Production of nanostructures on bulk metal samples by laser ablation for fabrication of low-reflective surfaces

Nanostructure formation on bulk noble metals (copper, gold and silver) by a femtosecond laser was studied aiming at the production of low-reflectivity surfaces. The target surface was irradiated with the beam of a 775 nm wavelength and 150 fs pulse duration Ti:sapphire laser. The fluence was in the 16–2000 mJ/cm² range, while the average pulse number was varied between 10 and 1000 depending on the scanning speed of the sample stage. The reflectivity of the treated surfaces was measured with a visible–near-infrared microspectrometer in the 450–800 nm range, while the morphology was studied with a scanning electron microscope. A strong correlation was found between the decreasing reflectivity and the nanostructure formation on the irradiated surface; however, the morphology of silver significantly differed from those of copper and gold. For the two latter metals a dense coral-like structure was found probably as a result of cluster condensation in the ablation plume followed by diffusion-limited aggregation. In the case of silver the surface was covered by nanodroplets, which formation was probably influenced by the ‘spitting’ caused by ambient oxygen absorption in the molten silver followed by its fast release during the resolidification.     

Pore collapse dynamics during laser annealing of metal surfaces

Empty pore collapse dynamics during laser annealing of metal surfaces is studied. A self-consistent model of pore radius variation in the melt is constructed. Pore size variation with the distance from the melting front is determined.     

Noble metal nanoparticles produced by nanosecond laser ablation

Silver and gold thin films were deposited by pulsed laser ablation in a controlled Ar atmosphere at pressures between 10 and 100 Pa. Different morphologies, ranging from isolated nanoparticle arrays up to nanostructured thin films were observed. Fast imaging of the plasma allowed deducing the expansion dynamics of the ablated plume. Plasma velocity and volume were used together with the measured average ablated mass per pulse as input parameters in a model to estimate the average size of nanoparticles grown in the plume. The nanoparticle size is expected to decrease from 4 nm down to 1 nm with decreasing Ar pressure between 100 and 10 Pa: this was confirmed by transmission electron micrographs which indicate a reduced dispersion of particle size over narrow size ranges. The production of substrates for surface enhanced Raman scattering whose performances critically depend on nanoparticle size, shape, and structure is discussed.     

Correlation between ablation efficiency and nanoparticle generation during the short-pulse laser ablation of metals

The mechanisms of material ablation and nanoparticle generation from metal samples exposed to intense short laser pulses are experimentally investigated. We performed measurements of the ablated volume using optical microscopy and the analysis of the ablation plume by fast imaging. The results confirm the existence of two distinguished ablation regimes as a function of the laser fluence, and give a deeper insight in the involved physical mechanisms. Thus, both regimes are found to be related to the relative amount of atoms and nanoparticles within the plume. Comparing the results obtained for copper and gold, it is possible to determine the influence of electron-lattice coupling on the sample heat regime and the resulting plume properties.     

Dynamics of excimer laser ablation of thin tungsten films monitored by ultrafast photography

The time course of laser light induced transport of tungsten films from a glass support is followed by ultrafast photography using delayed dye laser pulses. The photographs provide unambiguous evidence that the material transport in the 40–200 mJ/cm² intensity domain takes place via removal of solid pieces from the film material. These results are consistent with heat flow calculations which predict the overall melting of the metal layer above 380 mJ/cm². The series of photographs presented give detailed insight into the melting process and have revealed an unexpected in-flight phase separation of solid fracture pieces and molten droplets throughout the 200–900 mJ/cm² domain. The faster propagating molten droplets form a condensed halo in front of the solid pieces, thereby providing an efficient shield between the processing laser light and the solid phase.     

Experimental investigation of ablation efficiency and plasma expansion during femtosecond and nanosecond laser ablation of silicon

Femtosecond laser (Ti:sapphire, 100 fs pulse duration) ablation of silicon in air was compared with nanosecond laser (Nd:YAG, 3 ns pulse duration) ablation at ultraviolet wavelength (266 nm). Laser ablation efficiency was studied by measuring crater depth as a function of pulse number. For the same number of laser pulses, the fs-ablated crater was about two times deeper than the ns-crater. The temperature and electron number density of the laser-induced plasma were determined from spectroscopic measurements. The electron number density and temperature of fs-induced plasmas decreased faster than ns-induced plasmas due to different energy deposition mechanisms. Images of the laser-induced plasma were obtained with femtosecond time-resolved laser shadowgraph imaging. Plasma expansion in both the perpendicular and the lateral directions were compared. PACS 52.38.Mf; 52.30.-q     

Ultrafast laser ablation of metal films on flexible substrates

For the development of organic electronics on flexible substrates, we study the potentialities of direct laser patterning of conductive films deposited on plastic foils. The materials under study are silver and platinum films (100-nm thick) deposited on Kapton^® substrates. The experiments are done using a laser source operating at 1030 nm, 500 fs, under different irradiation conditions: single and multiple pulses at various frequencies. The laser ablation thresholds are measured and the ablation morphologies are analyzed with scanning electron microscopy. The results of these investigations show that photomechanical effects lead to delamination of the film and that depending of the irradiation conditions, incubation or heat accumulation effects can occur. The experimental results are compared to simulations based on the two-temperature model. Particularly we study the heat accumulation effects that can occur in the case of multiple pulses and that are detrimental for plastic substrates.     

Nanostructures fabricated on metal surfaces assisted by laser with optical near-field effects

Nanostructures on metal film surfaces have been written directly using a pulsed ultraviolet laser. The optical near-field effects of the laser were investigated. Spherical silica particles (500–1000 nm in diameter) were placed on metal films. After laser illumination with a single laser shot, nanoholes were obtained at the original position of the particles. The mechanism for the formation of the nanostructure patterns was investigated and found to be the near-field optical resonance effect induced by the particles on the surface. The size of the nanohole was studied as a function of laser fluence and silica particle size. The experimental results show a good agreement with those of the relevant theoretical calculations of the near-field light intensity distribution. The method of particle-enhanced laser irradiation allows the study of field enhancement effects as well as its potentialapplications for nanolithography. Received: 10 December 2002 / Accepted: 20 January 2003 / Published online: 28 May 2003 RID="*" ID="*"Corresponding author. Fax: +65-777/1349, E-mail: HUANG_Sumei@dsi.a-star.edu.sg     

Modelling the formation of nanostructures on metal surface induced by femtosecond laser ablation

We employ the particle-in-cell method to simulate the mechanisms of femtosecond (fs) laser interactions with a metallic target. The theoretical approach considers the solid as a gas of free electrons in a lattice of immobile ions and the laser fluences close to the ablation threshold. At first moments of the interaction, our simulations mapped out different nanostructures. We carefully characterized the rippling phase and found that its morphology is dependent on the distribution of the electron density and the period of the ripples depends on the laser intensity. The simulation method provides new insights into the mechanisms that are responsible for surface grating formation.     

Trapping and energy accomodation of atoms on metal surfaces

Expressions for the trapping and sticking probability and for the energy accomodation coefficient of atoms on metal surfaces are derived by using a Gaussian form for the distribution of the energy transfer. The results are in good agreement with recent numerical simulation calculations and experiments.     

Synthesis of nanoparticles by pulsed laser ablation of consolidated metal microparticles

This paper describes a novel process of nanoparticle synthesis by pulsed laser ablation of consolidated microparticles. Metal microparticles, including Cu, Al and Ag, are consolidated by a cold isobaric press with pressures up to a few hundred MPa before laser irradiation. Nanoparticles are then synthesized in air by high-power pulsed laser ablation of the microparticles using a Q-switched Nd:YAG laser. It is shown that the degree of compaction plays a significant role in determining the size of the produced nanoparticles. The effect of laser fluence and collector position on the distribution of particle size is examined. Photoacoustic deflection probing and nanosecond time-resolved visualization indicate that the novel process attains increased efficiency of laser-energy coupling with the target. PACS 81.07.-b; 81.07.Wx; 42.62.Cf     

Singlet-triplet conversion of He(1s2s) by metal surfaces

The singlet-triplet conversion of the excited He(1s2s) metastable atom, approaching the metal surface with relatively low work function, is delt with on the basis of an extended Newns-Anderson model allowing the atom to have spin singlet and triplet states. After a canonical transformation, we show that a virtual process through the ionic states, of both the positive and the negative ions, makes the conversion possible. The numerical results show that in a certain range of parameters, the conversion possibly takes place in a region rather far from the surface.     

Investigation of metal nanoparticles produced by laser ablation and their catalytic activity

Polydiphenylsilylenemethylene (PDPhSM) thin films, which are difficult to fabricate by conventional methods because of their insolubility and high melting point, have been synthesized by using laser-ablated metal nanoparticles for the thermal ring-opening polymerization of 1,1,3,3-tetraphenyl-1,3-disilacyclobutane (TPDC) in this paper. TPDC was first evaporated on silicon substrates and then exposed to metal (Pt, Cu and Ag) nanoparticles deposition by laser ablation prior to heat treatment. The catalytic activity of Pt, Cu and Ag nanoparticles has been studied. The results showed that the mean diameter of Pt nanoparticles was the smallest, Cu nanoparticles the moderate and Ag nanoparticles the biggest, while the polymerization efficiency for Pt nanoparticles was the highest, Cu nanoparticles the moderate and Ag nanoparticles the lowest. In addition, the penetration behaviours of Pt, Cu and Ag nanoparticles into the TPDC monomer films during laser ablation were different due to the particle size or the chemical interaction between metal nanoparticles and TPDC molecules.     

Large area metal dot matrices made by diffraction mask projection laser ablation

A route towards large area metal dot templates is presented. The templates are generated by diffraction mask projection laser ablation of thin metal films. The well ordered nanodot‐matrices are fast and easy to fabricate and show a high homogeneity over a large area. The metal nanodots are bound to the substrate, making them suitable for subsequent 3D nanostructure synthesis such as glancing angle deposition and nanowire growth mechanisms. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Ultrafast emission of ions during laser ablation of metal for 3D atom probe

The 3D atom probe(3DAP) is an imaging instrument based on the controlled field evaporation of single atoms from a sample having a tip shape with an end radius of 50 nm. In the fs laser assisted 3DAP the evaporation is induced by the laser pulses so that the physical process involved in this 3DAP analysis might correspond to the very early stages of the ablation process. In this paper we present the principle of the 3DAP and we discuss the existing models of the fs assisted evaporation. At last, we test the relevance of these models with pump-probe experiments on tungsten tips in the tomographic atom probe.     

Optical-to-acoustic energy conversion efficiency upon interaction of pulsed laser radiation with a liquid: II. Conversion efficiency measurement by holographic interferometry upon acoustooptic interaction

The efficiency of conversion of pulsed CO₂ laser energy to acoustic energy is measured with holographic interferometry. The method suggested is simple and does not require that fringe shifts in the interferogram be converted to local pressure values in the acoustic wave. The efficiency measured for the thermal mechanism of energy conversion is in good agreement with analytical calculations [1] where the temperature dependence of the volume thermal expansion coefficient of water is taken into account.     

The influence of thermal diffusion on laser ablation of metal films

Single-shot ablation thresholds of nickel and gold films in the thickness range from 50 nm to 7 m have been measured for 14 ns laser pulses at 248 nm, using photoacoustic shock wave detection in air. The metal films were deposited on fused silica substrates. The ablation threshold was found to increase linearly with film thickness up to the thermal diffusion length of the film. Beyond this point it remains independent of film thickness. The proportionality between threshold fluence and thickness allows the prediction of ablation thresholds of metal films from the knowledge of their optical properties, evaporation enthalpies and thermal diffusivities. Physically it proves that ablation is driven by the energy density determined by the thermal diffusion length. A simple thermodynamic model describes the data well. Thermal diffusivities, an essential input for this model, were measured using the technique of transient thermal gratings. In addition, the substrate dependence of the ablation threshold was investigated for 150 nm Ni films.     

纳秒激光烧蚀下液态金属表面微结构的形成(英文)

首次发现了在不同保护气体及多脉冲UV-IR激光的照射下,液态金属的微型突起和微结构的形成。测量表明,针对不同的金属和保护气,这种结构的单脉冲生成速率可达(5～20)μm/pulse,形成了长度为1～2 mm,直径约为焦点两倍的单个微型突起。最后,介绍了控制微结构形状的可能性,并讨论了它们的应用潜力。     

Plume temperature in the laser ablation of polyimide films measured by infrared emission spectroscopy

The infrared emission spectrum of the plume produced by KrF excimer laser ablation of polyimide films in air and in He was measured in the 680 to 1580 cm^–1 wavenumber range. Using 400 mJ/cm² laser pulses of 248 nm and 35 ns duration yielded a strong emission band characteristic of thev ₂ transitions of hot HCN molecules. Band counters calculations were carried out of thev ₂ emission expected from HCN in thermal equilibrium at various temperatures. They indicate that except for a slight deviation of the measured data from thermal equilibrium, the best fit of the observed results is obtained at a plume temperature of 2250±150K.     

Photoacoustic figure of merit for photothermal energy conversion efficiency

The photoacoustic signal detected near the non-illuminated back surface of a photothermal converter can be used for comparative studies of the conversion efficiency of the converter. The signal is caused not only by the thermal wave transmitted through the sample, but also by mechanical vibrations of the sample itself. While no absolute absorptance or emittance values can be obtained, the signal reflects the influences of both these quantities on the conversion efficiency. Thus this kind of method may lend itself to quality control of such converters, although direct comparisons between absorbing surfaces on different substrates are still problematic.