Coherent linking of periodic nano-ripples on a ZnO crystal surface induced by femtosecond laser pulses

We demonstrate the coherent linking of periodic nano-ripples formed on the surface of ZnO crystals induced by femtosecond laser pulses. By adjusting the distance between two laser scanning zones, the periodic nano-ripples induced by two separated laser writing processes can be coherently linked and the ZnO nanograting with much longer grooves is therefore produced. The length limitation of this kind of nanograting previously set by the laser focus size is thus overcome. The micro-Raman mapping technique is used to evaluate the quality of coherent linking, and the underlying physics is discussed. The demonstrated scheme is promising for producing large-size self-organized nanogratings induced by femtosecond laser pulses.     

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.     

Simple synthetic route for hydroxyapatite colloidal nanoparticles via a Nd:YAG laser ablation in liquid medium

Pulsed laser ablation (PLA) in liquid medium was successfully employed to synthesize hydroxyapatite (HAp) colloidal nanoparticles. The crystalline phase, particle morphology, size distribution and microstructure of the HAp nanoparticles were investigated in detail. The obtained HAp nanoparticles had spherical shape with sizes ranging from 5 to 20 nm. The laser ablation and the nanoparticle forming process were studied in terms of the explosive ejection mechanism by investigating the change of the surface morphology on target. The stoichiometry and bonding properties were studied by using XPS, FT-IR and Raman spectroscopy. A molar ratio of Ca/P of the prepared HAp nanoparticles was more stoichiometric than the value reported in the case of ablation in vacuum.     

Growth of ZnO nanoparticles and nanorods with ultrafast pulsed laser deposition

In this work, we study the application of ultrafast pulsed laser deposition (PLD) in ZnO nanomaterial synthesis, including nanoparticles and nanorods. PLD using long pulse (nanosecond) lasers has been widely used as a method for growing prototype materials. The recently-emerged ultrafast PLD is expected to be able to overcome the problem of large liquid droplet formation. Using near infrared and femtosecond laser pulses in ablation, we first characterize the ablation plume using a Langmuir probe and plasma optical emission spectroscopy. We then examine the structural properties of the nanoparticles generated during low-fluence ablation. Finally, we demonstrate that using nanoparticle aggregates as templates, assisted by plume-excited nitrogen radicals at a high fluence, high quality ZnO nanorods can be grown free of metal catalysts.     

Scanning the laser beam for ultrafast pulse laser cleaning of paint

Aligned arrays of N₂-encapsulated multilevel branched carbon nanotubes were synthesized using a simple one step CVD method by pyrolysis of ferrocene and acetonitrile. Electron energy loss spectroscopy (EELS) and elemental mapping studies reveal that gaseous nitrogen was encapsulated in the carbon nanotubes. Batch-type pyrolysis of catalysts induced flow fluctuation of the reaction gases, resulting in the growth of branched junctions. Molecular nitrogen extruded rapidly along conical catalyst particles inducing N₂ encapsulation inside the branched nanotubes. PACS 07.78.+s; 61.46.+w; 81.07.De; 81.15.Gh     

Ultrafast dynamic ellipsometry measurements of early time laser ablation of titanium thin films

The dynamics of laser ablated titanium thin films are investigated using a recently developed technique that measures time-resolved and one-dimensional spatially-resolved ablation dynamics in a single shot. Ultrafast dynamic ellipsometry, a technique based on space-shifted spectral interferometry, uses the time-dependent frequency of a chirped laser pulse to provide time encoding, allowing the picosecond probing of material dynamics in a single shot. With this technique, the sample is probed at two different incident angles with both s- and p-polarized light, which measures the motion of the material and any change in its complex refractive index. Ultrafast dynamic ellipsometry is applied to study the mechanism of initiation by laser-based optical detonators that employ the ablation of titanium thin films. The resulting data indicate that the titanium is ablated as a fragmented flyer and not as an expanding plasma.     

Cumulative and continuous laser vaporization synthesis of single wall carbon nanotubes and nanohorns

The conditions for the scaled synthesis of single wall carbon nanotubes (SWNTs) and single wall carbon nanohorns (SWNHs) by laser vaporization at high temperatures are investigated and compared using in situ diagnostics. An industrial Nd:YAG laser (600 W, 1–500 Hz repetition rate) with tunable pulse widths (0.5–50 ms) is utilized to explore conditions for high-yield production. High-speed videography (50000 frames/s) of the laser plume and pyrometry of the target surface are correlated with ex situ high resolution transmission electron microscopy analysis of the products for pure carbon targets and carbon/catalyst targets to understand the effects of the processing conditions on the resulting nanostructures. Carbon is shown to self-assemble into single-wall nanohorn structures at rates of ∼1 nm/ms, which is comparable to the catalyst-assisted SWNT growth rates. Two regimes of laser ablation, cumulative ablation by multiple pulses and continuous ablation by individual pulses, were explored. Cumulative ablation with spatially overlapping 0.5-ms pulses is favorable for the high yield and production rate of SWNTs at ∼6 g/h while continuous ablation by individual long laser pulses (∼20 ms) at high temperatures results in the highest yield of SWNHs at ∼10 g/h. Adjustment of the laser pulse width is shown to control SWNH morphology.     

Underwater excimer laser ablation of polymers

In this paper, we study the photoablation kinetic of poly (ethylene terephthalate) (PET), polycarbonate (PC), polyimide (PI) and polystyrene (PS) in both air and water. Compared to the results obtained in air, we highlight the decrease of the ablation threshold (AT) of polyesters in contact with water as a function of polymer chemical structure. In order to check the expected hydrolytic reaction of polyesters near the ablation threshold, the chemical modification of the polymer surfaces, as well the composition of the ablation products, were investigated after irradiation near the fluence of ablation threshold in air (air-F _t ) by X-ray photoelectron spectroscopy (XPS) and confocal Raman microspectroscopy. The morphology of polymers obtained by underwater irradiation and near the air-F _t was also examined using scanning electron microscopy (SEM). To understand the process and its dynamics in contact with water, we consider the model of temperature at the polymer-water interface based on the semi-analytical solution of the transit heat-diffusion equation.     

Near-damage threshold femtosecond laser irradiation of dielectric surfaces: desorbed ion kinetics and defect dynamics

We study the modification of fluoride single crystals after irradiation with femtosecond laser pulses for a range of incident intensities from well below to near damage threshold. The behavior of the desorbed positive ion yields, as analyzed by time-of-flight mass spectrometry, is corroborated with temporal characteristics of radiation induced defects in fluorides. The ion yield evolution upon repetitive irradiation (incubation) exhibits the typical reduction of the multi-shot damage threshold with increasing number of pulses. The experimental data point towards an exponential growth of the transient defect density as the origin of this effect. On the other hand, measurements of the time decay of transient defect fluorescence inside the transparent sample show that the defect lifetime may be even longer than tens of milliseconds. To account for the incubation and the increase of the radiation-target coupling efficiency, a model relating the defect lifetime to a pulse-by-pulse accumulation of transient defects is presented, based on a calculation of the free electron density.     

Laser-based techniques for living cell pattern formation

In the production of biosensors or artificial tissues a basic step is the immobilization of living cells along the required pattern. In this paper the ability of some promising laser-based methods to influence the interaction between cells and various surfaces is presented. In the first set of experiments laser-induced patterned photochemical modification of polymer foils was used to achieve guided adherence and growth of cells to the modified areas: (a) Polytetrafluoroethylene was irradiated with ArF excimer laser (λ=193 nm, FWHM=20 ns, F=9 mJ/cm²) in presence of triethylene–tetramine liquid photoreagent; (b) a thin carbon layer was produced by KrF excimer laser (λ=248 nm, FWHM=30 ns, F=35 mJ/cm²) irradiation on polyimide surface to influence the cell adherence. It was found that the incorporation of amine groups in the PTFE polymer chain instead of the fluorine atoms can both promote and prevent the adherence of living cells (depending on the applied cell types) on the treated surfaces, while the laser generated carbon layer on polyimide surface did not effectively improve adherence. Our attempts to influence the cell adherence by morphological modifications created by ArF laser irradiation onto polyethylene–terephtalate surface showed a surface–roughness dependence. This method was effective only when the Ra roughness parameter of the developed structure did not exceed the 0.1 micrometer value. Pulsed laser deposition with femtosecond KrF excimer lasers (F=2.2 J/cm²) was effectively used to deposit structured thin films from biomaterials (endothelial cell growth supplement and collagen embedded in starch matrix) to promote the adherence and growth of cells. These results present evidence that some surface can be successfully altered to induce guided cell growth.     

Molecular imaging by Mid-IR laser ablation mass spectrometry

Mid-IR laser ablation at atmospheric pressure (AP) produces a mixture of ions, neutrals, clusters, and particles with a size distribution extending into the nanoparticle range. Using external electric fields the ions can be extracted and sampled by a mass spectrometer. In AP infrared (IR) matrix-assisted laser desorption ionization (MALDI) experiments, the plume was shown to contain an appreciable proportion of ionic components that reflected the composition of the ablated target and enabled mass spectrometric analysis. The detected ion intensities rapidly declined with increasing distance of sampling from the ablated surface to ∼4 mm. This was rationalized in terms of ion recombination and the stopping of the plume expansion by the background gas. In laser ablation electrospray ionization (LAESI) experiments, the ablation plume was intercepted by an electrospray. The neutral particles in the plume were ionized by the charged droplets in the spray and enabled the detection of large molecules (up to 66 kDa). Maximum ion production in LAESI was observed at large (∼15 mm) spray axis to ablated surface distance indicating a radically different ion formation mechanism compared to AP IR-MALDI. The feasibility of molecular imaging by both AP IR-MALDI and LAESI was demonstrated on targets with mock patterns. Presented at the 9-th International Conference on Laser Ablation, 2007 Tenerife, Canary Islands, Spain     

Modeling of residual thermal effect in femtosecond laser ablation of metals: role of a gas environment

To describe the effect of significant enhancement in thermal energy retained in metal targets following femtosecond laser ablation in a gas environment, we develop a combined model based on both 2D thermal modeling of laser-induced target heating and dynamics of the ambient gas perturbed by multiphoton absorption of laser energy in close proximity to the target. Using our model, we find that thermal energy coupling to the sample is significantly enhanced due to laser-induced gas-dynamic motion in plasma. Another finding is that total thermal energy coupled to the sample due to gas-dynamic energy transfer and thermal energy conduction is close to that measured in our experiment.     

Ultrashort laser ablation of PMMA and intraocular lenses

The use of intraocular lenses (IOLs) is the most promising method to restore vision after cataract surgery. Several new materials, techniques, and patterns have been studied for forming and etching IOLs to improve their optical properties and reduce diffractive aberrations. This study is aimed at investigating the use of ultrashort laser pulses to ablate the surface of PMMA and intraocular lenses, and thus provide an alternative to conventional techniques. Ablation experiments were conducted using various polymer substrates (PMMA samples, hydrophobic acrylic IOL, yellow azo dye doped IOL, and hydrophilic acrylic IOL consist of 25% H₂O). The irradiation was performed using 100 fs pulses of 800 nm radiation from a regeneratively amplified Ti:sapphire laser system. We investigated the ablation efficiency and the phenomenology of the ablated patterns by probing the ablation depth using a profilometer. The surface modification was examined using a high resolution optical microscope (IOLs) or atomic force microscope—AFM (PMMA samples). It was found that different polymers exhibited different ablation characteristics, a result that we attribute to the differing optical properties of the materials. In particular, it was observed that the topography of the ablation tracks created on the hydrophilic intraocular lenses was smoother in comparison to those created on the PMMA and hydrophobic lens. The yellow doped hydrophobic intraocular lenses show higher ablation efficiency than undoped hydrophobic acrylic lenses.     

Cross patterning on MgO based on dislocations using femtosecond laser irradiation

We show a unique technique to form dense dislocations locally inside a MgO single crystal with a rock-salt type structure using femtosecond (fs) laser irradiation. Cross-shaped patterns of micrometer size, originating from densely introduced dislocations, are formed spontaneously around the focal point. We controlled the three-dimensional propagation of the dislocations by adjusting the pulse energy of the fs laser and NA of objective lens. The technique may open up a new field of dislocation technology for optical applications.     

A point source analytical model of inverse pulsed laser deposition

A simple analytical model for inverse pulsed laser deposition is proposed. In the model the motion of the evaporated material is assumed to emerge as from a point source located above the surface of evaporation at some distance. The obtained thickness profiles of inverse deposited films agree well with those calculated by the test particle Monte Carlo method. The proposed approach has been applied for analysis of experimental data on inverse pulsed laser deposition of graphite in nitrogen atmosphere with nanosecond pulses of laser fluences between 1 and 7 J/cm². The model describes well the thickness profiles and pressure dependence of film growth rate for inverse deposition.     

Interaction of short laser pulses with metals at moderate intensities

The developed model including hydrodynamics with electron heat conduction and electron–ion energy exchange, and cavitation breakup of stretched metastable liquid aluminum (Al) is compared with our laser experiments. For the first time the measured and calculated ablation thresholds agree well in both crater depth and fluence.     

Submicrometer-MOS capacitor with ultra high capacitance biased by Au nanoelectrodes

The ultimate limits of size of the current metal-oxide-semiconductor capacitors can be overcome by preparation of three-dimensional devices that can vertically be biased using one-dimensional metal nanostructures. Here, we present a general and efficient approach to the assembly and integration of Au nanocrystals into functional nanoelectrodes of three-dimensional submicrometer-MOS (0.35 μm²) capacitors, presenting an ultra high capacitance (24±1 pF). The Au nanocrystals were directly produced into a nanoporous template of anodized aluminum oxide that was evaluated, and the electrical characterization of this device corroborates the formation of the MOS capacitor. Flat band voltage is independent of sweep voltage range, and negligible hysteresis of capacitance-voltage curves is observed when sweep voltage ranges from positive to negative and turned again to positive bias. In addition, experimental results match theoretical analysis and indicate the presence of free surface charges stored in the Au nanostructures. The demonstrated ability to control the assembling of the nanocrystals and the results of electrical characterization indicate that the embedded Au nanoelectrodes have a high potential for memory applications based on three-dimensional devices.     

Study of laser fragmentation process of silver nanoparticles in aqueous media

Laser fragmentation of Ag nanoparticles in Ag hydrosol was studied by simultaneous measurements of the transmitted fluence of the incident laser beam and the time evolution of the surface plasmon extinction (SPE) spectra. The experiments showed that the laser fragmentation in a small volume of hydrosol proceeds during first 20 pulses and then reaches saturation. The value of the transmitted fluence corresponding to saturation increases with incident pulse fluence, but the impact of the first pulse applied to the hydrosols shows an optical limitation. Fluences above 303 mJ/cm² cause the formation of less stable, aggregating nanoparticles, while fluences below 90 mJ/cm² do not provide sufficient energy for efficient fragmentation. The interval of fluences between 90–303 mJ/cm² is optimal for fragmentation, since stable hydrosols constituted by small, non-aggregated nanoparticles are formed.     

CO2 laser-induced crystallization of sol–gel-derived indium tin oxide films

Indium tin oxide (ITO) thin films prepared by the sol–gel method have been deposited by the dip-coating process on silica substrates. CO₂ laser is used for annealing treatments. The electrical resistivity of sol–gel-derived ITO thin films decreased following crystallization after exposure to CO₂ laser beam. The topological and electrical properties of the irradiated surfaces have been demonstrated to be strongly related to the coating solution and to the laser processing parameters. Optimal results have been obtained for 5 dip-coating layers film from 0.4 mol/l solution irradiated by 0.6 W/m² laser power density. In this case, homogeneous and optically transparent traces were obtained with a measured sheet resistance of 1.46×10² Ω/□.     

Inverse pulsed laser deposition

Since the advent of pulsed laser deposition (PLD), several different target-substrate arrangements have been proposed. Besides the most common on-axis PLD, several off-axis geometries were studied, mainly to protect the substrate from the agglomerated species (clusters, droplets, particulates) of the plasma plume, which are detrimental to the homogeneity of films. Recently we introduced a novel geometry, termed inverse pulsed laser deposition (IPLD), in which the substrate is placed parallel to and slightly above the target plane. In this paper we summarize our results on this new geometry, and show how it can extend the perspectives of pulsed laser deposition, e.g., by improving the surface morphology of the films. Effects of ambient pressure are presented and exemplified on metallic and compound IPLD films, including Ti, CN _x , and Ti-oxides. AFM topographic images are used to prove that under optimized conditions IPLD is capable of growing compact and smooth films that are superior to PLD ones. A special—but easy-to-implement—IPLD arrangement is also introduced that considerably improves the homogeneity of IPLD films. In this geometry, the properties (e.g., deposition rate and roughness) of the films grown in the 1–25 Pa pressure domain are examined.