The electron temperature distribution of laser erosion plume after ablation of a tantalum target with excimer laser in vacuum

The erosion plume resulting from an ablation of a tantalum target in vacuum with an excimer laser radiation (308 nm) was studied using the Langmuir probe technique. The spatial and temporal dependencies of the electron probe currents were obtained in real time. The electron temperature of different plume regions was determined from a series of I–U characteristics taken at different distances between the probe and the target. We determined that the plume electron temperature is non-uniform and has a maximum value in front of the plume.     

Numerical modeling of ultrashort-pulse laser ablation of silicon

Silicon ablation by a single ultrashort laser pulse is simulated through a computer model. The agreement between results obtained through the model and experimental data found in the literature supports the hypothesis made by the authors in considering thermal evaporation as the dominant ablation mechanism in silicon. Two distinctive thresholds are defined for the ablation procedure leading to a better interpretation of experimental data. The dependence of ablation fluence thresholds on both wavelength and pulse width is discussed. An approximate analytical model describing the crater formation process is proposed and indicative results are presented.     

Elastic characterization of nanoporous gold foams using laser based ultrasonics

A resonance based laser ultrasonics technique is explored for the characterization of low density nanoporous gold foams. Laser generated zero group velocity (ZGV) lamb waves are measured in the foams using a Michelson interferometer. The amplitude spectra obtained from the processed time-domain data are analyzed using a theoretical model from which the foam Young’s modulus and Poisson’s ratio are obtained. The technique is non-contact and nondestructive, and the ZGV resonance modes are spatially localized, allowing for spatial mapping of the bulk sample properties. The technique may be suitable for process control monitoring and mechanical characterization of low density nanoporous structures.     

激光超声方法研究固-固界面波传播特性

对界面波的传播特性进行了理论及实验研究.首先探讨了界面波的求根问题，基于黎曼面分析，给出了求解界面波特征方程所有根的一般方法.理论上对三种常见的界面波——Stoneley波，Leaky Rayleigh及Leaky Interface波传播机理进行了分析，描述了三种界面波的波矢及位移势在两种介质中的状态.最后基于光弹效应原理, 利用全光学的激光超声手段对界面波进行了实验测量，实测结果与理论符合很好.     

Fractional ionization in plasmas produced by pulsed laser ablation

A pulsed infrared laser (Q-switched Nd:Yag) is employed to irradiate different metal targets having atomic number from Z =13 up to Z =82. The high laser fluence deposited on the metals, of the order of 100 v J/cm, produces high ablation yield and a plasma generation at the target surface. The emitted species are neutral and ionized atoms. Both components have been investigated in terms of yield emission, time-of-flight measurements and angular distribution. Results indicate that the main emission occurs mainly along the normal to the target surface, that the etching, at high fluence, is of the order of 10 v atoms/ pulse, that the atoms velocities are of the order of 10 v m/s, that the maximum ion energies are of the order of keV. During the laser irradiation, expanding and non-equilibrium plasma is produced in front of the target. The plasma has a fraction ionization depending by the metal species and generally within 10% and 80%. The plasma'temperature, at high fluence, can be theoretically calculated and reaches about 10 v K. The fractional ionization of the plasma, experimentally measured, has been investigated as a function of the laser fluence and of the energy binding of the target molecules. The ion emission yield is presented and discussed in view of the possibility to realize a laser ion source for ion accelerators.     

Diffraction modelling of laser ablation using transmission masks

We present an analysis of near-field diffraction effects in ablation with transmission masks, based on coupling a simplified form of the Fresnel–Kirchhoff diffraction integral with basic models for material removal. Modelling for square, hexagonal and circular proximity masks is described and compared with previously reported experiments on glass, silicon and polyimide using excimer, femtosecond and CO₂ lasers. The model has general applicability and can provide useful insight into the effect of near-field diffraction in ablation patterning.     

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利用时空分辨的测量技术,测定了XeCl紫外激光烧蚀金属Cu消融粒子的发射光谱随时间与空间的强度分布。通过在不同的氢气压强下拍摄到的XeCl308nm紫外激光烧蚀金属Cu消融粒子的发光羽照片,发现激光消融粒子发光羽的颜色在不同区域有不同的颜色,不同区域的发光羽颜色随环境气压的改变而变化。随环境气压的增大,发光羽不仅逐渐变小,而且逐渐变淡。对激光消融粒子发光羽的机理进行了探讨,激光消融粒子发光羽的发光动力学模式在不同区域有不同的发光模式,且随环境气压的变化而改变。定性地解释了所观察的实验现象。     

Formation of periodic structures by laser ablation of metals in liquids

Experimental results are presented on ablation of metals (W, Cu, brass and bronze) in a liquid environment (e.g., ethanol or water) by irradiation with either a pulsed copper vapor laser (0.51 μm) or a pulsed Nd:YAG laser (1.06 μm). The target material is ejected into surrounding liquid in the form of nanoparticles. In a certain range of laser parameters (fluence and number of laser shots) the surface of the solid target is composed of micro-cones having a regular structure. The distance between neighboring micro-cones in the structure depends on the laser spot size. The structures allow the observation of up-conversion of the laser frequency due to generation of the second harmonics in the eye retina.     

“烧蚀模式”激光推进的实验研究

 采用单脉冲激光进行了大气环境下激光烧蚀小钢珠实验,得到其推进效应参数,发现并分析了钢珠在不同放置位置（焦前与焦后）时不同的物理现象;为了得到一系列定量实验数据,采用自行研制的激光冲量靶仪进行了单脉冲激光烧蚀推进效应实验测试,得到了不同环境条件、不同靶材料的激光推进效应参数,并与国外的实验数据以及数值计算结果进行了比较。实验表明,靶材料和激光功率密度是影响冲量耦合系数的主要因素,冲量耦合系数随环境气压的降低而升高。     

The enclosed laser ablation of multiple stacks of laminated, metallised polymer

Enclosed laser ablation of single laminated, metallised polymer films (i.e. polymer, metal, adhesive, polymer layers respectively) has been studied in depth previously by the authors. It was found that most of the energy during this process was transmitted by the transparentised films. Therefore, it was proposed that multiple stacks of laminated metallised polymer samples could be transparentised with a single pass. In this paper, such experiments were carried out and stacks of 10 samples could be transparentised successfully with a single laser pass. The pulse energy incident upon each film was calculated from transmissivity measurements of each sample layer in a successfully demetallised stack, and the results were used to explain the behaviour observed from the experiments. Additional physical phenomena that were not present with single layer enclosed ablation have now been observed for the enclosed laser ablation of multiple layered stacks.     

Investigation of different liquid media and ablation times on pulsed laser ablation synthesis of aluminum nanoparticles

Aluminum nanoparticles were synthesized by pulsed laser ablation of Al targets in ethanol, acetone, and ethylene glycol. Transmission Electron Microscope (TEM) and Scanning Electron Microscope (SEM) images, Particle size distribution diagram from Laser Particle Size Analyzer (LPSA), UV-visible absorption spectra, and weight changes of targets were used for the characterization and comparison of products. The experiments demonstrated that ablation efficiency in ethylene glycol is too low, in ethanol is higher, and in acetone is highest. Comparison between ethanol and acetone clarified that acetone medium leads to finer nanoparticles (mean diameter of 30 nm) with narrower size distribution (from 10 to 100 nm). However, thin carbon layer coats some of them, which was not observed in ethanol medium. It was also revealed that higher ablation time resulted in higher ablated mass, but lower ablation rate. Finer nanoparticles, moreover, were synthesized in higher ablation times.     

Mathematical modeling of laser induced heating and melting in solids

An analytical method for treating the problem of laser heating and melting is developed in this paper. The analytical method has been applied to aluminum, titanium, copper, silver and fused quartz and the time needed to melt and vaporize and the effects of laser power density on the melt depth for four metals are also obtained. In addition, the depth profile and time evolution of the temperature of aluminum before melting and after melting are given, in which a discontinuity in the temperature gradient is obviously observed due to the latent heat of fusion and the increment in thermal conductivity in solid phase. Additionally, the calculated melt depth evolution of fused quartz induced by 10.6 μm laser irradiation is in good agreement with the experimental results.     

Hidden corrosion detection in aircraft aluminum structures using laser ultrasonics and wavelet transform signal analysis

Preliminary results of hidden corrosion detection in aircraft aluminum structures using a noncontact laser based ultrasonic technique are presented. A short laser pulse focused to a line spot is used as a broadband source of ultrasonic guided waves in an aluminum 2024 sample cut from an aircraft structure and prepared with artificially corroded circular areas on its back surface. The out of plane surface displacements produced by the propagating ultrasonic waves were detected with a heterodyne Mach-Zehnder interferometer. Time-frequency analysis of the signals using a continuous wavelet transform allowed the identification of the generated Lamb modes by comparison with the calculated dispersion curves. The presence of back surface corrosion was detected by noting the loss of the S(1) mode near its cutoff frequency. This method is applicable to fast scanning inspection techniques and it is particularly suited for early corrosion detection.     

Picosecond pulsed laser ablation and micromachining of 4H-SiC wafers

Ultra-short pulsed laser ablation and micromachining of n-type, 4H-SiC wafer was performed using a 1552 nm wavelength, 2 ps pulse, 5 μJ pulse energy erbium-doped fiber laser with an objective of rapid etching of diaphragms for pressure sensors. Ablation rate, studied as a function of energy fluence, reached a maximum of 20 nm per pulse at 10 mJ/cm², which is much higher than that achievable by the femtosecond laser for the equivalent energy fluence. Ablation threshold was determined as 2 mJ/cm². Scanning electron microscope images supported the Coulomb explosion (CE) mechanism by revealing very fine particulates, smooth surfaces and absence of thermal effects including melt layer formation. It is hypothesized that defect-activated absorption and multiphoton absorption mechanisms gave rise to a charge density in the surface layers required for CE and enabled material expulsion in the form of nanoparticles. Trenches and holes micromachined by the picosecond laser exhibited clean and smooth edges and non-thermal ablation mode for pulse repetition rates less than 250 kHz. However carbonaceous material and recast layer were noted in the machined region when the pulse repetition rate was increased 500 kHz that could be attributed to the interaction between air plasma and micro/nanoparticles. A comparison with femtosecond pulsed lasers shows the promise that picosecond lasers are more efficient and cost effective tools for creating sensor diaphragms and via holes in 4H-SiC.     

Preparation of nano-sized functional materials using laser ablation in liquids

We propose a convenient technique applicable for investigations of various functions of nanoparticles produced by laser ablation in liquids. It was demonstrated that nanoparticles of anatase-TiO₂, a electrode material for lithium secondary batteries, produced by laser ablation in acetone could be efficiently deposited on a substrate by using an electrophoresis technique. Analysis of the electrochemical properties of nanoparticles become much more facile with those deposited nanoparticles than with dispersed nanoparticles. In addition, it was demonstrated that comparison of the electrochemical properties between nanoparticles and microparticles were possible by means of this technique.     

Enhancement of ablation rate and production of colloidal nanoparticles by irradiation of metals with nanosecond pulsed laser in presence of external electric field

This paper presents the results of experimental study on the effect of electric field on the ablation rate during the nanosecond pulsed laser ablation of aluminum and copper in deionized water. The effect of electric field strength on the material removal rate and its mechanisms were investigated both in the electric field parallel and perpendicular to the laser beam path schemes. The ablation rate was estimated by measuring the dimensions of craters on the target induced by laser. The crater dimensions and optical properties of the produced colloidal nanoparticles were characterized by means of optical microscopy and UV–Vis absorption spectroscopy, respectively. The results indicate that pulsed laser ablation in the presence of an electric field significantly leads to higher material removal rate. The experimental results also confirm that the crater geometry extremely depends on the direction of the electric field with respect to the laser beam direction. The UV–Vis spectra show that the nanoparticles production efficiency increases with increasing the electric field strength.     

Laser ultrasonics detection of an embedded crack in a composite spherical particle

Laser ultrasonics was applied to the manufacturing control of the integrity (no failure) of coated spherical particles designed for High Temperature Reactors (HTR). This control is of major importance, since the coating of the nuclear fuel kernel is designed to prevent from the diffusion of fission products outside the particle during reactor operation. The SiC layer composing the coating is particularly important, since this layer must be an impenetrable barrier for fission products. The integrity of the SiC shell (no crack within the shell) can be assessed by the ultrasonic vibration spectrum of the HTR particle, which is significantly changed, compared to the reference spectrum of a defect-free particle. Spheroidal vibration modes of defect-free dummy particles with a zirconium dioxide (ZrO₂) core were observed in the 2-5 MHz range. A theoretical analysis is presented to account for the observed vibration spectra of defect-free or cracked HTR particles.     

Numerical modeling of laser shock peening with femtosecond laser pulses and comparisons to experiments

A physics-based model has been developed for laser shock peening (LSP) with femtosecond (fs) laser pulses (fs-LSP), which has never been reported in literature to the authors’ best knowledge. The model is tested by comparing simulations with measured plume/shock wave front transient propagations and the LSP-induced hardness enhancement layer thickness. Reasonably good agreements have been obtained. The model shows that fs-LSP can produce much higher pressure than LSP with nanosecond (ns) laser pulses (ns-LSP), and it can also generate very large compressive residual stress in the workpiece near-surface layer with a thickness up to ∼100 μm. The developed model provides a powerful guiding tool for the fundamental study and the practical applications of fs-LSP. This study, together with the recently reported work by Nakano et al. [Journal of Laser Micro/Nanoengineering 4(1) (2009) 35-38], has confirmed the feasibility of fs-LSP on both theoretical and experimental sides.     

Femtosecond laser writing of nanostructures on bulk Al via its ablation in air and liquids

We report on the formation of self-organized nanostructures (NS) on bulk Al under its ablation in air and liquids with femtoseconds (fs) laser pulses. In case of exposure into liquids, NS are regularly formed on the Al surface with an average period of about 200 nm, independent of the laser polarization. A dispersion of Al nanoparticles (NPs) into the liquid additionally occurs. Irregular nano-bumps are produced when the irradiation is performed in air. NP dispersions as well as NS formed on Al surface show a characteristic absorption peak in the near UV which has been attributed to plasmon oscillation of electrons. The wings of this peak extending to the visible, lead to a distinct yellow coloration of the processed Al surface and the liquid dispersions. Ultrafast laser processing of bulk Al in liquids may be potentially a promising technique for efficient production of nanosized aluminum.