing effect were investigated. The metalenses prepared by TPP with optimum parameters show a minimum full width at half maximum (FWHM) of 978 nm and a focusing efficiency of 1.02% with 628 nm light source. A line width of 980 nm was obtained under a white light source. The research results demonstrate that TPP can be a feasible and simplified approach for fabrication of metalenses, but for higher focusing efficiency and imaging resolution metalenses, some high refractive index photoresist materials being suitable for the TPP should be developed.

Modeling the microwave transmittance of glass fiber-reinforced plastics subjected to laser ablation

The microwave transmittance of glass fiber-reinforced plastic (GFRP) slabs subjected to continuous-wave laser ablation was studied in the framework of continuum mechanics. First, a one-dimensional physical model involving laser absorption, heat conduction, resin pyrolysis, thermal radiation, and convection heat transfer was established to obtain the temperature field. An experiment-based absorption coefficient was proposed to capture the bulk-to-surface absorption transition during laser ablation. Second, the complex dielectric constant was modeled using a solid-state kinetic model describing the graphitization of pyrolysis products. The microwave reflectivity and transmittance were calculated based on the dielectric constant distribution. The agreement of the temperature and microwave transmittance with the experimental results suggests the feasibility of the model. The influence of laser power density, material thickness, and tangential airflow velocity on microwave transmittance was studied based on the model. The microwave transmittance changed nonmonotonically with increasing slab thickness owing to the competition between different physical mechanisms. The existence of tangential airflow reduced the decrease in microwave transmittance, particularly for weaker lasers. This study provides a useful physical model for predicting the microwave transmittance performance of GFRP in extreme heat environments.

Classification and variation of fracture modes in laser shock hole-clinching

Laser shock hole-clinching is a high–strain rate mechanical joining process in which the metal foils are joined together based on plastic deformation generated by a laser-induced shock wave. In the process, fracture is a typical defect and seriously influences the clinching quality and production efficiency of joints. However, the classification and variation of fracture modes in laser shock hole-clinching are of less concern. In this study, the fracture mode of Cu-Fe joints in laser shock hole-clinching was experimentally investigated. Both optical microscopy and scanning electron microscopy were adopted to analyze the fracture surface morphology of clinched joints. The influence of laser power density, laser spot diameter, the initial grain size and thickness of metal foil, and spacer height on the fracture mode of joints was evaluated. It is revealed that the temperature increase caused by high–strain rate plastic deformation has no impact on the fracture behavior of the joining partners. Fracture always occurs on joining partner I, and it can be divided into four modes, including bottom surface fracture, bottom corner fracture, neck fracture, and mixed fracture. It is found that fracture on the bottom surface is seldom seen but mixed fracture accounts for most of the cracked specimens. Neck tensile fracture rarely appears alone, and it usually exists accompanied by fracture on the bottom corner. The fracture mode varies from a tensile fracture mode on the bottom corner to a mixed fracture mode and then to a shear fracture mode on the neck with the enhancement of laser power density. In addition, the initial grain size of joining partner I has a significant impact on the fracture mode of clinched joints. The fracture mode varies from bottom corner fracture to mixed fracture in relation to the change of mechanical properties of metal foil with an enlarged grain size. However, the mixed fracture mode always appears with the enlargement of both laser spot diameter and spacer height.

Characterization of multilayer coatings of aluminium, tungsten and molybdenum on steel substrate using laser-induced breakdown spectroscopy

Laser induced breakdown spectroscopy (LIBS) has been applied for the depth profiling of multilayer metallic coating structure which is simulated to the ITER-like wall materials in fusion devices such as EAST tokamak. The coatings were produced by magnetron sputtering and characterized by LIBS using a Nd:YAG laser at 1064 nm with pulse duration of 5 ns. The light from the resulting plasma was analyzed as a function of both number of laser pulse (N) and elemental spectral line intensity. The depth profile of the multilayer coating was determined by three different spectral signal processing methods; intensity background subtraction, intensity normalization and linear Pearson correlation. The intensity normalization method provides the best outcomes/solutions for thickness measurement and analysis of interfaces of multilayer coatings among other methods. The evolution of spectral intensity representing different materials was properly examined for 90 consecutive laser shots at the same spot. The ablation behaviour of the coating materials was analyzed in terms of ablation rate under the laser pulse of various fluences. In a much deep crater, some factors originating from laser ablation, such as material re-deposition and plasma shielding effect were analyzed. As a comparison, the depth, shape and diameter of craters were also measured using SEM and a profilometer. The influence of laser power density on the ablation rate and crater profile was investigated. The elemental composition, inside the crater, was verified by the EDX technique. The results indicate that LIBS has a potential power for in-situ depth profiling of multilayer coatings on the wall of the EAST.

Mechanical joining behavior of Cu–Fe dissimilar metallic foils in laser shock clinching

Laser shock clinching is a novel joining method derived from laser shock forming in which the metal foil is plastically deformed under the pulsed laser induced shock wave, and then two or more metal foils can be joined together based on plastic deformation. However, the present researches are less concerned with the mechanical joining behavior of metal foils during incremental impacts of multiple laser pulses. In the present study, the mechanical joining behavior of pure copper foil and pre-pierced stainless steel sheet in laser shock clinching was investigated. A finite element model was established to analyze the material flowing and clinching behavior of metal foils under multiple laser pulses. Based on the validated model, the deformation stages, thickness change, and shock wave propagation features were studied. The temperature rise during clinching was assessed considering both the compression by shock wave and plastic deformation at high strain rates. It is revealed that the laser shock clinching process can be divided into three deformation stages, that is, free bulging forming, radial expansion, and formation of interlock. Both experimental and numerical results prove that the formation of clinched joints relies on the plastic deformation of the joining partner I. The thinnest region of the joint locates at the material of the joining partner I in contact with the upper corner of the joining partner II. In addition, there is no obvious influence of temperature increase on the mechanical properties of joining partners. Moreover, the shock wave propagation characteristics along axial direction and the influence of laser power density on interlock value and thickness distribution were also discussed.

Influence of process parameters on forming quality of Cu-Fe joints by laser shock hole-clinching

Laser shock hole-clinching is a novel mechanical joining process in which the shock wave pressure induced by a high-energy pulsed laser is employed as a punch. This process is quite suitable for the joining of two dissimilar materials with significant differences in plasticity and strength. However, the effect of key process parameters on the forming quality of joints by laser shock hole-clinching is less concerned. In this study, the influence of laser power density, spacer height, number of laser pulse, and initial grain size of metal foils on the forming quality of Cu-Fe joints was experimentally investigated. The characteristics of interlock value and thinning rate distribution under various process parameters were systematically examined. It is revealed that laser shock hole-clinching process can be divided into the confined bulging and interlock formation stages. Based upon the examination of substandard joints, the defects in laser shock hole-clinching process are classified which contains no interlock, nonuniform interlock, bottom and neck fracture. It is found that the thinnest position of joints locates at the region in contact with the upper corner of the joining partner II in which extensive plastic deformation occurs related to the force acting during clinching process. The interlock value increases significantly as the enlargement of initial grain size, whereas the deviation also becomes larger due to the inhomogeneous plastic deformation for the metal foil with coarse grains. Moreover, the ratio of spacer height to the thickness of the joining partner II affects the thinning rate distribution and interlock value of joints. The spacer height equaling to the thickness of the joining partner II is recommended within the given parameters.

A rapid method for screening and determination test of methanol content in ethanol-based products using portable Raman spectroscopy

Raman spectroscopy is nondestructive sample preparation and suitable for use in quality control-related field inspections, particularly on ethanol-based products. The influence of laser power density (PD) and exposure time (ET) on the Raman spectra similarity, specificity, selectivity, and accurate measurements for screening and determination of methanol content were carried out in this study.Raman spectra of samples were measured using a Raman handheld instrument. Measuring scales of PD and ET were varied from 50 to 300 milliWatt (mW) and 1000–5000 ms (ms), respectively. The similarity of Raman-spectra of those samples was statistically analysed with the help of hierarchical cluster analysis (HCA) and cross-correlation function (Ccorrel). The subtracted Raman spectra and pure ethanol spectra were implemented to identify methanol content in samples of ethanol-based products specifically.The Raman spectra measured at PD of 200–300 mW and clustered in one group by ward linkage method showed a similarity level of 99.98%. The variation of ET between 1000 and 5000 ms on fix PD of 300 mW showed high precision (RSD <2%) with more than 99% similarity of Raman spectra. Rinsing of PD was found to increase the signal: noise ratio, while prolonged exposure time reduced the signal: noise ratio. The precision and accuracy measurements were obtained by setting the PD and ET values of 300 mW and 4000 ms, respectively. The subtracted Raman spectra with the spectra of absolute ethanol introduced a simultaneous screening of methanol containing alcohols and determination of both substances.

Research on Debonding Defects in Thermal Barrier Coatings Structure by Thermal-Wave Radar Imaging (TWRI)

In this paper, thermal-wave radar imaging (TWRI) is introduced to detect debonding defects in SiC-coated Ni-based superalloy plates. Linear frequency modulation signal (chirp) is used as the excitation signal which has a large time–bandwidth product. Artificial debonding defects in SiC coating are excited by the laser beam with the light intensity modulated by a chirp signal. Cross-correlation algorithm and chirp lock-in algorithm are introduced to extract the thermal-wave signal characteristic. The comparative experiment between TWRI reflection mode and transmission mode was carried out. Experiments are conducted to investigate the influence of laser power density, chirp period, and excitation frequency. Experimental results illustrate that chirp lock-in phase has a better detection capability than other characteristic parameters. TWRI can effectively detect simulated debonding defects of SiC-coated Ni-based superalloy plates.

Laser postionization of neutral molecules sputtered using bismuth and argon cluster primary ions.

In this study, the influence of two different cluster primary ions in laser secondary neutral mass spectrometry (Laser-SNMS) has been investigated. Despite the many advantages of Laser-SNMS, fragmentation of neutral organic molecules during both sputtering and photoionization has limited its efficiency for the study of large organic and biological molecules. Cluster ion sputtering, and in particular large argon gas cluster sputtering, has been proposed as a means of reducing this fragmentation. Molecules of 9-fluorenylmethoxycarbonyl-pentafluoro-l-phenylalanine were sputtered using Bi3+ and Ar2000+ cluster primary ions, and the desorbed neutral species ("secondary neutrals") were postionized using a 7.87 eV vacuum ultraviolet laser light fluorine excimer laser. By varying timing parameters and laser power density, time-of-flight and laser power density distributions were obtained to investigate the fragmentation and energy distributions of the sputtered neutral species. Changing from 30 keV Bi3+ sputtering to 10 keV Ar2000+ resulted in a significant reduction in fragmentation of the molecule as well as a suppression of the high background that results from metastable decay of highly excited ions, yielding significantly improved detection of the intact molecule and characteristic fragments. Analysis of the influence of laser power density and laser pulse delay time indicates a reduction of fragmentation in both the sputtering phase and the photoionization phase. This study demonstrates the importance of soft desorption for efficient laser postionization of large organic molecules and shows the potential for improving the efficiency of laser postionization by using large gas cluster ion sputtering.

Influence of laser power density on damage of comb by photodynamic therapy—simulation and validation of mathematical models

Photodynamic therapy (PDT) has poor therapeutic outcomes for the treatment of port-wine stain (PWS) lesions with long drug-light intervals (DLIs). This letter investigates the possibility of improving the treatment efficacy through increasing the laser power density using a computer simulation and a cock comb model. The computational model includes a Monte Carlo simulation for the laser distribution and a calculation of the singlet oxygen concentrations (1O2). Both simulation and experimental results show that increasing the power density from 100 to 140 mW/cm2 not only improves the PDT efficacy, but also results in the unwanted skin damage.

KrF excimer laser irradiated nanoporous TiO 2 layers for dye-sensitized solar cells: Influence of laser power density

This paper reports the performance of dye-sensitized solar cells (DSSCs) with nanoporous TiO2 photoanodes irradiated by KrF excimer laser beams of various power densities. The laser induces surface remelting and solidification to create textures on the TiO2 layers. After laser irradiation, TiO2 also undergoes a phase transition from anatase to rutile; the amount of the transformed phase increases with the laser power density. For dye-anchored TiO2 layers, light absorption increases first and then decreases as the laser power density increases. The assembled cell efficiency, strongly correlated with the photocurrent density, also increases first and then decreases as the laser power density rises. This indicates that laser treatment creates surface textures on TiO2 that improve light trapping in the dye-anchored TiO2 photoanodes, thereby increasing the photocurrent level and cell efficiency. For high laser irradiation power density, the ablation of the TiO2 layer becomes significant, leading to a decrease in cell efficiency. The surface remelting and solidification process reduces the density of the surface recombination centers on TiO2, resulting in an increase of open-circuit voltage.

Laser Surface Modification of Borochromizing C45 Steel

Laser Surface Modification of Borochromizing C45 SteelIn this study the test results for borochromized C45 steel after laser surface modification were presented. Influence of laser heat treatment on the microstructure and microhardness of surface layer was investigated. The process of borochromizing consisted of chromium plating followed by diffusion boronizing. The laser heat treatment (LHT) of multiple tracks in the helical line was carried out with CO2laser beam. The technological laser TRUMPF TLF 2600 Turbo CO2of the nominal power 2.6 kW was applied. Borochromizing was carried out with laser power density q = 41.40 kW/cm2and at laser beam scanning rate v = 0.67 m/min and v = 2.016 m/min. Measurements of microhardness were conducted using the Vickers' method and Zwick 3212 B hardness tester. Microstructure observations were performed by means of an optical microscope Metaval Carl Zeiss Jena and scanning electron microscope Tescan VEGA 5135. After laser heat treatment with re-melting a three-zone layer was obtained, which included: re-melted zone, heat affected zone and a core. Influence of laser treatment parameters on thickness of melted zone and microstructure of the surface layer was tested. The microhardness tested along the axis of track of the surface layer after laser modification was about 800-850 HV. The results of tests showed influence of laser power density and scanning rate on microstructure and properties of borochromized layers.

The Effects of Laser Texturing Parameters on Wear Properties of Rollers with Fractal Geometry

The processing parameters of laser texturing are linked with the wear property of roller surface based on the relationship between the fractal parameters and surface roughness. The influence of laser power density, scanning speed and pulse width on the roughness is analyzed though experiments. The effect of fractal parameters on the wear rate of the roller is analyzed with fractal geometry model for wear prediction. Condition of achieving engineering surface with optimal fractal dimension is presented and the expression of the optimal fractal dimension is derived. It shows that the roughness of laser textured surface and normalized wear rate are mainly determined by laser power density and fractal dimension, respectively. Moreover, to get the textured surface with smallest nominal wear rate, processing parameters of laser texturing, especially the laser power density, should be controlled properly besides the material properties of the roller. The result will provide guidance for the processing.

Estudio del endurecimiento superficial por láser de fundiciones grises

Three different ferritic-pearlitic gray cast irons have been laser hardened. The influence of laser power density and scanning speed on their microstructural modification, affected depth and surface hardness have been evaluated. It was observed that when the laser energy density surpasses a certain critical value, a surface layer melts and a ledeburitic microstructure of very high hardness is obtained. Below it, a second layer of martensitic microstructure also of high hardness, is produced.

Ignition of pyrotechnic mixture by means of a laser diode. Part I: Numerical modelling

AbstractA numerical model of the heating and ignition of a reactive solid by a laser beam has been developed. A transient, two‐dimensional heat equation was solved numerically using an explicit scheme. A nonlinear source term (Arrhenius equation) complicates the analytical resolution of this type of problem. Laser beam absorption is considered in a few micrometer depth. Influence of depth absorption coefficient is investigated. Influence of laser power density, lasering time and thermal diffusivity on ignition are examined by this model. The developed numerical model has been used to design a laser ignition system for explosive substances. It has been shown that ignition by a 0.6 W laser diode at a fiber optics output is feasible.