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.     

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.     

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     

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     

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.     

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.     

Hardnes and resistivity of amorphous carbon thin films deposited by laser ablation

Amorphous carbon thin films were deposited by laser ablation of a graphite target, using the fundamental line of a 5 ns Nd:YAG laser. Deposition was carried out as a function of the plasma parameters (mean kinetic ion energy and plasma density), determined by means of a planar probe. In the selected working regimes the optical emission from the plasma is mainly due to atomic species, namely C⁺ (426.5 nm); however, there is also emission from other atomic species and molecular carbon. The hardness and resistivity could be varied in the range between 10 and 25 GPa, and 10⁸ and 10¹¹ Ω cm, respectively. The maximum values were obtained at a 200 eV ion energy and 6×10¹³ cm⁻³ plasma density, where the maximum quantity of C–C sp³ bonds was formed, as confirmed by Raman spectroscopy.     

High repetition rate femtosecond laser nano-machining of thin films

Thin film laser micromachining has been utilized for repairing semiconductor masks, creating solar cells and fabricating MEMS devices. A unique high repetition rate femtosecond fiber laser system capable of variable repetition rates from 200 KHz to 25 MHz along with helium gas assist was used to study the effect of pulse repetition rate and pulse energy on femtosecond laser machining of gold-coated silicon wafer. It was seen that high repetition rates lead to smaller craters with uniform line width. Craters created at 13 MHz pulse repetition rate with 2.042 J/cm² beam energy fluence measured 110 nm in width and had a heat affected zone of 0.79 μm. It was found that pulse repetition rate only played a significant role in the size of the heat affected zone in the lower pulse energy ranges. In the future, a 1 W laser system will be acquired to find the optimal repetition rate that would create the minimal feature size with the least heat affected zone. Using this kind of setup along with techniques such as radial polarization and a different gas assist may enable us to create sub 100 nm feature size with good quality.     

Enhanced magnetization and ferrimagnetic behavior of normal spinel ZnFe2O4 thin film irradiated with femtosecond laser

We report on spatially selective change of magnetism from paramagnetic to ferrimagnetic-like behaviors in normal spinel ZnFe₂O₄ thin film under irradiation with 780 nm femtosecond laser pulses. The distribution of Zn²⁺ and Fe³⁺ ions in the irradiated region on the film surface becomes disordered because of local heating to high temperatures, and the metastable phase of ZnFe₂O₄ is frozen in by the rapid quenching after irradiation, resulting in the formation of the ferrimagnetic phase. The ferrimagnetic phase reverts to the paramagnetic state by annealing at 800°C. The present technique is useful for two-dimensional patterning of magnetic thin films.     

Nonlinearity and time-resolved studies of ion emission in ultrafast laser ablation of graphite

We have investigated the laser fluence dependence of the ion emission process in ultrafast laser ablation of graphite using a time-of-flight technique. Two regimes of ion emission have been identified: (1) a highly nonlinear laser absorption process accompanied by generation of a transient electrical field on the surface and collisionless emission of ions due to electrostatic repulsion; (2) a saturation regime for laser power absorption characterised by nearly equal kinetic energy of ejected carbon clusters. We also show the effect of the surface temperature on the emitted clusters’ stability and the influence of nonlinearity on the intensity autocorrelation traces.     

Thickness distribution of thin amorphous chalcogenide films prepared by pulsed laser deposition

Amorphous chalcogenide thin films were prepared from As₂Se₃, As₃Se₂ and InSe bulk glasses by pulsed laser deposition using a KrF excimer laser. Thickness profiles of the films were determined using variable angle spectroscopic ellipsometry. The influence of the laser beam scanning process during the deposition on the thickness distribution of the prepared thin films was evaluated and the corresponding equations suggested. The results were compared with experimental data.     

Influence of the plasma parameters on the properties of aluminum oxide thin films deposited by laser ablation

The plasma produced by the ablation of a high purity Al₂O₃ target, using the fundamental line (1064 nm) of a Nd:YAG laser, was characterized. The laser fluence was varied in order to study its effect on the characteristics of the produced plasma as well as on the properties of the material deposited. Optical emission spectroscopy (OES) was used to determine the type of excited species present in the plasma. The mean kinetic energy of the ions and the maximum plasma density were determined from the time of flight (TOF) curves, obtained with a planar Langmuir probe. The obtained results reveal that the fast peak in the probe curve could be attributed to Al III, while the slow peak corresponds to the Al II. Aluminum oxide thin films were then deposited under the same conditions of the diagnosed plasma, in an attempt to correlate the plasma parameters with the properties of the deposited material. It was found that when Al II ion energies are lower than 461.0 eV the films deposited have structural characteristics similar to that of α-Al₂O₃, whereas at ion energies greater than 461.0 eV amorphous material was obtained.     

SBN thin films growth by RF plasma beam assisted pulsed laser deposition

SBN thin films were grown on MgO and Silicon substrates by PLD and RF-PLD (radiofrequency assisted PLD) starting from single crystal Sr_0.6Ba_0.4Nb₂O₆ and ceramic Sr_0.5Ba_0.5Nb₂O₆ stoichiometric targets. Morphological and structural analyses were performed on the SBN layers by AFM and XRD and optical properties were measured by spectroellipsometry. The films composition was determined by Rutherford Backscattering Spectrometry. The best set of experimental conditions for obtaining crystalline, c-axis preferential texture and with dominant 31° in-plane orientation relative to the MgO (100) axis is identified.     

Optical extinction for determining the size distribution of gold nanoparticles fabricated by ultra-short pulsed laser ablation

This work presents a method, based on measurements of the optical extinction spectra, to determine the size of spherical gold nanoparticles produced using the femtosecond laser ablation process in deionized water. By using an improved theoretical model that modifies the contribution of the free electrons to the dielectric function introducing a size-dependent term, it is possible to fit the full experimental extinction spectrum considering a certain size distribution. Additionally, in order to obtain complementary measurements of the size distribution, TEM analysis was performed. The results obtained showed that the predominant nanoparticle size distribution ranges from 1 to 11 nm in terms of radii. An optical extinction measurement together with an appropriate theoretical model based on Mie’s theory represents a simple, low-cost, fast and easy method to describe a multimodal size distribution of spherical gold nanoparticles.     

Microstructure and electrical conductivity of YSZ thin films prepared by pulsed laser deposition

Yttria-stabilized zirconia (YSZ) is the most common solid electrolyte material used e.g. in ceramic fuel cells. Thin films of YSZ were deposited on c-cut sapphire single crystals by pulsed laser deposition using a KrF excimer laser focused on a polycrystalline 8 mol% Y₂O₃-stabilized ZrO₂ target. Depending on the substrate temperature and the oxygen background pressure during deposition, different microstructures are obtained. XRD and high-resolution SEM revealed the formation of dense amorphous films at room temperature. At 600°C preferentially (111) oriented polycrystalline films consisting of densely agglomerated nm-sized grains of the cubic phase resulted. Grain size and surface roughness could be controlled by varying the oxygen background pressure. RBS and PIXE evidenced congruent transfer only for a low number of pulses, indicating a dynamical change of the target stoichiometry during laser irradiation. The in-plane ionic conductivity of the as-deposited crystalline films was comparable to bulk YSZ. The conductivity of initially amorphous YSZ passes a maximum during the crystallization process. However, the relative changes remain small, i.e. no significant enhancement of ionic conductivity related to the formation of a nanocrystalline microstructure is found.     

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.     

Electrical properties of boron-doped diamond-like carbon thin films deposited by femtosecond pulsed laser ablation

We report on electrical measurements and structural characterization performed on boron-doped diamond-like carbon thin films deposited by femtosecond pulsed laser deposition. The resistance has been measured between 77 and 300 K using four probe technique on platinum contacts for different boron doping. Different behaviours of the resistance versus temperature have been evidenced between pure DLC and boron-doped DLC. The a-C:B thin film resistances exhibit Mott variable range hopping signature with temperature. Potential applications of DLC thin films to highly sensitive resistive thermometry is going to be discussed.     

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.     

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.