Optodynamic study of multiple pulses micro drilling

This paper describes an analysis of pulsed lasers micro-drilling of different metals. Study focuses to an optodynamic phenomenon which appears as thermal effects induced by laser light pulses and leads to dynamic process manifested as ultrasonic shock waves propagating into the sample material. The shock waves are detected by a non-contact optical method by using arm compensated Michelson. Monitoring of the main parameters of the micro drilling such as material ablation rate and efficiency was realized by analysis of the optodynamic signals. The process is characterized by decreasing ablation rate that leads to the finite hole depth. The experimental part of study comprehends a comparison between various metals. In order to describe decreasing ablation rate a theoretical model based on the energy balance is proposed. It considers the energy/heat transfer from the laser beam to the material and predicts a decreasing drilling rate with an increasing number of successive laser pulses. According to the proposed model, the finite depth of the hole appears as a consequence of the increasing surface area through which the energy of the laser beam is conducted away to the material around the processed area. Decreasing ablation rate and the finite hole depth predicted by model were in good agreement with the experimental results.     

飞秒激光诱导硒化锌晶体表面自组织生长纳米结构

 以250 kHz高重复频率钛宝石飞秒激光聚焦到硒化锌晶体表面,利用扫描电子显微镜观测飞秒激光辐照后晶体的表面结构。发现线偏振激光辐照的区域形成了自组织周期性纳米结构,其周期为160 nm左右,并且可以通过改变激光的偏振方向调节纳米光栅结构的取向;当晶体相对于激光光束以10 mm/s速度移动,经激光扫描后,在晶体表面形成了长程类布拉格光栅。当飞秒激光光束为圆偏振光时,辐照区域形成均匀的纳米颗粒。     

Functional activity of TiO2 nanocolloids generated from diverging high power laser beams

ABSTRACT

When a pulsed laser gets tightly focused onto a target-liquid interface, plasma formation occurs which leads to nucleation of nanoparticles based on the choice of target and its affinity to the surrounding liquid. Here, we report the generation of Titanium Oxide nanoparticles at titanium–water interface from defocused diverging laser beam rather than from converging tightly focused beam. This is done to tackle the laser-induced fragmentation of ejected nanoparticles in the beam path which widens the nanoparticulate statistics as laser energy gets consumed by the nanoparticles thereby causing a reduction in the ablation efficiency of the target material. The use of diverging laser beam effectively takes most of the ablated species away from the beam path and improvises on the ablation phenomena with yield up to 4?mg/h. The utility of the derived nanoparticulates under such conditions is then checked from its photocatalytic activity which shows 70% photodegradation of environmental pollutants.     

Wavelength dependence of soft tissue ablation by using pulsed lasers

Pulsed laser ablation of soft biological tissue was studied at 10.6-, 2.94-, and 2.08-μm wavelengths. The ablation effects were assessed by means of optical microscope, the ablation crater depths were measured with reading microscope. It was shown that Er:YAG laser produced the highest quality ablation with clear,sharp cuts following closely the patial contour of the incident beam and the lowest fluence threshold. The pulsed CO2 laser presented the moderate quality ablation with the highest ablation efficiency. The craters drilled with Ho:YAG laser were generally larger than the incident laser beam spot, irregular in shape, and clearly dependent on the local morphology of biotissue. The blation characteristics, including fluence threshold and ablation efficiency, varied substantially with wavelength. It is not evident that water is the only dominant chromophore in tissue.     

Effects of green laser fluence on the characteristics of graphene nanosheets synthesized by laser ablation method in liquid nitrogen medium

We have studied the effects of laser fluence on the characteristics of graphene nanosheets produced by pulsed laser ablation technique. In this work, The second harmonic of a Q-switched Nd:YAG laser at 532 nm wavelength and 5 Hz repetition rate with different laser fluences in the range of 0.5–1.8 J/cm² was used to irradiate the graphite target in liquid nitrogen medium. The products of ablation were characterized using Fourier transform infrared spectroscopy, field emission scanning electron microscopy, X-ray diffraction pattern, UV–Vis absorption spectroscopy, Raman spectrum and transmission electron microscopy. The Raman spectroscopy indicates that the quality of the graphene nanosheets was decreased while their structure defects were increased as the laser fluence was increased from 0.5 to 1.4 J/cm². Our results suggest that the amount of defects and the number of layers in graphene nanosheets can be changed by adjusting the laser fluence. This study could be a useful guidance for producing of high quality of graphene nanosheets by laser ablation method.     

Femtosecond pulse laser-induced self-organized nanostructures on the surface of ZnO crystal

This paper reports self-organized nanostructures observed on the surface of ZnO crystal after irradiation by a focused beam of a femtosecond Ti：sapphire laser with a repetition rate of 250kHz. For a linearly polarized femtosecond laser, the periodic nanograting structure on the ablation crater surface was promoted. The period of self-organization structures is about 180 nm. The grating orientation is adjusted by the laser polarization direction. A long range Bragglike grating is formed by moving the sample at a speed of 10μm/s. For a circularly polarized laser beam, uniform spherical nanoparticles were formed as a result of Coulomb explosion during the interaction of near-infrared laser with ZnO crystal.     

Laser ablation of copper and aluminium in air

The ablation behavior of copper alloy and aluminium irradiated in air by 1.06 m, 10 ns pulsed laser with power density of 6.4×10⁹W/cm² was studied using scanning electron microscopy (SEM), MCS-RBS and X-ray microanalysis. Evidence of bulk vaporization via bubble formation was observed for the copper alloy under the laser irradiation. Silver-enrichment microregions were found in the ablation crater created by the laser shots on the copper alloy sample. Material removal rates of these materials were determined by crater shape-profile measurement. Using self-similar solutions of the gas-dynamic equations, gas-dynamic parameters of the vaporization waves are obtained. These parameters are used to calculate material removal rates and impulse coupling coefficients of these materials under the pulsed laser irradiation. The calculated mass removal rates and the coupling coefficients are compared with the corresponding experimentally determined values. The surface kinetic energy of the irradiated area on the Al sample is estimated. Possible mechanisms for laser ablation of the materials under study are discussed.     

电场中不同能量密度激光烧蚀制备纳米硅晶粒分布特性

在室温和10 Pa氩气环境中,引入平行于靶面方向的直流电场,通过改变脉冲激光能量密度烧蚀单晶硅靶,在与羽辉轴线呈不同角度的衬底上沉积纳米硅晶薄膜。利用扫描电子显微镜和拉曼散射谱对沉积样品进行分析,结果表明:随着激光能量密度的增加,位于相同角度衬底上的晶粒尺寸和面密度逐渐变大;在同一激光能量密度下,零度角处衬底上的晶粒尺寸和面密度最大,且靠近接地极板处的值比与之对称角度处略大。通过朗缪尔探针对不同能量密度下烧蚀羽辉中硅离子密度变化的诊断、结合成核区内晶粒成核生长动力学过程,对晶粒分布特性进行了分析。     

Crater geometry characterization of Al targets irradiated by single pulse and pulse trains of Nd:YAG laser in ambient air and water

High intensities laser pulses are capable to generate a crater when irradiating metal targets. In such condition, after each irradiation significant ablation occurs on the target surface and as a result a crater is formed. The crater characterization is very important specifically for some applications such as micromachining. In this paper, the crater formation in metal targets was studied experimentally. The planar aluminum 5052 targets were irradiated by frequency doubled (532 nm), Q-switched Nd:YAG (∼6 ns) laser beam in ambient air and distilled water. A crater was produced after each irradiation and it was characterized by an optical microscope. Different laser intensities as well as pulse trains were applied for crater formation. The effects of laser characteristics in crater geometry were examined. The depth of the craters was measured by optical microscope and the diameter (width) was characterized by processing of the crater image. The results were explained in terms of ablation threshold and plasma shielding. The results show that the crater geometry extremely depends on the laser pulse intensity, the number of laser pulses, and ambient.     

Femtosecond- and nanosecond-pulse laser ablation of bariumalumoborosilicate glass

Laser ablation with femtosecond pulses (130 fs, wavelength 800 nm, repetition rate 2 Hz) was compared with nanosecond-pulse ablation (10 ns, wavelength 266 nm, repetition rate 2.5 Hz) of bariumalumoborosilicate glass in air using the direct focusing technique. Different ablation thresholds and heat-affected zones were observed. The lateral and vertical machining precision was evaluated. Single nanosecond laser pulses in the far UV resulted in a bubble or a circular hole in the centre of the illuminated spot, depending on the applied fluence. The ablation behaviour in the case of near-IR femtosecond pulses contrasted to this. Bubble formation was not detected. It needed repeated pulses at the same spot to modify the surface until material removal could be observed (incubation). Cavity dimensions of less than the beam diameter were achieved in this case.     

Hybrid laser processing for microfabrication of glass

Hybrid laser processing for the precision microfabrication of glass materials, in which the interaction of a conventional pulsed laser beam and a medium on the material surface leads to effective ablation and modification, is reviewed. A major role of the medium is to produce strong absorption of the conventional laser beam by the material. Simultaneous irradiation by a vacuum ultraviolet (VUV) laser beam that possesses an extremely small laser fluence and an ultraviolet (UV) laser greatly improves the ablation quality and modification efficiency for fused silica (VUV-UV multiwavelength excitation process). The metal plasma generated by the laser beam effectively assists high-quality ablation of transparent materials by the same laser beam, resulting in microstructuring, cutting, color marking, printing, and selective metallization of glass materials (laser-induced plasma-assisted ablation (LIPAA)). The detailed discussion presented here includes the ablation mechanism of hybrid laser processing. Received: 18 December 2002 / Accepted: 20 January 2003 / Published online: 28 May 2003 RID="*" ID="*"Corresponding author. Fax: +81-48/462-4682, E-mail: ksugioka@postman.riken.go.jp     

Gold nanoparticles produced by laser ablation in water and in graphene oxide suspension

The comparison between two different approaches based on the use of the laser ablation in medium to synthetise gold nanoparticles is presented and discussed. Deionised water as well as a graphene oxide (GO) suspension in deionised water have been employed as solution to produce gold nanoparticles by laser ablation. In the former case, the nanoparticles assembly has been stabilised by using surfactants, but in the latter case to avoid undesired effects the use of chemicals was not necessary and Au reduced graphene oxide (Au-rGO) nanocomposites have been obtained. The structure, size and composition of the gold nanoparticles and of the Au–rGO nanocomposites have been monitored by UV–Vis–NIR absorption spectroscopy and Raman spectroscopy, the transmission and scanning electron microscopies and the X-ray energy-dispersive spectroscopy. The presented methodology of Au rGO nanocomposites preparation could represent a green alternative on the production of metallic nanoparticles in biocompatible environment.     

纳秒紫外重复脉冲激光烧蚀单晶硅的热力学过程研究

采用波长为355 nm的纳秒紫外重复脉冲激光对单晶硅片进行了盲孔加工实验, 观测了随脉冲增加激光烧蚀硅片的外观形貌和盲孔孔深、孔径的变化规律, 并对紫外激光辐照硅片的热力学过程进行了分析. 研究结果表明:紫外激光加工硅盲孔是基于热、力效应共同作用的结果, 热效应会使得硅材料熔化、气化甚至发生电离产生激光等离子体,为材料的去除提供条件;激光等离子体冲击波以及高温气态物向外膨胀会对熔化材料产生压力致使其向外喷射,为重复脉冲的进一步烧蚀提供了条件;力效应主要沿着激光传输的方向,垂直于硅表面,使得去除部位主要集中在孔的深度方向,达到较高的孔径比,实验观察孔径比可达8:1;此外,激光等离子体的产生也阻止了激光对靶面的作用,加之随孔深的增加激光发生散焦,使得烧蚀深度有一定的限制,实验观察烧蚀脉冲个数在前100个时加工效率较高.     

Elaboration and characterization of bimetallic nanoparticles obtained by laser ablation of Ni75Pd25 and Au75Ag25 targets in water

A YAG laser operating at the second harmonic wavelength (532 nm, 10 Hz, 8 ns and 40 mJ) was used to elaborate bimetallic nanoparticles by laser ablation of Ni₇₅Pd₂₅ and Au₇₅Ag₂₅ targets in water. TEM–EDX, UV–Vis spectroscopy and PIXE measurements were performed to obtain information on their mean sizes, size distributions and chemical composition as a function of the time of laser ablation. The surface of the laser impacted regions of the targets were characterized by RBS in order to check their composition after the laser ablation. The so-obtained bimetallic nanoparticles always show a homogeneous composition. However, while the composition of Au–Ag nanoparticles was found to be very similar to the one of the alloy target, the composition of the Ni–Pd nanoparticles can be different from the nominal composition of the alloy target. Segregation phenomena can be invoked to explain the difference between the Ni–Pd nanoparticles and the Au–Ag nanoparticles compositions obtained in the same conditions. However, an influence of chemical reactions occurring in the high pressure plasma created locally at liquid–solid interface (called ‘reactive quenching’) cannot be completely ruled out.     

Measurement of ablative laser propulsion parameters for aluminum, Co–Ni ferrite and polyurethane polymer

Laser ablation propulsion is a form of beam-powered propulsion in which a pulsed laser ablates a target material thus producing thrust. We report in this work the measurements of various parameters related to laser-induced micropropulsion in toluene diisocyanate-based polyurethane polymer, aluminum and Co–Ni ferrite. The targets were irradiated by a Q-switched pulsed Nd–YAG laser at 1064 nm (pulse duration 5 ns) under atmospheric conditions. A contact-free optical triangulation method was used to measure the laser ablation induced thrust in the samples. The measurements and calculations depict that Co–Ni ferrite is better in terms of critical propulsion parameters C _m and I _sp. It has been observed that the propulsion parameters depend on the energy per pulse of the incident laser beam.     

Robust plasmonic substrates

We report on resonant infrared laser ablation of polystyrene using single 8 ps pulses at a wavelength of 3.31 μm generated by a MgO:PPLN optical parametric amplifier pumped by a Nd:YLF laser. We determined the single-pulse ablation threshold to be 0.46 J/cm², about a factor of five smaller than in previous free-electron-laser studies. Time-resolved imaging of the laser–target interaction reveals that the detailed dynamics of the ablation process begin with thermal expansion of a large volume of hot material from which a less dense plume of polymeric material evaporates. This plume disappears on a time scale of 0.75 μs and the hot polymer material recedes back into the crater from which it was expelled. Subsequently, and on a much longer time scale, structural alterations in the ablation crater continue to evolve for at least another millisecond. Our results suggest that single picosecond pulses are effective for the ablation of polymers and exhibit dynamics similar to those observed in studies using a free-electron laser.     

Femtosecond laser ablation characteristics of nickel-based superalloy C263

Femtosecond laser (180 fs, 775 nm, 1 kHz) ablation characteristics of the nickel-based superalloy C263 are investigated. The single pulse ablation threshold is measured to be 0.26±0.03 J/cm² and the incubation parameter ξ=0.72±0.03 by also measuring the dependence of ablation threshold on the number of laser pulses. The ablation rate exhibits two logarithmic dependencies on fluence corresponding to ablation determined by the optical penetration depth at fluences below ∼5 J/cm² (for single pulse) and by the electron thermal diffusion length above that fluence. The central surface morphology of ablated craters (dimples) with laser fluence and number of laser pulses shows the development of several kinds of periodic structures (ripples) with different periodicities as well as the formation of resolidified material and holes at the centre of the ablated crater at high fluences. The debris produced during ablation consists of crystalline C263 oxidized nanoparticles with diameters of ∼2–20 nm (for F=9.6 J/cm²). The mechanisms involved in femtosecond laser microprocessing of the superalloy C263 as well as in the synthesis of C263 nanoparticles are elucidated and discussed in terms of the properties of the material.     

355 nm nanosecond pulsed Nd:YAG laser profile measurement, metal thin film ablation and thermal simulation

Laser ablation of nickel, gold and copper thin film on glass substrates has been investigated using a nanosecond pulsed Nd:YAG laser operating at 355 nm in air with a Gaussian intensity profile. The exact beam profile was measured through mechanical scanning with a photodiode. A small beam defect was observed, which can affect the machining performance at higher pulse energies. The ablation thresholds of the films were calculated from the crater diameter values. The effect of the pulse repetition rate and the film thickness was also studied. At high pulse repetition rates heat accumulation was observed and the ablation threshold decreased with the film thickness. Both cases resulted in higher diameters.     

Effect of doping and annealing on the physical properties of ZnO:Mg nanoparticles

Well-dispersed undoped and Mg-doped ZnO nanoparticles with different doping concentrations at various annealing temperatures are synthesized using basic chemical solution method without any capping agent. To understand the effect of Mg doping and heat treatment on the structure and optical response of the prepared nanoparticles, the samples are characterized using X-ray diffraction (XRD), energy-dispersive X-ray (EDX), UV–Vis optical absorption, photoluminescence (PL), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) measurements. The UV–Vis absorbance and PL emission show a blue shift with increasing Mg doping concentration with respect to bulk value. UV–Vis spectroscopy is also used to calculate the band-gap energy of nanoparticles. X-ray diffraction results clearly show that the Mg-doped nanoparticles have hexagonal phase similar to ZnO nanoparticles. TEM image as well as XRD study confirm the estimated average size of the samples to be between 6 and 12 nm. Furthermore, it is seen that there was an increase in the grain size of the particles when the annealing temperature is increased.     

Sub-15 femtosecond laser-induced nanostructures emerging on Si(100) surfaces immersed in water: analysis of structural phases

Nanoscale periodic rifts and subwavelength ripples as well as randomly nanoporous surface structures were generated on Si(100) surfaces immersed in water by tightly focused high-repetition rate sub-15 femtosecond sub-nanojoule pulsed Ti:sapphire laser light. Subsequent to laser processing, silicon oxide nanoparticles, which originated from a reaction of ablated silicon with water and aggregated on the exposed areas, were etched off by hydrofluoric acid. The structural phases of the three types of silicon nanostructures were investigated by transmission electron microscopy diffraction images recorded on focused ion beam sections. On nanorift patterns, which were produced at radiant exposure extremely close to the ablation threshold, only the ideal Si-I phase at its original bulk orientation was observed. Electron diffraction micrographs of periodic ripples, which were generated at slightly higher radiant exposure, revealed a compression of Si-I in the vertical direction by 6 %, which is attributed to recoil pressure acting during ablation. However, transitions to the high-pressure phase Si-II, which implies compression in the same direction at pressures in excess of 10 GPa, to the metastable phases Si-III or Si-IV that arise from Si-II on pressure relief or to other high-pressure phases (Si-V–Si-XII) were not observed. The nanoporous surfaces featured Si-I material with grains of resolidified silicon occurring at lattice orientations different from the bulk. Characteristic orientational relationships as well as small-angle grain boundaries reflected the rapid crystal growth on the substrate.