Millimeter-long nanofibers of PMMA spun at super-high speed by ablation with a single pulse of a KrF excimer laser

A new form of matter removal in laser ablation is reported. Polymethylmethacrylate (PMMA) nanofibers are obtained when a PMMA target is irradiated with a single pulse of a KrF excimer laser, whose beam is sharply imaged on a square of side the order of 140 μm, so that a strong intensity gradient is produced. The fluence threshold at which fibers appear, 3 J/cm², is much larger than the ablation threshold, approximately 0.8–1 J/cm². Above this fluence, the melt depth is then large enough and the temperature profile is such that explosive boiling is obtained. The model suggests an expulsion of energetic droplets from the intense pressure of the plume to the exterior of the spot. For the transient melt of a polymeric viscoelastic liquid resulting from UV-laser excitation, such droplets provide the heads of the jets pulled from the melt bath, giving rise, after solidification, to nanofibers. The speed of fiber spinning is extremely high (∼800 m/s) and unusual properties of the laser-produced nanofibers may be expected. Received: 16 April 2002 / Accepted: 17 April 2002 / Published online: 19 July 2002     

Liquid Pressure Varifocus Lens

A liquid pressure varifocus lens that consists of a thin polymer film and liquid is described. The lens can be continuously changed from a concave to a convex shape by driving the liquid pressure in the lens. The dynamic range of the focal length of the convex lens is from 50 to 250 mm. A beam spot verifies the variable focal function on a photographic film exposed by a He-Ne laser. In addition, the application of the laser for processing is demonstrated to manufacture Fe compressed powder sheets by normal pulse YAG laser. Its melting marks along a focus change are formed with a minimum width of 0.1 mm.     

Visualization of thermal cutting fluid flows

Outside of the fields where flow visualization is traditionally applied, there exist many processes where fluid phenomena are critical. Here, we survey flow visualization work with a focus on two thermal metal cutting processes. These two processes – plasma-arc cutting and gas assisted laser cutting – account for a large fraction of the means by which steel is cut in our world. Plasma-arc cutting utilizes an electric arc transferred between a cathode and the steel being cut to produce a high temperature gas jet that melts and removes metal. In gas assisted laser cutting, the assist jet is often high-pressure supersonic nitrogen for stainless steel, or near-atmospheric pressure, low-speed oxygen for carbon steel. Visualization of these millimeter-range diameter jets helps to understand the different roles that the assist gas has in these cutting processes, particularly with how the jets interact with the metal being cut. We describe experimental techniques for visualization of the arc jet and gas assist jet, as well as the liquid metal flows being removed from the cut and the gas flow in the torch itself. These visualizations overcome the small physical scales of the process, the bright illumination from the arc itself, and harsh high-temperature environment. The results lend perspective and understanding of the physical phenomena important to process control.     

Studying the mechanisms of steel sheet perforation by the radiation of a continuous-wave fiber laser

Aspects of steel sheet perforation are studied experimentally. Calculations show that the mechanisms of thin sheet perforation change as the focal spot size is increased at a constant laser power. If the spot is small, the melt is removed and the film is disrupted by steel boiling in the spot center. With larger spots, the melt is removed by the force of gravity. The hole diameter grows along with the focal spot size and sheet thickness and is reduced upon an increase in laser power.     

强激光条件下5CB液晶中的激光诱导衍射环现象

强激光经5CB液晶传播时会产生非线性自相位调制现象,利用532 nm和1 064 nmCW激光研究了强激光条件下5CB液晶中的激光诱导衍射环现象,并对强激光在5CB液晶中产生自相位调制的现象和机理进行了讨论.当532 nm激光和1 064 nm激光功率密度分别大于10 kW/cm2和300 W/cm2时,接收白屏上有较明显的衍射环现象|当激光分别持续作用数十毫秒和数百毫秒量级时间时,接收屏上的衍射环现象消失.分别利用基尔霍夫衍射积分公式的菲涅耳近似和夫朗和费近似形式对5CB中激光自相位调制和激光诱导衍射环进行了数值模拟,数值结果与实验结果符合的较好.基于热传导理论定性分析了5CB液晶在不同波长和入射条件下的三阶非线性系数.结果表明：在强激光入射条件下,热效应是自相位调制的主要原因,这种三阶非线性系数除了与液晶的吸收系数和作用时间相关,还与激光作用面积甚至激光诱导指向矢转动过程相关.     

Single-pulse drilling study on Au,Al and Ti alloy by using a picosecond laser

Picosecond laser single pulse ablation of Au, Al and Ti alloy (Ti6Al4V) was experimentally investigated with a laser pulse width of 10 ps at a wavelength of 1064 nm for potential industrial micromachining applications. The diameters, depths and morphologies of the drilled craters were studied. Two novel phenomena were found: as hole diameters decreased with fluence, a change of slope of the trend line indicated a change in ablation mechanism for Al and Ti alloy, metallic materials with short electron-phonon coupling times (<10 ps), while Au showed no such transition: an isolated island structure was also observed on Au due to significant melt expulsion. A one-dimensional two-temperature model has been used to discriminate different ablation phenomena. It is shown that metallic materials with different electron–phonon coupling constant have different ablation characteristics in the ps regime. This study could be very helpful for metallic material micromachining with high repetition rate ps lasers pulses which indicates that high throughput may be achieved as well as good machining quality.     

CO2 laser heating of surfaces: Melt pool formation at surface

The melt pool formation during the heating of titanium and steel surfaces by a moving CO₂ laser beam is examined. The repetitive pulses are introduced in the simulations and the Marangoni effect in the melt pool is incorporated in the model study. The influence of laser scanning speed and the laser intensity parameter on the melt pool size is also considered. The enthalpy–porosity method is adopted to account for the phase change in the irradiated spot. It is found that the influence of laser scanning speed on the melt pool size is considerable, which is more pronounced for laser beam parameter β=1. The melt pool size is smaller for stainless steel as compared to that corresponding to titanium.     

激光表面纹理化过程中的Rayleight-Taylor不稳定性

 用表面纹理化的热模型,分析了固体表面的熔化和重固化的物理过程,计算了在纹理化过程中的表面温度,穿透深度,熔化深度,相面移动速度等物理量,提出了Rayleight-Taylor不稳定性形成周期化结构的机理。     

Ablation with a single micropatterned KrF laser pulse: quantitative evidence of transient liquid microflow driven by the plume pressure gradient at the surface of polyesters

A microscopic flow of a transient liquid film produced by KrF laser ablation is evidenced on targets of PET and PEN. Experiments were done by using single pulses of the excimer laser beam micropatterned with the aid of submicron projection optics and grating masks. The samples of various crystalline states, ablated with a grating-forming beam (period Λ=3.7 μm), were precisely measured by atomic force microscopy, in order to evidence any deviation from the ablation behavior predicted by the current theory (combination of ablation curve and beam profile). This was confirmed by comparing various behaviors dependent on the polymer nature (PC, PET and PEN). PC is a normally ablating polymer in the sense that the ablated profile can be predicted with previous theory neglecting liquid-flow effects. This case is called ‘dry’ ablation and PC is used as a reference material. But, for some particular samples like crystalline PET, it is revealed that during ablation a film of transient liquid, composed of various components, which are discussed, can flow under the transient action of the gradient of the pressure of the ablation plume and resolidify at the border of the spot after the end of the pulse. This mechanism is further supported by a hydrodynamics theoretical model in which a laser-induced viscosity drop and the gradient of the plume pressure play an important role. The volume of displaced liquid increases with fluence (0.5 to 2 J/cm²) and satisfactory quantitative agreement is obtained with the present model. The same experiment done on the same PET polymer but prepared in the amorphous state does not show microflow, and such an amorphous sample behaves like the reference PC (‘dry’ ablation). The reasons for this surprising result are discussed. Received: 31 October 2002 / Accepted: 4 November 2002 / Published online: 22 January 2003 RID="*" ID="*"Present address: ST Microelectronics, Crolles, France RID="**" ID="**"Corresponding author. Fax: +33-556/84-6645, E-mail: s.lazare@lpcm.u-bordeaux1.fr     

Viscous liquid expulsion in nanosecond UV laser ablation: From “clean” ablation to nanostructures

This paper reviews recent results on modeling UV nanosecond laser ablation. Particular attention is given to a viscous liquid flow driven by ablation pressure. Based on the analysis of the Navier-Stokes equation, various strongly different manifestations of this phenomenon are explained. These are (i) a “clean” laser ablation, when the laser spot has a clean sharp spot border free of resolidified melt dross; (ii) a new form of material removal in laser ablation, expulsion on a poly(methyl methacrylate) target of long (up to 1 mm) nanofibers with a radius about 150–200 nm to the exterior of the spot under the action of a single pulse of a KrF excimer laser; and (iii) a new method of laser surface nanostructuring, the formation of a surface foam having a structure of micropores interconnected by nanofilaments with diameters of about 100 nm as a result of single-pulse KrF laser ablation of biopolymer films.     

Analysis of hybrid Nd:Yag laser-MAG arc welding processes

In the hybrid laser-arc welding process, a laser beam and an electric arc are coupled in order to combine the advantages of both processes: high welding speed, low thermal load and high depth penetration thanks to the laser; less demanding on joint preparation/fit-up, typical of arc welding. Thus the hybrid laser-MIG/MAG (Metal Inert or Active Gas) arc welding has very interesting properties: the improvement of productivity results in higher welding speeds, thicker welded materials, joint fit-up allowance, better stability of molten pool and improvement of joint metallurgical quality. The understanding of the main relevant involved physical processes are therefore necessary if one wants for example elaborate adequate simulations of this process. Also, for an efficient use of this process, it is necessary to precisely understand the complex physical phenomena that govern this welding technique. This paper investigates the analysis of the effect of the main operating parameters for the laser alone, MAG alone and hybrid Laser/MAG welding processes. The use of a high speed video camera allows us to precisely characterize the melt pool 3D geometry such as the measurements of its depression and its length and the phenomena occurring inside the melt pool through keyhole-melt pool-droplet interaction. These experimental results will form a database that is used for the validation of a three-dimensional thermal model of the hybrid welding process for a rather wide range of operating parameters where the 3-D geometry of the melt pool is taken into account.     

Ripple formation in the chromium thin film during laser ablation

The beam of a nanosecond pulse laser tightly focused to a line was applied for the back-side ablation of the chromium thin film on a glass substrate. The stripe ablated with a single laser pulse had sharp edges on both sides and ridges of the melted metal around it. The partially overlapping pulses formed a wide cleaned area with a complicated structure made of the metal remaining from the ridges. Regular structures, ripples, were developed when laser fluence was slightly above the single-pulse removal threshold and the shift between pulses was less than half width of the line ablated with a single laser pulse. The ripples were located periodically (∼4 μm) and were orientated perpendicularly to the long axis of the beam spot. Their orientation did not depend on the laser beam polarization. Different models of the ripple formation in the thin metal film were considered, and instability of the moving vapor-liquid-solid contact line during evaporation of thin liquid films appears to be the most probable process responsible for the observed phenomena. Formation of regular gratings with the unlimited line length was experimentally implemented by using the above-mentioned technique.     

Glass modification by continuous-wave laser backside irradiation (CW-LBI)

We report on a permanent change in the physical properties inside glass that is rapidly heated and quenched with a continuous-wave (CW) laser beam. The absorption of the glass was enhanced by laser heating, and the heated spot moved by thermal radiation and conduction. To trigger the heating, an absorbent material was placed on the backside of a glass plate and irradiated through the glass. The laser beam can modify borosilicate glass with a high aspect ratio (∼100:1) at a rate of ∼130 mm/s. The modified zone consists of two concentric cylindrical zones and is crack-free.     

Analysis of melt ejection during long pulsed laser drilling

In pulsed laser drilling, melt ejection greatly influences the keyhole shape and its quality as well, but its mechanism has not been well understood. In this paper, numerical simulation and experimental investigations based on 304 stainless steel and aluminum targets are performed to study the effects of material parameters on melt ejection. The numerical method is employed to predict the temperatures, velocity fields in the solid, liquid, and vapour front, and melt pool dynamics of targets as well. The experimental methods include the shadow-graphic technique, weight method, and optical microscope imaging, which are applied to real-time observations of melt ejection phenomena, measurements of collected melt and changes of target mass, observations of surface morphology and the cross-section of the keyhole, respectively. Numerical and experimental results show that the metallic material with high thermal diffusivity like aluminum is prone to have a thick liquid zone and a large quantity of melt ejection. Additionally, to the best of our knowledge, the liquid zone is used to illustrate the relations between melt ejection and material thermal diffusivity for the first time. The research result in this paper is useful for manufacturing optimization and quality control in laser-material interaction.     

Effect of spot size on cone formation in a XeCl laser ablation of polyethersulfone films

Radiation from the UV excimer lasers, with the fluence above the ablation threshold, can etch the polymer surfaces by photoablation. In some cases different microstructures may appear on the surface during the laser ablation. In this paper the effect of the laser spot size on the cone formation on polyethersulfone films has been investigated. The experiments have been performed with a XeCl laser at the wavelength of 308 nm and at the fluences of 70 and 100 mJ/cm² at air. For the investigation of the effect of the laser spot size on cone formation, the samples were irradiated at two different laser spot sizes of w₁ and w₂ = 0.1 w₁. The morphology of the processed surface was studied by scanning electron microscopy (SEM). It has shown that the shape, size and density of cones change with the change of the laser spot size. Also, the number of pulses and the pulse repetition rate which are needed for threshold of cone formation are affected by the laser beam spot size on the surface.     

Pulsed laser heating calculations incorporating vapourization

We present a scheme for solution of the heat flow equation in one-dimension incorporating melting and vapourization produced under pulsed laser irradiation. The method can be applied to pure materials as well as multilayered structures such as deposited films. The variation of physical properties with temperature can be easily taken into account. Results of calculation are presented for aluminium and for chromium and antimony layers deposited on aluminium. As a consequence of excessive vapourization at high energy densities, the melt depth and the melt duration do not increase beyond a certain limit. The resolidification front velocity is strongly dependent on energy density and can be controlled in an experiment by a careful choice of laser parameters. Some recent experimental data on laser treated chromium films are discussed in light of our calculations.     

Numerical analysis of gas-dynamic instabilities during the laser drilling process

The use of high-pressure gas jets in the laser-drilling process has significant influence on the melt ejection mechanism. These jets are highly unstable and this directly relates to the gas pressure and the geometry of the hole being drilled. The evolution of gas-dynamic instabilities during the laser-drilling process was investigated numerically. A minimum length nozzle (MLN) with a 300 μm throat diameter was modelled at various gas pressures, with the gas jet impinging on a range of simulated holes with different aspect ratios. The simulations predict the formation of surface pressure fluctuations that have a broad spectrum due to both the turbulent nature of the jet and the blunt shock oscillation on the surface. The surface pressure variations and the blunt shock oscillation govern the gas dynamic conditions inside the hole, which strongly influence the melt ejection phenomena during the laser-drilling process.     

Comparative visualized investigation of impact-driven high-speed liquid jets injected in submerged water and in ambient air

This paper is a comparative study on the characteristics of high-speed liquid jets injected in surrounding water and air using shadowgraph technique. One of the main objectives is to investigate the effects of liquid’s physical properties, used to generate the high-speed liquid jets, on jet generation’s characteristics. Moreover, comparative investigations on effects of those liquid jets after injected in water and air are reported. The high-speed liquid jets were generated by the impact of a projectile launched by a horizontal single-stage power gun. The impact-driven high-speed liquid jets were visualized by shadowgraph technique and images were recorded by a high-speed digital video camera. The process of impact-driven high-speed liquid jet injection in air and water, oblique shock waves, jet-induced shock waves, shock waves propagation, the bubble behavior, bubble collapse-induced rebound shock waves and bubble cloud regeneration were clearly observed. It was found that different properties of liquid (surface tension and kinematic viscosity) affect the jet maximum velocity and shape of the jet. Bubble behaviors were only found for the jet injected in water. From the shadowgraph images, it is found that the maximum average jet velocity, expansion and contraction velocities of bubble in axial direction increase when the value of the multiplied result of surface tension by kinematic viscosity increases. Therefore, surface tension and kinematic viscosity are the significant physical properties that affect characteristics of high-speed liquid jets.     

An opto-mechanical nanoshell–polymer composite

Metal nanoshells, which are nanoparticles consisting of a dielectric core surrounded by a metal shell, have an optical response dictated by the plasmon resonance. This optical resonance leads to large extinction cross-sections, which are typically several times the physical cross-section of the particles. The wavelength at which the resonance occurs depends on the core and shell sizes, allowing nanoshells to be tailored for applications. In this paper, we demonstrate how incorporating nanoshells transforms a thermoresponsivepolymer into a photothermally responsive nanoshell–polymer composite. When the thermoresponsive polymer, co-N-isopropylacrylamide-acrylamide (NIPAAm-co-AAm), is heated, the polymer undergoes a reversible decrease in volume. Pristine NIPAAm-co-AAm does not inherently absorb visible or near infrared light. However, by incorporating metal nanoshell particles with a resonance that has been placed at 832 nm into the NIPAAm-co-Aam, nanoshell–polymer composite hydrogels are fabricated. When the composite is illuminated with a diode laser at 832 nm, the nanoshells absorb light and convert it to heat. This induces a reversible and repeatable light-driven collapse of the composite with a weight change of 90% after illumination at 1.8 Wcm^-2. Received: 18 July 2001 / Published online: 10 October 2001     

Modelling of the solid–liquid interface during laser processing using an inverse methodology

This study presents a methodology for estimating the melt depth during laser processing of solid materials. The determination of the melt depth is treated as an inverse heat conduction problem, which includes the solid and liquid phases. The conjugate gradient method is applied to treat the inverse problem using the available temperature measurements. Without the inverse methodology the melt depth is very difficult to obtain with precision. The proposed method can also be applied during microthermal machining to determine the location of the solid–liquid interface and the temperature distributions of the two phases by using scanning thermal microscopy.