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Applied Surface Science (v.247, #1-4)

Subject Index (pp. viii-xxx).

Author Index (pp. i-vii).

Production of gas phase zinc oxide nanoclusters by pulsed laser ablation by I. Ozerov; A.V. Bulgakov; D.K. Nelson; R. Castell; W. Marine (pp. 1-7).

We present experimental results on the photoluminescence (PL) of gas-suspended zinc oxide nanoclusters prepared during ablation of sintered ZnO targets by a pulsed ArF laser in the presence of oxygen ambient gas. The PL spectra in the UV spectral region correspond to the exciton recombination in the nanoclusters which are crystallized and cooled down to the temperature of the ambient gas in the ablation chamber. The time evolution of the spectra as well as their dependence on the ambient gas pressure are discussed.

Keywords: PACS; 61.80.Ba; 78.55.Et; 81.05.Ys; 81.15.FgZinc oxide; Nanoclusters; Photoluminescence; Laser ablation

Improving the photoluminescence of thin films by nanostructuring the rare-earth ion distribution by R. Serna; A. Su�rez-Garcia; M. Jimenez de Castro; C.N. Afonso (pp. 8-17).

Pulsed laser deposition has been used to prepare nanostructured rare-earth-doped films. Alternate deposition from the host (Al₂O₃) and dopant (Er, Yb) targets has been used to obtain artificially structured films in which the rare-earth ion–ion separation is controlled in the nanometer scale (0.5–9nm). In the case of Er³⁺–Er³⁺ interactions, it has been found that self-quenching is greatly reduced for separations larger than 3nm. The control of the laser fluence used for Er ablation is also important, and higher fluences lead to higher luminescence efficiencies. The Er and Yb co-doped films have been designed with the Er³⁺ and Yb³⁺ ions distributed either in the same layer or in different layers separated from 1 to 3nm. The emission intensity in these co-doped films can be up to two orders of magnitude higher than that reported for films doped only with Er. The results evidence that structuring the rare-earth distribution in the nanometer scale is a very efficient means to reduce rare-earth clustering. Finally, it will also be shown that nanostructuring offers a valuable method to analyze ion–ion interactions such as energy transfer mechanisms.

Keywords: Photoluminescence; Nanostructuring; Rare-earth ion distribution; Thin films

Random lasing from surface modified films of zinc oxide nanoparticles by A. Stassinopoulos; R.N. Das; E.P. Giannelis; S.H. Anastasiadis; D. Anglos (pp. 18-24).

The photoluminescence properties of zinc oxide (ZnO) ceramic thin films, prepared by spin coating of ZnO nanoparticle aqueous suspensions, were studied with emphasis on the influence of film structure and surface morphology on the observation of random laser action. Surface processing employing laser annealing transforms the particulate grain structure of the as-deposited films into a porous channel-like network. This modification was shown to be critical for achieving random laser action as it favors efficient coupling of the pump light into the film material.

Keywords: Random laser; Surface processing; Zinc oxide

Iron–iron oxide core–shell nanoparticles synthesized by laser pyrolysis followed by superficial oxidation by F. Dumitrache; I. Morjan; R. Alexandrescu; V. Ciupina; G. Prodan; I. Voicu; C. Fleaca; L. Albu; M. Savoiu; I. Sandu; E. Popovici; I. Soare (pp. 25-31).

Iron-based core–shell nanostructures were synthesized by laser pyrolysis in a two-steps procedure. In a first step, using a cross-flow configuration, the laser radiation was heating a gas phase mixture containing iron pentacarbonyl (vapors) entrained by an ethylene flow, which plays also the role of an energy transfer agent. Secondly, a carefully controlled in situ passivation of the freshly formed pyrophoric iron nanoparticles created a protective iron oxide shell. The produced nanoparticles (22nm size diameters) with core–shell features were analyzed by TEM, XRD, SAED and Raman spectroscopy. Majoritary iron and gamma iron oxide/magnetite and minoritary carbon phases were identified. In laser pyrolysis experiments in which the reaction temperature was increased, the catalyzed homogeneous nucleation and growth of carbon nanotubes in the gas phase was observed and is presented here for the first time.

Keywords: Laser pyrolysis; Passivation; Iron core–oxide shell nanocomposite; Carbon nanotube

Structuring of metallic bi- and tri-nano-layer films by laser interference irradiation: control of the structure depth by A. Lasagni; F. M�cklich (pp. 32-37).

A periodical structuring on bi-metallic thin layer films was performed by irradiation in air with an Nd:YAG laser operating in pulsed mode. The samples consist on Fe–Al, Cu–Al, Ni–Al and Fe–Al–Fe over a glass substrate. The influence of the laser fluence, the layer thickness, and the substrate on the structure depth (SD) was studied. A thermal simulation was carried out to estimate the temperature distribution and the quantities of the molten material in each layer. It was found that the maximal SD in the lower energy regime is reached at the threshold laser fluence value at which the topography type changes from a low to a high structured profile type. In addition, it was found that as the thickness of the second layer and the difference between the melting points of the metallic layers increase, the structure depth also increases. In the case of bi-layer films, the structuring of the samples is conducted by the expansion of the Al and the glass–substrate at the interference peaks. Nevertheless, if a third metallic layer is placed over the substrate, the structure depth decreases as the thickness of this layer increases. This occurs because the third layer helps to evacuate the heat from the first two layers much faster than the glass substrate and therefore decreasing the structure depth.

Keywords: Structure depth; Topography; Laser interference; Fe/Al; Cu/Al; Ni/Al

Detailed studies of the plume deflection effect during sub-ps laser ablation of Si target by A. Dima; A. Perrone; A. Klini (pp. 38-44).

For the first time the plume deflection effect during the irradiation of silicon target with sub-ps KrF laser pulses was investigated. The morphological changes of the laser ablated target and the corresponding plume deflection angle have been studied in vacuum with different laser fluences and number of pulses. In accordance with earlier results obtained by nanosecond laser, we found a strong correlation between the morphological changes on the irradiated target and the observed plume deflection angle. As the number of laser pulses increases, the formation of well-defined silicon columnar microstructures oriented towards the laser beam has been observed on the targets ablated with fluences between 0.1 and 1.7J/cm² (2.2�10¹¹ and 37.4�10¹¹W/cm² of power density, respectively). It has also been observed that the plume deflection effect is present for laser fluences between 0.1 and 1.2J/cm², with a maximum deflection angle always higher than 20�. Simultaneously, an array of silicon substrates placed on a hemi-cylindrical holder was used for deposition experiments, in order to study the influence of the plume deflection on the thickness distribution profile of the deposited films. Profilometric analyses of the deposited silicon films revealed a non-uniform material deposition. Comparison with the results obtained by nanosecond lasers is reported and discussed.

Keywords: Plume deflection; PLD; Silicon; SEM

Number density and size distribution of droplets in KrF excimer laser deposited boron carbide films by T. Sz�r�nyi; R. Stuck; F. Antoni; E. Fogarassy (pp. 45-50).

This paper contributes to the old problem of droplet formation by providing detailed quantitative data on the population statistics of particulates deteriorating the surface of boron carbide films produced by PLD. Films of 180±30nm thicknesses have been fabricated by KrF excimer laser ablation of a sintered B₄C target in high vacuum. Number densities and size distributions of the particulates are given for 14 films, deposited at laser fluences tuned between 2 and 14Jcm⁻², as a result of evaluating SEM images of the same magnification taken from minimum five different areas (altogether (2–4)×10⁻²mm²) of each film. The overwhelming majority of the droplets is small with diameters, d≤1μm. The number density of those with d≤0.5μm ranges from (1 to 7)×10⁴mm⁻². The effect of gradual deterioration of the target surface as a result of prolonged irradiation screens practically totally the effect of changing fluence. The number of droplets of diameters in excess of 1μm increases linearly with increasing number of shots/site resulting in maximum surface coverage values as high as 36%.

Keywords: PLD; Thin films; Boron carbide; B; ₄; C; Particulates; Droplets; Size distribution; Histograms

Femtosecond pulsed laser ablation of group 4 carbides by R. Teghil; L. D’Alessio; A. De Bonis; A. Galasso; P. Villani; M. Zaccagnino; A. Santagata; D. Ferro (pp. 51-56).

The pulsed laser ablation of titanium, zirconium and hafnium carbides has been performed by a Nd:glass laser with a pulse duration of 250fs. The gaseous phase produced from the ablation has been analysed by emission spectroscopy and ICCD imaging. The results evidence large differences in the plasma characteristics in the case of femtosecond or nanosecond ablation. In particular, in the femtosecond and nanosecond plumes, the energy and the velocity of neutral and ionised particles are very different. The ablation mechanism seems to be very different, including, in the case of femtosecond ablation, the delayed emission from the target of large and slow particles. The morphology of the deposited films is clearly related to the characteristics of the plasma. Preliminary results show a morphology consisting of a large number of spherical particles with diameters ranging from 50 to 200nm.

Keywords: Pulsed laser ablation; Carbides; fs ablation

Thermal properties characterization of conductive thin films and surfaces by pulsed lasers by J. Martan; N. Semmar; C. Leborgne; E. Le Menn; J. Mathias (pp. 57-63).

An experimental system for investigation of thermal properties of thin films and material surfaces especially with high thermal conductivity was developed. This system is based on photothermal method using pulsed laser with nanosecond pulse duration in the UV range, and a high speed IR photodetector.Calibration of the experimental system, enabling absolute temperature measurement, is described. The system was calibrated for Cu, Al, Ti, Ni samples and Au/Ni layers on bronze substrates, where the mean sensibility of the detector varied from 31μV/K for Cu and Au to 116μV/K for Ti.First are shown measured thermal responses and deduced effusivities for pure metals in a ‘bulk’ form, which were investigated to validate the experimental accuracy of the system.Second, for a specific application of electrical contacts in microelectronics multilayer Au/Ni coatings on bronze substrates were studied. Thermal effusivity values plotted versus time after laser pulse enable estimation of the effusivity variation from the surface coating to the substrate. The effusivity of surface Au/Ni coatings with thickness 0.8/2μm was estimated to 25800Ws^1/2m⁻²K⁻¹ and with thickness 0.2/2μm was estimated to 21400Ws^1/2m⁻²K⁻¹.

Keywords: Thermal properties characterization; Thin surfaces; Pulsed lasers

A correlated study of laser produced plume expansion dynamics and thin film growth of manganates by S. Amoruso; M. Angeloni; G. Balestrino; N. Boggio; R. Bruzzese; P.G. Medaglia; A. Tebano; M. Vitiello; X. Wang (pp. 64-70).

Thin films of La_0.67Ba_0.33MnO₃ were deposited on NdGaO₃ by pulsed laser deposition with in situ RHEED diagnostics. A strong dependence of structural and electrical transport properties of the films on the background oxygen pressure was observed. Electrical resistance versus temperature and X-ray diffraction measurements were used to characterise the deposited films. Deposition at low background oxygen pressure (≈10⁻¹Pa) resulted in a good structural quality with an atomically flat surface, but poorer transport properties compared to films grown at higher pressure (≈50Pa). These observations have been correlated with the characteristics of the plume expansion by studying pulsed laser ablation of manganate samples into oxygen background gas by optical emission spectroscopy and ion probe diagnostics. These studies have been carried out using the parent LaMnO₃ compound as target material in presence of an oxidizing atmosphere.

Keywords: Manganate; Metal–insulator; Laser; Plume expansion

Synthesis of nickel nanoparticles and nanoparticles magnetic films by femtosecond laser ablation in vacuum by S. Amoruso; G. Ausanio; C. de Lisio; V. Iannotti; M. Vitiello; X. Wang; L. Lanotte (pp. 71-75).

We report on the generation of Ni nanoparticles and the synthesis of magnetic nanoparticles films by 100femtosecond (fs) Ti:sapphire laser ablation in vacuum. The nanoparticles sizes have been studied by atomic force microscopy, showing a mean size of ≈40nm. Vibrating sample magnetometry analysis has been performed to characterize magnetic properties of the deposited nanoparticles layers. The hysteresys loops clearly indicate that the Ni nanoparticles film behaves as a system of isolated magnetic particles. Our results evidence the potentiality of fs laser ablation, in vacuum, for the generation of nanoparticles and the deposition of nanoparticles films of magnetic materials.

Keywords: Magnetic nanoparticles; Ferromagnetic films; Laser deposition

Magnetic domains in Co-cluster assembled films deposited by LECBD by F. Dumas-Bouchiat; H.S. Nagaraja; F. Rossignol; C. Champeaux; A. Catherinot (pp. 76-82).

Cobalt aggregates prepared using a cluster beam generator have been deposited on Si(100) substrate leading to thin films of randomly assembled Co nanoparticles which exhibit a spherical shape with a mono-dispersed diameter distribution centred around 9nm. Films with thickness ranging from 50 to 550nm are investigated using magnetic force microscopy (MFM) and results show the presence of twisted magnetic domains. An in-plane magnetic field applied during the growth of the layer leads to the formation of magnetic stripe domains but we observe a similar behaviour if an in-plane magnetic field is applied after the deposition. This indicates that probably the magnetic field applied during the film growth does not drive its magnetic structure. Finally, the measured variation of magnetic domain width D reveals at dependence, where t is the film thickness, and is independent of the magnetic history of the films.

Keywords: Co-cluster films; Low energy cluster beam deposition; Laser ablation; Magnetic force microscopy; Twisted magnetic domains

Pulsed laser deposition of pepsin thin films by G. Kecskem�ti; N. Kresz; T. Smausz; B. Hopp; A. N�gr�di (pp. 83-88).

Pulsed laser deposition (PLD) of organic and biological thin films has been extensively studied due to its importance in medical applications among others. Our investigations and results on PLD of a digestion catalyzing enzyme, pepsin, are presented. Targets pressed from pepsin powder were ablated with pulses of an ArF excimer laser ( λ=193nm, FWHM=30ns), the applied fluence was varied between 0.24 and 5.1J/cm². The pressure in the PLD chamber was 2.7×10⁻³Pa. The thin layers were deposited onto glass and KBr substrates. Our IR spectroscopic measurements proved that the chemical composition of deposited thin films is similar to that of the target material deposited at 0.5 and 1.3J/cm². The protein digesting capacity of the transferred pepsin was tested by adapting a modified “protein cube�? method. Dissolution of the ovalbumin sections proved that the deposited layers consisted of catalytically active pepsin.

Keywords: PACS; 81.15.Fg; 68.55.Jk; 87.68.+zPulsed laser deposition; Thin films; Pepsin; FTIR

Epitaxial growth of La_0.7Ca_0.3MnO₃ thin films by KrF excimer laser assisted metal organic deposition process by K. Daoudi; T. Tsuchiya; I. Yamaguchi; T. Manabe; S. Mizuta; T. Kumagai (pp. 89-94).

Epitaxial La_0.7Ca_0.3MnO₃ (LCMO) films were prepared on SrTiO₃ single-crystal substrates by metal organic deposition (MOD) process. A KrF excimer laser irradiation at a substrate temperature of 500°C is successfully used instead of thermal annealing at high temperature to perform the epitaxial growth of the LCMO thin films. The epitaxial growth was confirmed by X-ray diffraction (XRD: θ–2 θ scans and pole figure analysis). Morphology and surface properties of the obtained films were investigated by scanning electron microscopy and atomic force microscopy. The temperature coefficient of resistance (TCR) was calculated from the temperature dependence of the resistance measurements. A TCR of about 6% was obtained for the excimer laser annealed films which is considered a good value for bolometric applications.

Keywords: LCMO; MOD; Epitaxial growth; Low temperature; Excimer laser; Bolometer

SnO₂ nanostructured films obtained by pulsed laser ablation deposition by C. Ristoscu; L. Cultrera; A. Dima; A. Perrone; R. Cutting; H.L. Du; A. Busiakiewicz; Z. Klusek; P.K. Datta; S.R. Rose (pp. 95-100).

Nanostructured tin oxide films were deposited on Si(100) substrates at room temperature using the PLAD method. Depositions were achieved by using a XeCl^* laser, with a fluence of 10J/cm² to ablate a SnO₂ target either in vacuum or in an atmosphere containing a relatively low partial pressure (10⁻² to 100Pa) of O₂. A range of spectroscopic diffraction and real space imaging techniques, SEM, EDS, XRD, and XPS were used in order to characterize the surface morphology, structure, and composition of films. XPS results indicate that the film deposited in vacuum comprises a mixture of Sn, SnO and SnO₂. In contrast, films deposited in oxygen comprise only SnO and SnO₂. This indicates that the deposition of tin oxide films in relatively low partial pressures of O₂ prevents the accumulation of unreacted tin metal within the resulting film. SEM studies reveal a sub-micron grain size structure, while XRD analyses indicate that the samples are crystalline, with no evidence for the presence of amorphous material.In this paper we discuss the significance and implications of these findings in terms of the optimization of tin oxide films for use in gas sensing devices and other technological applications.

Keywords: Oxide films; PLAD; SEM; XRD; XPS

Wave front dislocations appearance under the laser beam self-action in liquid crystal by Svitlana Subota (pp. 101-106).

We present theoretical study of optical singularity birth and behaviour in an initially smooth wave front of the incident astigmatic Gaussian light beam. Linearly polarised light beam illuminates a homeotropically aligned nematic liquid crystal cell. Strong director anchoring at the cell walls is assumed. Director reorientation profile is found numerically solving Euler-Lagrange equations. We found the threshold intensity for light induced Fredeericksz-type transition. The threshold intensity of light beam is appeared to increase with increasing of beam asymmetry under the constant value of laser beam area and cell thickness. The results are compared with those calculated using Gaussian-like trial function.Utilizing the Huygens-Fresno principle we calculate the propagation of the distorted light beam after the liquid crystal cell. It is found that with distance increasing we can observe at first the dipole, then the quadrupole and then again the dipole of optical vortices whis unit charge. Thus, the trajectory of zero amplitude resembles a deformed rubber ring symmetrical in the xz-, yz-planes and stretched along z-axis.

Keywords: PACS; 61.30Gd 42.65.-k 42.70.DfNematic liquid crystal; Phase singularities; Light induced Fredeericksz transition

Spin dynamics and lifetime of exciton polaritons in CuCl by H. Rahimpour Soleimani; S. Cronenberger; O. Cr�gut; J.-P. Likforman; M. Gallart; T. Ostatnicky; P. Gilliot; B. H�nerlage (pp. 107-114).

In CuCl, biexcitons may be excited from the semiconductor ground state by two circularly polarized exciton polaritons of opposite helicity. We use this property to develop a new method in order to determine the spin-relaxation dynamics by means of non-degenerate pump–probe experiments. A pump beam first excites a spin-polarized exciton population. Then, we measure the induced absorption of a circularly polarized probe beam at the exciton biexciton transition. It depends on its polarization and the induced absorption dynamics gives information on the spin state of the exciton polariton population. When comparing results obtained in a (σ⁺σ⁺) configuration with that of a (σ⁺σ⁻) configuration, we determine the time evolution of the pseudo-spin of the exciton polaritons as well as their lifetime.

Keywords: Exciton-spin dynamics; Spin flip; CuCl

Gravimetric and profilometric measurements of the ablation rates of photosensitive polymers at different wavelengths by Th. Dumont; R. Bischofberger; T. Lippert; A. Wokaun (pp. 115-122).

The ablation rates of two polyimides (PMDA and Durimid™) and one triazene polymer were studied by gravimetric (quartz microbalance) and profilometric (profilometer) methods at irradiation wavelengths of 193, 248 and 308nm. The ablation rates determined by the two methods are discussed in the context of the absorption behavior of the materials. Furthermore, the consistence of the two experimental methods is discussed for the ablation rates of Durimid™ and the triazene polymer. The gravimetric measurements revealed a good correlation between the ablation rate and the absorption properties of the examined materials. The comparison of the gravimetric and the profilometric measurements suggest a significant mass removal, e.g. by formation of gaseous products, prior to the detection of changes in the surface morphology.

Keywords: Polyimides; Quartz microbalance; Profilometer

RF plasma reactive pulsed laser deposition of boron nitride thin films by B. Mitu; P. Bilkova; V. Marotta; S. Orlando; A. Santagata (pp. 123-127).

Thin films of boron nitride (BN) have been deposited on Si(100) substrates by reactive pulsed laser ablation (PLA) of a boron target in the presence of a 13.56MHz radio frequency (RF) nitrogen plasma. The gaseous species have been deposited at several substrate temperatures, using the on-axis configuration. The film properties have been investigated by Scanning Electron Microscopy, Atomic Force Microscopy, Fourier Transformed Infrared Spectroscopy, and X-ray diffraction characterization techniques, and compared to those resulting from the conventional PLA method. The behavior of hexagonal-BN and cubic-BN phases grown by PLA as function of substrate temperature is also reported.

Keywords: Reactive pulsed laser deposition; RF plasma; Nitrides; Thin films

Pulsed laser deposition of HfO₂ and Pr_xO_y high-k films on Si(100) by M. Ratzke; D. Wolfframm; M. Kappa; S. Kouteva-Arguirova; J. Reif (pp. 128-133).

Pulsed laser deposition was used to grow thin films of the high-k materials praseodymium oxide (Pr_xO_y) and hafnium oxide (HfO₂) on Si(100) due to its experimental simplicity and flexibility. Most important factors for technical application, such as film morphology and interface quality, were investigated by optical microscopy, atomic force microscopy and Raman spectroscopy.During the growth process typical splashes, originating from the laser–target interaction, are embedded into the growing layer. The size of these splashes appears to depend strongly on the laser wavelength (355, 532, 1064nm). The microscopic morphology of layers of both materials shows a dependence on substrate temperature, which is much more pronounced in case of HfO₂.Raman spectra of the films show relatively sharp phonon peaks, a single one for Pr_xO_y and a rich spectrum for HfO₂, clearly evidencing crystalline areas. This is corroborated by substrate Raman spectra which indicate a stressed interface, pointing to epitaxial Pr_xO_y and HfO₂ film growth, respectively, during the initial stages of growth.

Keywords: PACS; 77.55; 81.15.FgHigh-k gate dielectric material; Praseodymium oxide; Hafnium oxide; Pulsed laser deposition

Behaviour of a planar Langmuir probe in a laser ablation plasma by B. Doggett; C. Budtz-Joergensen; J.G. Lunney; P. Sheerin; M.M. Turner (pp. 134-138).

We have investigated some aspects of the behaviour of planar Langmuir probes in the supersonic plasma flow produced by laser ablation of solid materials in vacuum. The ablation was done using a 26ns, 248nm excimer laser, irradiating a silver target at 1Jcm⁻². We have compared the behaviour of the probe when it is orientated perpendicular and parallel to the plasma flow. In particular, we have shown that it is possible to adapt an analytical model, developed for plasma immersion ion implantation, to quantitatively describe the variation of the ion current with probe bias for the case when the plasma flow is along the probe surface. The electron temperature was also measured.

Keywords: Laser ablation plasma; Excimer laser; Plasma immersion ion implantation

PLD thin films obtained from CrO₃ and Cr₈O₂₁ targets by F. Guinneton; O. Monnereau; L. Argeme; D. Stanoi; G. Socol; I.N. Mihailescu; T. Zhang; C. Grigorescu; H.J. Trodahl; L. Tortet (pp. 139-144).

Thin films of chromium oxides mixtures have been grown on sapphire substrates using pulsed laser deposition (PLD). The work originates from the goal of enhancing the CrO₂ concentration in the films deposited from targets made of various chromium oxides. Their stability depends on temperature and pressure conditions. The PLD experiments are reviewed with the aim of discussing the influence of the target composition and of the deposition conditions upon the resulted films. The targets and corresponding thin films have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM) and Raman spectroscopy. The Cr₈O₂₁ targets are the most stable, which allows the preparation of thin films with a reproducible CrO₂ content of about 25% and a satisfactory surface morphology.

Keywords: Chromium oxides; PLD; Thin films; XRD; SEM; AFM; Raman spectroscopy

Preparation of tin oxide films on various substrates by excimer laser metal organic deposition by T. Tsuchiya; K. Daoudi; I. Yamaguchi; T. Manabe; T. Kumagai; S. Mizuta (pp. 145-150).

Tin oxide (SnO₂) thin films were prepared on various substrates by the excimer laser metal organic deposition (ELMOD) and the thermal metal organic deposition (MOD) processes. When the amorphous SnO₂ film prepared at 300°C on (100) Si, and (1010) sapphire substrates was irradiated by the KrF excimer laser at a fluence of 200mJ/cm² and 25°C, polycrystalline SnO₂ films were obtained. When the amorphous SnO₂ film prepared at 300°C on the (100) SrTiO₃ and (100) TiO₂ substrates was irradiated by the KrF excimer laser at the same conditions, a (110) oriented SnO₂ film on the SrTiO₃ substrate and a (100) oriented SnO₂ film on the TiO₂ substrate were obtained. Using the pole–figure measurements, a SnO₂ film on TiO₂ substrate was found to be epitaxially grown. On the other hand, when a thermal MOD process was used, all the product films on their substrates were of the polycrystalline phase whereas the orientation of the films depended on the substrate. Polycrystalline and epitaxial growth of the SnO₂ films by the ELMOD process are also discussed.

Keywords: SnO; ₂; Excimer laser; MOD; Epitaxial growth; Si; TiO; ₂; photothermal reaction

Pulsed laser deposition of thin films of various full Heusler alloys Co₂MnX (X=Si, Ga, Ge, Sn, SbSn) at moderate temperature by E. Valerio; C. Grigorescu; S.A. Manea; F. Guinneton; W.R. Branford; M. Autric (pp. 151-156).

This work presents pulsed laser deposition of cobalt-based Heusler thin films Co₂MnX (X=Si, Ga, Ge, Sn, SbSn) on different substrates (Si, GaAs, InAs). The deposition processes developed in vacuum (about 10⁻⁵Pa) to avoid oxidation of the films and targets. The temperature of the substrates during the depositions was kept below 500K to minimise interface interdiffusion. From X-ray diffraction, we found that the films are crystalline and slightly oriented. The stoichiometric composition of the films was further checked by EDS, while the size and density of droplets were determined by SEM. The magnetic properties of the films are consistent with those of the bulk material used as target.

Keywords: Pulsed laser deposition; Ferromagnetic alloys; Crystallised Heusler thin film

Double-pulse LIBS in bulk water and on submerged bronze samples by A. De Giacomo; M. Dell’Aglio; F. Colao; R. Fantoni; V. Lazic (pp. 157-162).

In this work laser-induced breakdown spectroscopy (LIBS) has been applied in bulk water using a double-pulse laser source. As in the case of former experiments in air, the use of the double-pulse technique allows for enhancing line emission intensity and reducing the duration of the continuum spectrum, thus increasing the overall analytical performances of the technique. Tap water analysis of Na and Mg dissolved cations has been performed to investigate the capability of the technique, but most significant results have been obtained in determining the composition of submerged bronze targets by laser ablation of their surface in seawater. When the plasma is generated by double-pulse laser, the ablated matter is strongly confined by the water vapor inside the cavitations bubble. The confinement of the plasma leads to higher values of excitation temperature and holds the conditions suitable for chemical analysis (homogeneity and LTE) longer than what happens in gaseous media. The double-pulse experiments performed directly in bulk water point out the features of LIBS technique for real analytical applications in situ, such as the water quality assessment and the investigation of irremovable submerged objects.

Keywords: LIBS; Water; Copper alloy

Influence of ambient medium on femtosecond laser processing of silicon by S. Besner; J.-Y. Degorce; A.V. Kabashin; M. Meunier (pp. 163-168).

Femtosecond laser radiation (800nm, 250fs, 1mJ/pulse) has been used to treat a Si surface in different ambient environments, namely vacuum, air and water. We show that ablation threshold and crater characteristics (diameter “ D�? and shape of craters) are similar for the three media at low laser fluences, suggesting an identical radiation-related mechanism of material removal. In contrast, at high fluences the characteristic dependence of the crater size D²( F) in the semi-logarithmic scale becomes non-linear, starting from F=10, 4 and 2J/cm² for vacuum, air and water, respectively, while the shape of craters becomes different for these media. The non-linear phenomena are ascribed to plasma-related ablation effects. Possible mechanisms of material removal are discussed.

Keywords: Femtosecond laser ablation in liquid; Crater characteristics; Silicon

(Zr, Sn)TiO₄ thin films for application in electronics by M. Nistor; F. Gherendi; M. Magureanu; N.B. Mandache; A. Ioachim; M.G. Banciu; L. Nedelcu; M. Popescu; F. Sava; H.V. Alexandru (pp. 169-174).

Thin films of zirconium tin titanate, (Zr_0.8Sn_0.2)TiO₄ (ZST) were deposited using a pulsed electron beam source based on a channel-spark discharge for target ablation. An advanced degree of crystallization was obtained for the films deposited on alumina substrate post-annealed at 1000�C. The crystalline lattice constants of the films are very close to those of the target material, which confirms the same stoichiometry in ZST films and in the bulk.

Keywords: Zirconium tin titanate; Pulsed electron deposition; Thin films

Multiple-layer laser deposition of steel components using gas- and water-atomised powders: the differences and the mechanisms leading to them by Andrew J. Pinkerton; Lin Li (pp. 175-181).

Functionally-graded or composite components have been recognised as having immense potential for many industries. The flexibility of direct metal deposition (DMD) has the potential to expand this to microstructurally graded components, but accurate control of the process is a problem. In this work, the effects of using different powder morphologies as a control mechanism for microstructure and other material properties are investigated experimentally. For the first time, comparison of the characteristics of two different gas-atomised (GA) and water-atomised (WA) materials is undertaken in order to evaluate the significance of the different DMD characteristics originating from the difference in atomisation method. 316L and H13 materials and a 1.2kW CO₂ DMD system are used. Three primary factors for the differences are identified: increased vaporisation of the powder, a hotter melt pool and less powerful outward Marangoni flow when using water-atomised powder. The reasons for these, the influence they have on process characteristics and final material properties, and the circumstances under which they occur are discussed.

Keywords: Laser; Direct Metal Deposition; Steel; Powder; Functionally graded

Thickness distribution of carbon nitride films grown by inverse-pulsed laser deposition by L. �gerh�zi; Zs. Geretovszky; T. Sz�r�nyi (pp. 182-187).

Recently, we proposed a new PLD geometry, termed as inverse PLD, in which the backward motion of the ablated species was utilized for film growth on substrates lying in the target plane. Qualitative measurements revealed that in this geometry the growth rate of carbon nitride films was comparable to or even exceeded that of the traditional geometry in the critical pressure domain of reactive PLD. The knowledge of the lateral distribution of the growth rate is of primary importance for both applications and modelling. Here first quantitative data on the dependence of the lateral distribution of deposition rate of carbon nitride films, fabricated by KrF excimer laser ablation of a rotating graphite target, on N₂ pressure are reported. Thickness distributions of films grown on Si substrates within the 0.5–50Pa pressure window have been recorded along the axes of symmetry of the laser spot by stylus profilometry. At all background pressures, the thickness decreases exponentially with increasing distance from the ablating laser spot. The elliptical symmetry typical at low pressures and near to the spot shifts to circular with increasing pressure and distance. The existence of flip-over suggests that recondensation of the plasma species without scattering on the surrounding atmosphere effectively contributes to IPLD film growth.

Keywords: Inverse pulsed laser deposition; Carbon nitride; Thin films; Growth process; Laser plasma

In situ studies of complex PLD-grown films using hard X-ray surface diffraction by P.R. Willmott; C.M. Schlep�tz; B.D. Patterson; R. Herger; M. Lange; D. Meister; D. Maden; Ch. Br�nnimann; E.F. Eikenberry; G. H�lsen; A. Al-Adwan (pp. 188-196).

A novel ultra-high vacuum chamber for growing films via pulsed laser deposition and studying the growth processes in situ using hard synchrotron radiation is presented. The chamber is mounted onto a 5-circle surface diffractometer and contains a large cylindrical-section beryllium window, which allows an extensive range of reciprocal space to be probed. The chamber is primarily used to perform surface diffraction measurements, for which much of the most valuable information derives from the weakest signals. With this in mind, we have employed a photon-counting area pixel detector system, which reduces data acquisition times by one to two orders of magnitude and concomitantly increases the potential volume of data that can be acquired for any given sample before the surface degrades due to surface contamination or radiation damage. Examples are presented of initial results obtained using this system.

Keywords: PACS; 61.10.Nz; 68.47.Gh; 68.55.Ac; 81.15.FgPulsed laser deposition; Thin films; Complex metal oxides; X-ray diffraction; Surface diffraction; Synchrotron radiation

Can thin perovskite film materials be applied as model systems for battery applications? by M.J. Montenegro; M. D�beli; T. Lippert; S. M�ller; A. Weidenkaff; P.R. Willmott; A. Wokaun (pp. 197-203).

The catalytic performance of carbon-based perovskite gas diffusion electrodes and thin films of identical perovskite catalyst phases (La_0.6Ca_0.4CoO3−_δ (LCCO), La_0.7Ca_0.3MnO3−_δ (LCMO) and La_0.7Ca_0.3Mn_0.9Ni_0.1O3−_δ (LCMNO)) has been compared. The thin films were deposited on MgO(100), MgO(110), and MgO(111) by pulsed reactive crossed-beam laser ablation (PRCLA). The quality of the films was determined by analyzing the composition as well as the crystallographic phase structure and texture of the electrodes. The catalytic activity for both the oxygen reduction and oxygen evolution reactions is evaluated from polarization curves for the selected electrodes. The compositional analysis of the films indicates that the ablation is congruent mainly for the LCCO, and that the films are nearly stoichiometric in their composition. The crystallographic orientation of the grown films is affected by the substrate. Under the selected deposition conditions it was not possible to grow parallel epitaxial films on MgO(110) and MgO(111). The key factors for epitaxy are besides the lattice misfit, also chemical/electronic interactions between film and substrate. The predominant crystallographic orientation is the one corresponding to the next lower surface/interfacial energy. The prepared films act as bifunctional catalysts, and the electrochemical experiments indicate that different exposed surfaces affect the performance of the electrode. The most active electrode was found to be the (100) textured film.

Keywords: Pulsed reactive crossed-beam laser ablation; Perovskite; Electrocatalysis

Real-time analysis of UV laser-induced growth of ultrathin oxide films on silicon by spectroscopic ellipsometry by Patrik Patzner; Andrey V. Osipov; Peter Hess (pp. 204-210).

In this work atomic oxygen was created by the photochemical dissociation of O₂ with a F₂ laser at 157nm and of N₂O with an ArF laser at 193nm. With this technique ultrathin (<6nm) amorphous silicon oxide films (a-SiO_x and a-SiO₂) were grown onto hydrogen terminated Si(111), Si(100), and amorphous silicon (a-Si) substrates. The oxidation process was monitored in real-time by spectroscopic ellipsometry. Different ellipsometric models are applied to characterize the interface. The influence of temperature, atmosphere, and of the surface morphology of the substrate was studied. Although the kinetics of oxidation is affected by the gas phase chemistry and temperature, the oxidation conditions do not influence appreciably the structure of the growing interface. The interface formed by this low-temperature oxidation process seems to be characteristic for the structure of the substrate surface. It consists of amorphous silicon with a low content of oxygen (a-SiO_x). The thickest a-SiO_x interface layer grew on the Si(111):H surface with a thickness of (0.8�0.3) nm and a stoichiometric index x of (0.40�0.15), whereas on Si(100):H a (0.40�0.15)nm thick SiO_x interface of nearly pure amorphous silicon was observed. Additionally X-ray photoelectron spectroscopy (XPS) and FTIR spectroscopy measurements were carried out to confirm the ellipsometric results.

Keywords: Real-time analysis; Spectroscopic ellipsometry; Amorphous silicon oxide

High laser-fluence deposition of organic materials in water ice matrices by “MAPLE�? by Bo Toftmann; Katarzyna Rodrigo; J�rgen Schou; Roman Pedrys (pp. 211-216).

Matrix assisted pulsed laser evaporation (MAPLE) is a deposition technique for organic material. Water ice was used as a matrix for the biotechnologically important guest material, polyethylene glycol (PEG), for concentrations from 0.5 to 4wt.%. The target was irradiated with 6ns laser pulses at 355nm at a fluence of 2.5–12J/cm². Even at this high fluence, Fourier transform infrared spectroscopy (FTIR) indicates a chemical structure of the deposit close to that of the un-irradiated PEG. Matrix assisted laser desorption and ionization (MALDI) and gel permeation chromatography (GPC) show that the mass distribution of the deposited PEG is similar to that of the starting material. Optical pictures of the films show particle structures of PEG of a size up to 5–10μm. The deposition rate measured with a quartz crystal microbalance is typically of the order of 1ng/(cm²shot).

Keywords: Laser fluence; MAPLE; PEG

Processing of mussel adhesive protein analog thin films by matrix assisted pulsed laser evaporation by R. Cristescu; T. Patz; R.J. Narayan; N. Menegazzo; B. Mizaikoff; D.E. Mihaiescu; P.B. Messersmith; I. Stamatin; I.N. Mihailescu; D.B. Chrisey (pp. 217-224).

Mussel adhesive proteins are a new class of biologically-derived materials that possess unique biocompatibility, bioactivity, and adhesion properties. We have demonstrated successful thin film growth of 3,4-dihydroxyphenyl-l-alanine modified poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (DOPA modified- PEO-PPO-PEO) block copolymer, a mussel adhesive protein analog, using matrix assisted pulsed laser evaporation. We have demonstrated that the main functional groups of the mussel adhesive protein analog are present in the transferred film. The effect of increasing of chain length of the mussel adhesive protein analog on film structure was also examined. These novel polymer thin films could have numerous medical and technological applications if their thin film properties are similar to what is found in bulk. This is the first report of successful MAPLE deposition of this material as thin films.

Keywords: Mussel adhesive protein; DOPA-polymer conjugates; MAPs analog thin films; Matrix assisted pulsed laser evaporation

Mechanical and tribological characterization of tetrahedral diamond-like carbon deposited by femtosecond pulsed laser deposition on pre-treated orthopaedic biomaterials by A.-S. Loir; F. Garrelie; C. Donnet; J.-L. Subtil; M. Belin; B. Forest; F. Rogemond; P. Laporte (pp. 225-231).

Femtosecond pulsed laser deposition (PLD) has been performed using a mode-locked Ti:sapphire laser including an amplification stage (150fs, 800nm, 1kHz) to deposit tetrahedral-amorphous carbon films (ta-C) on AISI 316L stainless steel and ultra high molecular weight polyethylene, in perspective to extend the wear resistance of materials used in hip joints. Ta-C films have been elaborated in high vacuum conditions at room temperature. The diamond-like coated silicon substrates exhibit high wear resistance (in the 10⁻⁸–10⁻⁹mm³(Nm)⁻¹ range) with moderate hardness (in the 20–30GPa range), which may be favorable for the accommodation motion between contacting surfaces in a hip joint. In situ sputter cleaning of the orthopaedic substrates in argon plasma prior to carbon deposition has been investigated, leading to the enhancement of the adhesion of the films onto the stainless steel substrates. The adhesion properties of films deposited in various conditions on metallic substrates have been studied by tensile tests. The tribological behavior of the coatings deposited on cleaned substrates have been widely investigated in a pin-on-flat configuration in ambient air and Ringer solution. Finally, a DLC thin film with an homogeneous thickness has been deposited on hemispherical surface of 22.2mm in diameter of a stainless steel femoral head of a hip prosthesis, whose wear behavior will be quantified using a hip joint simulator during one million of cycles (corresponding to the human activity during one year).

Keywords: Femtosecond pulsed laser deposition; Biomaterials; Tetrahedral diamond-like carbon; Hip joint

Femtosecond laser induced X-ray emission from metal alloys, polymers and color filters by Koji Hatanaka; Ken-ichiro Yomogihata; Hiroshi Ono; Hiroshi Fukumura (pp. 232-237).

Various material surfaces were irradiated on a moving stage with focused laser pulses from a conventional 1kHz femtosecond laser system, and X-ray emission spectra were measured during the laser ablation of the materials. Sharp K or L characteristic X-ray lines from the elements contained in the materials were clearly observed in a range of 2–15keV. Signals due to copper and zinc were recognizable within a few minutes when a brass surface was irradiated. Poly(vinyl chloride) gave a marked emission originating from chlorine. When a color glass filter was irradiated, the detection of cobalt and arsenic was possible even though the amounts of these components were estimated to be less than 1wt.% by using an electron probe microanalyzer. Time-integrated emission spectra in the visible region were also monitored during the femtosecond laser ablation of these materials. The emission spectra in the visible region were complicated owing to peaks originating from air components and white continuum emissions. Thus, the elemental analysis by femtoseond laser induced X-ray is considered to be useful for some samples. The etched trenches left at the surfaces after the laser ablation were examined with an optical microscope. The trench width varied with the materials, which may be attributed to changes in the irradiation area giving maximum counts of X-ray emission.

Keywords: XXX; XXX; XXX

2D calculations of the thermal effects due to femtosecond laser-metal interaction by S. Valette; R. Le Harzic; N. Huot; E. Audouard; R. Fortunier (pp. 238-242).

Experimental results previously published have shown that the radial heat affected zone (HAZ) for a 500nm thick Al sample in the femtosecond case is less than 2μm [Appl. Phys. Lett. 80 (2002) 3886]. The spread of radial thermal effects for femtosecond pulses is calculated, by developing a two-dimensional version of the two temperature model (TTM). The case of an axi-symmetrical geometry is simulated by a finite element technique. A physical metallurgy approach is used to define and to estimate the radial HAZ width from the calculations. The evolution of the temperature as a function of time is studied, leading to a radial HAZ width of 220nm for 500nm thick Al samples. Similar calculations in the nanosecond regime are also performed and compared to the femtosecond case.

Keywords: Femtosecond laser; Modelling; Two Temperature Model; Thermal effects

Mechanism of ultrashort laser ablation of metals: molecular dynamics simulation by N.N. Nedialkov; S.E. Imamova; P.A. Atanasov; P. Berger; F. Dausinger (pp. 243-248).

A theoretical model is developed and a molecular dynamics simulation technique is applied for the description of ultrashort laser ablation of metals. The ablation of Al, Ni, and Fe using 0.1, 0.5 and 5ps laser pulses at wavelengths of 248 and 800nm is studied. The process is investigated at fluences up to 0.5J/cm². The analysis based on the temporal evolution of the ablation, the temperature, and the pressure distributions into the material reveals that a thermo-mechanical mechanism (spallation) takes place near the threshold. However, phase explosion is found to be the dominant mechanism of material removal at fluences higher than several hundreds of mJ/cm². The influence of the laser parameters (wavelength and pulse duration) is obtained and discussed. The ablation depth as a function of the laser fluence and the ablation threshold value are evaluated and compared with the experimental data available. Good agreement between the theory and experiments is observed.

Keywords: Ultrashort laser ablation; Metals; Molecular dynamics simulation

Femtosecond laser ionization mass spectrometric analysis of layers grown by pulsed laser deposition by Florenta Costache; Markus Ratzke; Dirk Wolfframm; J�rgen Reif (pp. 249-255).

Femtosecond laser-induced charged particle emission and detection by time-of-flight mass spectrometry are used for elemental analysis and in-depth profiling of multi-layered metal-oxide-semiconductor (MOS) structures grown by pulsed laser deposition (PLD): thin films of a high- k material – praseodymium oxide, Pr₆O₁₁ – deposited on a silicon(100) surface and covered by an aluminum contact layer. An efficient emission of singly charged positive ions to large clusters was found at low incident fluence. The simultaneous monitoring of the main ion yields with the number of pulses exhibits pronounced anti-correlations, revealing several interfaces. This indicates not only that the layers were resolved but also additional layers were detected. Surface morphology investigation shows that, upon single- or multi-pulse exposure, localized holes and nano-particles were created on the irradiated area. The multiple interfaces may account for in-homogeneities within the PLD layer and/or a surface rearrangement due to the nano-particles formation. We demonstrate that, with this method, a depth resolution in the nanometer range can be achieved. Fundamental mechanisms of femtosecond laser ablation from hetero-structures are addressed.

Keywords: Femtosecond laser ablation; Time-of-flight mass spectrometry; PLD layers; Metal-oxide-semiconductor structure

The influence of the laser spot size and the pulse number on laser-induced backside wet etching by R. B�hme; K. Zimmer (pp. 256-261).

The laser-induced backside wet etching (LIBWE) of transparent solids at the interface to absorbing liquid is a new promising method for laser microstructuring. The influence of the laser spot size and the applied pulse number to the etch rate were investigated in detail for fused silica and two different liquids. Additional to the significant rise of the etch rate with increasing spot size considerable incubation effects have been observed at low laser fluences and pulse numbers. Based on the bubble formation during LIBWE processing, a relation between the bubble collapse time and the etch rate was ascertained. This relation fits the etch rate dependence on the spot size well. It is assumed that the deposition of decomposition products from the bubble accounts for the spot size influence the etch rate.

Keywords: PACS; 81.65.C; 81.05.K; 79.20.D; 71.80.B; 68.03.F; 47.55.D; 43.25.Y; 42.70.C; 42.55.LExcimer laser; Laser etching; Pulse number; Laser spot size; Etch rate

Tungsten oxide nanoparticles synthesised by laser assisted homogeneous gas-phase nucleation by O. Alm; L. Landstr�m; M. Boman; C.G. Granqvist; P. Heszler (pp. 262-267).

Tungsten oxide nanoparticles were generated by excimer (ArF) laser assisted chemical vapor deposition from WF₆/H₂/O₂/Ar gas mixtures. The deposited particles were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, and transmission electron microscopy. The deposition rate as a function of the partial pressures of the reactants and of the laser fluence was measured by X-ray fluorescence spectroscopy. The mean diameter of the deposited tungsten oxide particles varied with the experimental parameters and was typically 23nm. Particles with a higher degree of crystallinity were observed at a laser fluence exceeding 130mJ/cm², and X-ray amorphous particles were obtained below 110mJ/cm². The amorphous tungsten oxide had a stoichiometry ranging from WO_2.7 to WO₃. Deposits were formed only when hydrogen was present in the gas mixture.

Keywords: Tungsten oxide; Nanostructured materials; LCVD

A comparative study of wire feeding and powder feeding in direct diode laser deposition for rapid prototyping by Waheed Ul Haq Syed; Andrew J. Pinkerton; Lin Li (pp. 268-276).

Metal powder feeding has been used widely in various rapid prototyping and tooling processes such as direct laser deposition (DLD) and layered engineered net shaping (LENS) to achieve near net shape accuracy. Although powder recycling has been practiced, the material usage efficiency has been very low (normally below 30%). This study compares the process characteristics, advantages and disadvantages of wire- and powder-feed DLD. A 1.5kW diode laser is used to build multiple layer parts, which are compared and analysed in terms of surface finish, microstructure and deposition efficiency. Scanning electron microscopy (SEM), X-ray diffraction and optical microscopy are used for the material characterisation. The microstructure of samples from both the methods is similar, with some porosity found in powder-feed components, but the surface finish and material usage efficiency is better for wire-feed samples. The deposition angle is found to be critical in the case of wire feeding and the characteristics of different feed angles are explored. Possible reasons for the different characteristics of the two deposition techniques are discussed.

Keywords: Wire feeding; Powder feeding; Rapid prototyping; Laser deposition

An analytical model for striation formation in laser cutting by Lee Mein Wee; Lin Li (pp. 277-284).

Regular patterns (striation) are often observed on the cut surfaces in laser cutting of metallic materials. Although there have been various investigations on the understanding of the striation formation mechanism, the models so far are inadequate for the explanation of the observed phenomena, although it has been established that melt film thickness has a strong influence over the formation of striation. A two-dimensional, analytical model is reported in this paper. Focus is placed on the effect of laser parameters such as power, scan speed and spot sizes. Power absorption over the cut front, effect of oxidation and melt film thickness are predicted in the model.

Keywords: Striation formation; Laser cutting; Metallic materials

Multiple-layer laser direct writing metal deposition in electrolyte solution by L.M. Wee; L. Li (pp. 285-293).

The development of the miniaturized product, such as microelectronic component marks a new era in processing technologies. A wide range of miniaturized products is being fabricated by high-yield, cost-effective and high-resolution rapid prototyping (RP) techniques in which the product design and development cycle has been accelerated in the absence of tooling or apparatus. In this paper, a new, laser-based thin film deposition within a fluid environment for growing prototypes is described. The effects of the laser-processing conditions and fluid properties on the material deposition rate as well as microstructure features are presented. Also, the mechanism involved in the new process is discussed.

Keywords: Microfabrication; Electroplating; Roughness

Corrosion mechanism of laser-melted AA 2014 and AA 2024 alloys by Z. Liu; P.H. Chong; A.N. Butt; P. Skeldon; G.E. Thompson (pp. 294-299).

The corrosion performance of laser-melted AA 2014-T6 and AA 2024-T351 alloys, using a 2kW CW CO₂ laser, has been examined to gain insight into the factors influencing pitting corrosion resistance. Examination of laser-melted surfaces in terms of microstructure and phase analysis was performed using scanning electron microscopy (SEM), with associated elemental analysis by energy dispersive X-ray (EDX) spectroscopy, electron probe microanalysis (EPMA) and X-ray diffraction (XRD). Pitting corrosion resistance was evaluated using potentiodynamic anodic polarisation in 1M NaCl solution. The work revealed that there was an improvement of pitting corrosion resistance for the laser-melted AA 2014-T6 alloy, but no improvement for AA 2024-T351 alloy. It indicated that the refinement of the microstructure, per se, with finer intermetallic particles, did not play an important role in corrosion performance. More importantly, the extension of copper solubility in the α-Al matrix, leading to an increased corrosion potential, was considered to be the key factor responsible for the corrosion behaviours of the laser-melted aluminium alloys. For the AA 2014-T6 alloy, due to the cathodic nature of the Al₂Cu phase relative to the α-Al solution, the rise of corrosion potential of the α-Al solution reduced the galvanic coupling between the Al₂Cu and α-Al matrix. As a result, the driving force for pitting corrosion in the α-Al solution was reduced. For the AA 2024-T351 alloy, due to the anodic nature of the Al₂CuMg phase relative to the α-Al solution, the driving force for pit initiation at the Al₂CuMg phase was enhanced. Therefore, the laser melting promoted the pitting corrosion of the AA 2024-T351 alloy.

Keywords: AA 2014-T6 alloy; AA 2024-T351 alloy; Laser surface melting; Pitting corrosion

CO₂ and diode laser welding of AZ31 magnesium alloy by Jinhong Zhu; Lin Li; Zhu Liu (pp. 300-306).

Magnesium alloys are being increasingly used in automotive and aerospace structures. Laser welding is an important joining method in such applications. There are several kinds of industrial lasers available at present, including the conventional CO₂ and Nd:YAG lasers as well as recently available high power diode lasers. A 1.5kW diode laser and a 2kW CO₂ laser are used in the present study for the welding of AZ31 alloys. It is found that different welding modes exist, i.e., keyhole welding with the CO₂ laser and conduction welding with both the CO₂ and the diode lasers. This paper characterizes welds in both welding modes. The effect of beam spot size on the weld quality is analyzed. The laser processing parameters are optimized to obtain welds with minimum defects.

Keywords: AZ31 magnesium alloy; Laser welding; Welding modes

Free-electron laser surface processing of titanium in nitrogen atmosphere by E. Carpene; M. Shinn; P. Schaaf (pp. 307-312).

Surface treatment by means of pulsed laser beams in reactive atmospheres is an attractive technique to enhance the surface features, such as corrosion, wear resistances and hardness. Among all laser types, the free-electron laser (FEL) is relatively new for materials processing, its main peculiarity being the versatile pulse structure, with high repetition rates (the so-called macropulse). We have employed the FEL at the Jefferson Lab (Virginia, USA) to irradiate pure titanium substrates in nitrogen atmosphere. The influence of various experimental parameters (macropulse duration, spot overlap and laser fluence) on the nitrogen incorporation and the resulting microstructures has been investigated. It will be shown that the laser treatment is not only a successful way to form titanium nitride, but also that a proper set of the experimental parameters can efficiently control the growth mechanism and the crystallographic texture of the resulting nitride phase, leading to the synthesis of highly oriented δ-TiN(200) layers. As it will be shown, the surface features strongly depends on the irradiation conditions: in particular a very smooth, crack-free surface can be obtained with small spot overlap and long macropulses (1000μs), while the roughness increases with large overlap and short macropulses (<250μs). Accordingly, the degree of crystallographic texture is higher in the samples with the smother surface. Although the nitrogen content at the surface is rather independent of the irradiation conditions (being always close to 50at.%), the hardness profiles of the TiN coating is influenced by the laser spot overlap and consequently by nitrogen gradient.

Keywords: PACS; 61.80.Ba; 81.40.Ef; 81.65.LpFree-electron laser; Titanium; Titanium nitride; Laser nitriding

Surface laser-glazing of plasma-sprayed thermal barrier coatings by C. Batista; A. Portinha; R.M. Ribeiro; V. Teixeira; M.F. Costa; C.R. Oliveira (pp. 313-319).

Atmospheric plasma-sprayed (APS) ZrO₂–8%WtY₂O₃ thermal barrier coatings (TBCs) were subjected to a CO₂ continuous wave laser-glazing process in order to generate an external dense layer produced by different processing parameters. For that purpose, different beam scanning speeds and track overlapping were chosen.Surface roughness has been reduced significantly after laser-glazing. Despite the surface crack network, all laser-glazed specimens presented a fully dense and porous free external layer with a columnar microstructure. Surface cracks along the densified layer were found to have tendency to be oriented in two perpendicular directions, one in the direction of the laser beam travel, the other perpendicular to it. Moreover, the cracks parallel to the beam moving direction are found to be on the overlapping zone, coinciding with the edge of the subsequent track. The cracks along the densified layer are vertical and tend to branch and deviate from the vertical direction within the porous PS coating. The largest overlapping allied to the smallest amount of irradiated energy generated the most uniform layer with the shortest crack branches within the PS coating. For the as-sprayed coating, the XRD results revealed mainly t′ non-transformable tetragonal zirconia with a small percentage of residual monoclinic zirconia. All glazed coatings presented only t′ non-transformable tetragonal zirconia with some variations on preferable crystal orientation.

Keywords: Thermal barrier coating; Laser-glazing; Plasma spraying; Zirconia

Microstructure characterisation and process optimization of laser assisted rapid fabrication of 316L stainless steel by J. Dutta Majumdar; A. Pinkerton; Z. Liu; I. Manna; L. Li (pp. 320-327).

In the present study, laser assisted fabrication of 316L stainless steel has been attempted using a high power (1.5kW) continuous wave diode laser. The main process variables for the present study were applied power density, scan speed and powder feed rate. A detailed microstructural study of the surface and cross-section of the fabricated layer were carried out using optical and scanning electron microscopy to understand the influence of laser parameters on microstructure of the surface and interface between the successive layers. The microstructure of the top layer was equiaxed, the near substrate region was fine dendritic, however, at the interface between two successive layers, it was coarsened. The morphology and degree of fineness of the microstructure was found to vary with laser parameters. The range of grain size (maximum grain size–minimum grain size) was taken as a measure of homogeneity. It was found that with increasing the scan speed, the range of grain size was minimized. Micro-porosities were present in the microstructure that reduced with increasing scan speed and found to be minimum at a medium powder feed rate. The optimum processing conditions have been established by correlating the characteristics of the fabricated layer with process parameters.

Keywords: Laser; Fabrication; Microstructure; Stainless steel

Alloying the X40CrMoV5-1 steel surface layer with tungsten carbide by the use of a high power diode laser by L.A. Dobrzański; M. Bonek; E. Hajduczek; A. Klimpel (pp. 328-332).

The paper presents the effect of alloying with tungsten carbide on properties of the X40CrMoV5-1 steel surface layer, using the high power diode laser (HPDL). Selection of laser operating conditions is discussed, as well as thickness of the alloying layer, and their influence on structure and chemical composition of the steel. Analysis of the influence of the process conditions on the thicknesses of the alloyed layer and heat-affected zone is presented.

Keywords: Hot-work tool steel; Surface layer; Alloying; Laser HPDL

Laser cleaning: an alternative method for removing oil-spill fuel residues by M.P. Mateo; G. Nicolas; V. Pi�on; A. Ramil; A. Ya�ez (pp. 333-339).

Cleaning methods employed in last oil spills usually require direct contact or the intervention of external agents that can lead to additional contamination and damage of treated surfaces. As an alternative, a laser-based methodology is proposed in this work for controlled removal of fuel residues caused by the accident of Prestige tanker from rocks, as well as tools and equipment employed in fuel retaining and elimination procedures.Ablation thresholds of fuel crust and underlying material have been investigated with the aim to establish operational parameters that preserve the structural integrity and identity of the latter. The clean-up process was controlled by the self-limiting nature of the process or by laser-induced plasma spectroscopy. Contaminated, no contaminated and cleaned areas of the samples have been characterized by complementary microscopy techniques to help in the task of optimizing the laser cleaning procedure and checking the effectiveness of the removal process.

Keywords: Laser cleaning; Oil spills; Laser-induced plasma spectroscopy

Femtosecond laser fabrication of microspike-arrays on tungsten surface by Tomokazu Sano; Masato Yanai; Etsuji Ohmura; Yasumitsu Nomura; Isamu Miyamoto; Akio Hirose; Kojiro F. Kobayashi (pp. 340-346).

Microspike-arrays were fabricated by irradiating a femtosecond laser on a tungsten surface through a mask opening in air. The natural logarithms of the calculated intensity distributions diffracted at the edge of the mask opening were qualitatively consistent with the experimental results of the shape and arrays of microspikes fabricated. The shape and the array of microspikes depend on the intensity distribution diffracted at the edge of the mask opening. This microspike-array has the potential to be used as a source of micro emitter tips.

Keywords: Microspike-array; Femtosecond laser; Micro emitter

Corrosion behaviour of laser-melted magnesium alloys by G. Abbas; Z. Liu; P. Skeldon (pp. 347-353).

The present paper reports studies of the corrosion behaviour of magnesium alloys, AZ31, AZ61 and WE43 following laser surface melting (LSM) with a 2kW continuous wave CO₂ laser to depth about 1mm. The laser-melted samples were immersed in 5wt.% sodium chloride solution of pH 10.5 for 10 days, with open circuit potential measurements recorded in the initial 24h. LSM resulted in improvement of the corrosion resistance of the alloys, with weight losses reduced by about 30, 66 and 87% for the AZ31, AZ61 and WE43 alloys, respectively. Rutherford backscattering spectroscopy (RBS) and nuclear reaction analysis (NRA) revealed that the average compositions of the alloys, following removal of surface oxide, were relatively unaffected by laser melting, indicating that any losses of magnesium or gains of oxygen only affected superficial layers. The improved corrosion resistance following LSM is associated with refinement of the alloy microstructure due to the rapid cooling of the melted layer, which results in increased concentration of alloying elements in solid solution in matrix regions and more uniform distributions of the corrosion-resistant β-phase that can accumulate as a protective layer.

Keywords: Corrosion; Magnesium alloys; LSM

Boundary conditions for 3D dynamic models of ablation of ceramics by pulsed mid-infrared lasers by A. Vila Verde; Marta M.D. Ramos (pp. 354-361).

We present and discuss a set of boundary conditions (BCs) to use in three-dimensional, mesoscopic, finite element models of mid-infrared pulsed laser ablation of brittle materials. These models allow the study of the transient displacement and stress fields generated at micrometer scales during and after one laser pulse, where using conventional BCs may lead to some results without physical significance that can be considered an artefact of the calculations. The proposed BCs are tested and applied to a micrometer-scale continuous model of human dental enamel under CO₂ radiation (10.6μm, 0.35μs pulse, sub-ablative fluence), giving rise to the following results: the highest stress is obtained at the irradiated surface of the model, at the end of the laser pulse, but afterwards it decreases rapidly until it becomes significantly lower than the stress in a region 2.5μm deep in the model; a thermally induced vibration in the material is predicted. This non-intuitive dynamics in stress and displacement distribution cannot be neglected and has to be considered in dynamic laser ablation models, since it may have serious implications in the mechanisms of ablation.

Keywords: Mesoscopic modelling; Laser ablation; Enamel; Finite elements; Infrared laser; Boundary conditions

Characterisation and corrosion performance of laser-melted 3CR12 steel by P.H. Chong; Z. Liu; P. Skeldon; P. Crouse (pp. 362-368).

Dual-phase ferritic–martensitic 3CR12 steel, exhibiting an excellent corrosion resistance under ambient conditions, is a potential substitute for FeCrNi stainless steels in many applications. However, the steel suffers pitting corrosion in Cl⁻-containing environments, usually associated with the presence of non-metallic inclusions. The present work investigates the possibility of improvement of the pitting corrosion performance of hot-rolled and annealed 3CR12 steel using a laser surface melting (LSM) technique. The laser processing was carried out using a 3kW CW Nd:YAG laser with 10 and 13mm line beam profiles. Pitting resistance in de-aerated 3.5% NaCl solution was substantially increased by LSM. The degree of improvement is associated with the formation of ferrite and reduction of martensite in the melt region, the extent of which depends on the processing parameters. Observation of pitting morphologies of the laser-melted surfaces showed that corrosion was initiated at the ferrite grain boundaries, where the martensite or possibly impurities were located, compared with more random pitting in the banded microstructure of the initial steel.

Keywords: Laser surface melting; Martensite; Ferrite grain boundaries

Laser cleaning of ancient textiles by Romina Belli; Antonio Miotello; Paolo Mosaner; Laura Toniutti (pp. 369-372).

In this work, we tried to answer two questions: (i) are laser pulses, prior to SEM investigation, a suitable tool to recognize fibres contained in archaeological findings? (ii) are laser pulses a suitable tool to properly clean those textiles? The anaerobic environment (i.e. the peatery), aiding the preservation of such handmade textiles, often impregnates the textiles, in such a way that the normal chemical procedures for cleaning are not sufficient; besides the removal of the material could make the sample brittle. Laser pulses may offer an alternative method of cleaning. In this experimental work (performed on cotton samples), laser radiation effects have been investigated using several combinations of the two main laser parameters namely energy density and number of pulses. Since textile findings are quite rare, modern samples undergone to an artificial ageing have been previously treated. Scanning electron microscopy (SEM) has been used to examine the fibres before and after the treatment. This paper reports on early stages of research regarding fibre response to UV laser irradiation detected with SEM. The obtained experimental results can be considered as a first step of a more detailed study aiming at dirt and consolidant removal without impinging the original textiles.

Keywords: Laser pulse; Scanning electron microscopy; Textiles

Mechanical and electrochemical properties of multiple-layer diode laser cladding of 316L stainless steel by J. Dutta Majumdar; A. Pinkerton; Z. Liu; I. Manna; L. Li (pp. 373-377).

In the present investigation, a detailed mechanical and electrochemical properties of multiple-layer laser clad 316L stainless steel (from the powders produced by gas atomized route) has been carried out. Multiple-layer laser cladding of 316 L stainless steel has been conducted using a diode laser. The mechanical property (microhardness) of the fabricated product has been evaluated using a microhardness testing machine and correlated with the process parameters. The electrochemical property, mainly pitting corrosion resistance of the fabricated layer corresponding to maximum microhardness (in a 3.56% NaCl solution) has been evaluated using standard potentiodynamic polarization testing. The microhardness of the laser assisted fabricated layers was found to vary from 170 to 278VHN, increased with decrease in applied power density and increase in scan speed and was higher than that of conventionally processed 316L (155VHN). The superior microhardness value is attributed to grain refinement associated with laser melting and rapid solidification. The critical potential to pit formation ( E_PP1) was measured to be 550mV saturated calomel electrode (SCE) and superior to the conventionally processed 316L stainless steel (445mV (SCE)).

Keywords: Microhardness; Pitting corrosion; Laser; 316L Stainless steel

Laser densification of TiO₂ films prepared by aerosol assisted vapour deposition by Yiquan Wu; Kwang-Leong Choy; Larry L. Hench (pp. 378-383).

This paper concerns the microstructural development and phase transition of the aerosol deposited nanocrystalline titanium dioxide films by laser densification. Dense titanium dioxide films have been prepared using laser densification of porous films, which have been deposited using aerosol assisted spray deposition (AASD) at low temperature from a colloidal suspension and a metalorganic precursor. The AASD process involves spraying atomized aerosol precursor towards a heated zone where the droplets undergo chemical reactions at the heated substrate to deposit a solid film. Processing parameters can be varied to control microstructure, porosity, grain size and phase of the titanium dioxide films. The effect of the processing parameter of laser energy density has been investigated and 7.6–7.8Jmm⁻² is an optimal energy density for laser densification of titanium dioxide films prepared by the AASD. X-ray diffraction (XRD) was used to analyze the crystalline phase of titanium dioxide films before and after laser irradiation and heat treatment. The microstructure of laser irradiated films and the morphology of nanoparticle precursor were characterized using scanning electron microscopy (SEM) and transmitting electron microscopy (TEM). Laser densification has been shown to be a promising technique to densify porous films rapidly without deleterious effect on thermally sensitive substrates.

Keywords: Aerosol; Laser densification; Titanium oxide; Electrospraying

Pulsed laser deposition of crystalline LaB₆ thin films by V. Craciun; D. Craciun (pp. 384-389).

The deposition of LaB₆ thin films by the pulsed laser deposition (PLD) technique was investigated. X-ray photoelectron and Auger electron spectroscopy (XPS, AES), X-ray diffraction and reflectivity were used to characterize the properties of the deposited films. It has been found that crystalline films could be grown only by using laser fluences around 10J/cm² or higher and substrate temperatures in excess of 800°C. Cubic LaB₆ films ( a=0.4157nm) exhibiting a strong (100) texture were deposited under a residual vacuum better than 1×10⁻⁶Torr at 850°C. These films were smooth, with surface roughness values below 1.4nm and mass densities around 4.88g/cm³, very close to the theoretical LaB₆ density of 4.71g/cm³. XPS and AES investigations showed that the outermost 2–3nm of the surface region contained a significant amount of oxygen and La–O and B–O bonds. Once this surface region was removed by sputtering, the oxygen content decreased to values below 10%.

Keywords: Thin films; LaB; ₆; Laser ablation; Refractive coatings; X-ray reflectivity

Fast third-order optical nonlinearities in metal alloy nanocluster composite glass: negative sign of the nonlinear refractive index by E. Cattaruzza; G. Battaglin; F. Gonella; G. Mattei; P. Mazzoldi; R. Polloni; B.F. Scremin (pp. 390-395).

The fast component of the nonlinear refractive index, n₂, of a composite film formed by alloy nanoparticles embedded in silica was measured by Z-scan technique, in conditions free from cumulative heating effects. By using a single 6ps long laser pulse at 1Hz, 527nm of wavelength, a negative sign of n₂ was evidenced. An explanation of this result is suggested, based on the dielectric confinement effect. The fast refractive index value, in modulus, is of the order of 10⁻¹⁰cm²/W.

Keywords: PACS; 42.65.−k; 78.66.Sq; 78.67.Bf; 81.05.PjGlass-based composites; Nanoparticles; Nonlinear optics; Ion implantation

Laser-induced epitaxial recrystallization after alkali-ion implantion into α-quartz by S. Gąsiorek; S. Dhar; K.P. Lieb; P. Schaaf (pp. 396-400).

Ion implantation and recrystallization of quartz may be a very important process for photonic applications. We have investigated how laser irradiation can be used to epitaxially recrystallize layers of α-quartz, which were amorphized by 175keV Rb⁺ or 250keV Cs⁺ ion implantation at a fluence of 2.5×10¹⁶cm⁻². The samples were irradiated with pulses of a XeCl excimer laser (wavelength 308nm, pulse length 55ns, energy density 3.2–5J/cm²). The thickness of the amorphous layer and the quality of the recrystallized layer were analyzed by Rutherford Backscattering Channeling Spectroscopy. Partial epitaxial recrystallization was found for all laser-irradiated samples that proceeded faster for higher laser energy density, however no full epitaxy was achieved up to the maximum laser energy of 5J/cm². The degree of epitaxy was more or less the same for 20 or 200 laser shots.

Keywords: PACS; 61.80.Jh; 81.15.Np; 42.70.Ce; 42.55.Lt; 42.62−bEpitaxy; Ion implantation; Quartz; Excimer laser irradiation

Thin (001) tungsten trioxide films grown by laser deposition by N.E. Stankova; P.A. Atanasov; T.J. Stanimirova; A. Og. Dikovska; R.W. Eason (pp. 401-405).

Highly textured thin (001) tungsten trioxide (WO₃) films have been deposited on (001) SiO₂ substrates by XeCl excimer laser ablation of ceramic targets. The influence of the oxygen pressure and substrate temperature on the composition, crystallinity and optical properties of the films has been studied. The films were grown at pressures of 1–25Pa and substrate temperatures from room temperature up to 670°C. The best crystallinity was obtained for the films grown at 10Pa and 500°C as confirmed by XRD analyses and Raman spectroscopy. Optical transmission as high as 83% has been measured in the visible and near-infrared spectral regions. The ordinary index of refraction at wavelength of 633nm increases with the growth temperature and reaches 1.97 at the optimum growth conditions.

Keywords: Pulsed laser deposition; WO; ₃; thin films; Crystal structure; Optical properties

Femto- and nanosecond laser treatment of doped polymethylmethacrylate by J. Kr�ger; S. Martin; H. M�debach; L. Urech; T. Lippert; A. Wokaun; W. Kautek (pp. 406-411).

Femto- and nanosecond laser ablation of polymethylmethacrylate (PMMA) and PMMA doped with a linear absorber was investigated in the infrared spectral region. Ablation thresholds were determined and incubation phenomena were identified. The ‘degree’ of incubation was calculated employing a phenomenological model. The influence of the pulse duration on the machining quality of the polymers was examined. The presence of an absorbing chromophore is not a prerequisite for a controllable fs-laser structuring in contrast to the ns-treatment. Surface swelling always accompanied ablation.

Keywords: PACS; 79.20.D; 42.62.C; 68.55.L; 61.82.PLaser ablation; Laser beam machining; Doping thin films; Polymers-radiation effects

Photoluminescence characterization of pure and Sm³⁺-doped thin metaloxide films by V. Kiisk; I. Sildos; S. Lange; V. Reedo; T. T�tte; M. Kirm; J. Aarik (pp. 412-417).

Photoluminescence (PL) of pure and Sm³⁺-doped TiO₂, ZrO₂ and HfO₂ thin films were studied in the temperature range of 6–300K. The thin (100–250nm) films were prepared by using both the atomic layer deposition (ALD) technique and the sol–gel spin-coating process. The ion implantation was applied to dope the ALD-grown films with Sm³⁺ ions, whereas an in situ doping was used in the sol–gel process.The PL was excited via band-to-band transitions by using several pulsed lasers as well as tuneable synchrotron radiation in the energy range of 4–20eV. PL excitation spectra and decay kinetics were recorded.The dominating intrinsic emission of undoped materials was attributed to the radiative recombination of self-trapped excitons (STE). In doped materials, a broadband emission superposed by Sm³⁺ emission lines of a well-pronounced fine structure was observed under laser excitation. The broadband emission was attributed to the recombination of various bound excitonic states.The PL excitation spectra of doped and undoped films showed different behaviour, which was attributed to the damage produced by ion implantation and uncontrolled impurities incorporated into the films in sol–gel process.Relaxation of electronic excitations and the energy transfer process to Sm³⁺ ions is discussed.

Laser-induced modification of metal nanoparticles formed by laser ablation technique in liquids by N.V. Tarasenko; A.V. Butsen; E.A. Nevar (pp. 418-422).

The effects of laser irradiation of silver colloids prepared by laser ablation technique in acetone have been studied. The laser irradiation was performed using laser radiation at different wavelengths (532, 266, 400 and 800nm). Additional irradiation of colloids resulted in the changes of particles morphology, which were monitored by absorption spectroscopy and transmission electron microscopy methods. The experimental conditions favoring a dimension reduction of the initial particles and a formation of spherical size-controlled nanoparticles were found as well as irradiation conditions aiding the fabrication of nanowires. It was found that both the mean size of the nanoparticles and their stability could be controlled by changing the laser ablation and post-irradiation regimes.

Keywords: Nanoparticles; Colloids; Laser-induced modification

Morphological and structural characterization of CrO₂/Cr₂O₃ films grown by Laser-CVD by P.M. Sousa; A.J. Silvestre; N. Popovici; O. Conde (pp. 423-428).

This work reports on the synthesis of chromium (III, IV) oxides films by KrF laser-assisted CVD. Films were deposited onto sapphire substrates at room temperature by the photodissociation of Cr(CO)₆ in dynamic atmospheres containing oxygen and argon. A study of the processing parameters has shown that partial pressure ratio of O₂ to Cr(CO)₆ and laser fluence are the prominent parameters that have to be accurately controlled in order to co-deposit both the crystalline oxide phases. Films consistent with such a two-phase system were synthesised for a laser fluence of 75mJcm⁻² and a partial pressure ratio of about 1.

Keywords: PACS; 81.15.Fg; 81.15.Kk; 81.05.JeLaser-assisted CVD; Photodissociation; CrO; ₂; /Cr; ₂; O; ₃; two-phase system

Anatase phase TiO₂ thin films obtained by pulsed laser deposition for gas sensing applications by E. Gy�rgy; G. Socol; E. Axente; I.N. Mihailescu; C. Ducu; S. Ciuca (pp. 429-433).

Anatase phase titanium dioxide (TiO₂) thin films were grown by pulsed laser deposition on 〈001〉 SiO₂ substrates. An UV KrF* ( λ=248nm, τ_FWHM≅20ns, ν=2Hz) excimer laser was used for the irradiation of the TiO₂ targets. The substrates were kept at room temperature or heated during the film deposition at values within the 100–500°C range. The crystalline quality of the films and their chemical composition were investigated by X-ray diffractometry and energy dispersive X-ray spectroscopy. The optical properties were studied by a double beam spectrophotometer in the spectral range of 400–1200nm. At substrate temperatures higher than 300°C, the structure of the deposited thin films changes from poorly to well crystallised, corresponding to the tetragonal TiO₂ anatase phase. The average optical transmittance in the visible-infrared spectral range of the films is higher than 85% which makes them suitable for sensor applications.

Keywords: Anatase phase TiO; ₂; thin films; Pulsed laser deposition; Optical properties; Optical sensors applications

Laser processing and characterization of ZnS–Cu thin films by V. Khomchenko; L. Fedorenko; N. Yusupov; V. Rodionov; Yu. Bacherikov; G. Svechnikov; L. Zavyalova; N. Roshchina; P. Lytvyn; M. Mukhlio (pp. 434-439).

ZnS films were deposited using an original chemical method under atmospheric pressure on to a glass and ceramics substrates. The ZnS films were doped with Cu, Cl by a thermal diffusion method from a ZnS–Cu, Cl powder. Laser processing realized by a pulse nitrogen laser (337nm) with a pulse time 5ns. The power density varied from 10 to 32MW/cm². The morphology of films was investigated by atomic force microscopy (AFM). It was found that the luminescent properties and the surface morphology of the ZnS–Cu film undergo a change starting with the power density of 20MW/cm². The analysis of results shows that optimum dose is 23MW/cm². There is a laser ablation of the layer surface ZnS film during laser processing at the power densities greater than 20MW/cm². Basic roughness of the surface film was 55nm.The laser processing resulted in the smoothing of surface. The roughness decreases to 27nm. The emission and the excitation spectra measured before the laser processing and later are different. The excitation spectra consist of two bands at 337 and 375nm. The laser processing changes the ratio of the bands intensities. The emission band maximum is 525nm. The maximum shifts to 521nm after the laser irradiation. The emission band shape changes too. It becomes asymmetric because of increase short-wave wing. An analysis of the available data is accomplished taking into account two alternative possibilities: an inhomogeneous distribution of the emission centers along the direction of the growth of the film and the laser processing of ZnS–Cu film.

Keywords: Laser processing; ZnS–Cu thin film; Luminescence; Atomic force microscopy

Phase change dynamics in a polymer thin film upon femtosecond and picosecond laser irradiation by J. Bonse; S.M. Wiggins; J. Solis; T. Lippert (pp. 440-446).

The influence of the pulse duration on the laser-induced changes in a thin triazenepolymer film on a glass substrate has been investigated for single, near-infrared (800nm) Ti:sapphire laser pulses with durations ranging from 130fs up to 2.6ps. Post-irradiation optical microscopy has been used to quantitatively determine the damage threshold fluence. The latter decreases from ∼800mJ/cm² for a 2.6ps laser pulse to ∼500mJ/cm² for a pulse duration of 130fs. In situ real-time reflectivity (RTR) measurements have been performed using a ps-resolution streak camera set-up to study the transformation dynamics upon excitation with single pulses of duration of 130fs and fluences close to the damage threshold. Very different reflectivity transients have been observed above and below the damage threshold fluence. Above the damage threshold, an extremely complicated behaviour with oscillations of up to 100% in the transient reflectivity has been observed. Below the damage threshold, the transient reflectivity decreases by as much as 70% within 1ns with a subsequent recovery to the initial level occurring on the ms timescale. No apparent damage could be detected by optical microscopy under these irradiation conditions. Furthermore, within the 395–410mJ/cm² fluence range, the transient reflectivity increases by ∼10%. The analysis of these results indicates that the observed transformations are thermal in nature, in contrast to the known photochemical decomposition of this triazenepolymer under UV irradiation.

Keywords: Laser irradiation; Phase change dynamics; Polymer

Etching of CuInSe₂ thin films—comparison of femtosecond and picosecond laser ablation by David Ruthe; Klaus Zimmer; Thomas H�che (pp. 447-452).

In this paper, the processing of the new absorber material for thin film solar cells, CuInSe₂ (CIS), and the generation of trenches by femtosecond and picosecond laser etching is investigated. Threshold fluences and processing parameters for selective thin film ablation were ascertained. TEM, EDXS and Raman investigations were used to study the generation of defects in the CuInSe₂ crystal lattice near to the surface due to laser processing. Femtosecond as well as picosecond laser ablation cause only little material modification of this compound semiconductor and allow high quality laser scribing for photovoltaic applications.

Keywords: Femtosecond; Picosecond; Laser; CuInSe; ₂; Ablation

The wettability modification of bio-grade stainless steel in contact with simulated physiological liquids by the means of laser irradiation by L. Hao; J. Lawrence; L. Li (pp. 453-457).

Early surface events that occur rapidly upon implantation of a biomaterial into biological fluids determine the subsequent response. These involve wetting by physiological liquids followed by adsorption of proteins and cells to the biomaterials surface. A CO₂ laser and high power diode laser (HPDL) were used to modify the surface properties of the material and thus manipulate the wettability of the material and its interaction with physiological liquids. The contact angles, θ, of selected test liquids including simulated physiological liquids shows that the wettability of the stainless steel improved after CO₂ and HPDL treatment. The determined adhesion work of stainless steel towards stimulated physiological fluid enhanced after laser treatment, implying better interaction with the biological liquids. It is demonstrated that the laser could be a novel and controllable technique for enhancing the biocompatibility of bio-grade stainless steel.

Keywords: Laser; Wettability; 316LS stainless steel; Surface energy

Nanoscale evaluation of laser-based surface treated 12Ni maraging steel by J. Grum; J.M. Slabe (pp. 458-465).

Maraging steels are used in several high-tech areas. Among them are highly thermo-mechanically loaded vital parts of die casting dies for pressure die casting of aluminium and magnesium alloys. From the economic point of view, the operation life of dies is extremely important to the price of the castings. Operational life can be successfully extended by a regular maintenance of die parts. Laser surfacing is a very promising process for rebuilding of worn out surfaces of vital die parts. In this research, the state in the maraging steel 1.2799 (DIN) after the application of laser surfacing process has been analysed using scanning electron microscope. The analysis revealed diverse microstructure through-depth of the laser-surfaced specimens. On the basis of the estimated size and volume fraction of the nano-precipitates in the individual microstructure zones located through-depth of the heat-affected zone, a through-depth variation of microhardness was predicted. The results are supported by Vickers microhardness tests. It was confirmed that the mechanical properties of the 1.2799 maraging steel strongly depend on the characteristic at the nano or micro level. Some of the results obtained can be also applied to laser surface heat treatment of maraging steels.

Keywords: Maraging steel; Nanoscale evaluation; Laser surfacing; Precipitation hardening

Electrical and structural properties of La_0.8Sr_0.2Mn_0.5Co_0.5O3±_δ films produced by pulsed laser deposition by Nini Pryds; Bo Toftmann; J�rgen Schou; Peter Vang Hendriksen; S�ren Linderoth (pp. 466-470).

La_0.8Sr_0.2Mn_0.5Co_0.5O₃ (LSMCO) films for the use as contact layers or protective coatings in solid oxide fuel cells (SOFC) have been deposited on glass substrates by pulsed laser deposition (PLD). PLD is an obvious technique for thin film production of complex oxides, because of the ability to transfer material stoichiometrically from a multicomponent target to a growing film. In the present study, films were deposited at substrate temperatures of 473 and 573K and in different oxygen background pressures. The influence of the process parameters, in particular of the oxygen background gas pressure, on the electrical conductivity and structure of the films is investigated.

Keywords: PLD; Thin film; Solid oxide fuel cell; Oxides; Ablation

Crystallization of hydrogenated amorphous silicon–carbon films by means of laser treatments by G. Ambrosone; U. Coscia; S. Lettieri; P. Maddalena; C. Minarini; V. Parisi; S. Schutzmann (pp. 471-476).

Laser annealing of hydrogenated amorphous silicon–carbon films with carbon content, x=C/(C+Si), ranging from 0.08 to 0.28, deposited on Corning glass, has been carried out by a pulsed frequency doubled Nd:YAG (532nm) laser and a pulsed KrF excimer (248nm) laser using a fluence of 242mJ/cm². The results show that the laser radiation of 532nm induces in the samples only the growth of silicon crystallites and the degree of crystallinity decreases with increasing x. In the laser treated films at 248nm, the degree of crystallinity is enhanced with x and cubic SiC crystallites are detectable in samples with x≥0.18. Dark conductivity decreases with x for Nd:YAG laser treated films, while it is approximately constant for KrF laser treated ones. The effects of laser annealing on the crystallization process appear to be correlated with the optical properties of the as deposited films.

Keywords: Silicon–carbon alloys; Pulsed laser treatment; Crystallization

Effect of sub-micrometer polymer gratings generated by two-beam interference on surface plasmon resonance by M. Csete; Cs. Vass; J. Kokavecz; M. Goncalves; V. Megyesi; Zs. Bor; M. Pietralla; O. Marti (pp. 477-485).

Sub-micrometer periodic gratings were generated on thin poly-carbonate films spin-coated onto thin flat silver layer covered BK7 substrates by two-beam interference realized with the fourth harmonic of the Nd:Yag laser. The 416nm period of the laser-induced grating was half of the applied master grating, the modulation depth was increased by the number of laser pulses at a given intensity. Attenuated total reflection measurements were performed in case of plasmon propagation directions parallel and perpendicular to the grooves, and on rotated gratings oriented at angles making the grating coupling effect detectable. Atomic force microscopy (AFM) was applied to determine the modulation depth of the gratings. It was shown that the periodically corrugated polymer surfaces resulted in the coupling of the surface plasmons already when the structure period was very close to the plasmon wavelength. The small shift of the resonance positions proved that the modulation depth cannot be explained by material removal, the material rearrangement plays an important role in the grating formation. These experimental results correlate with our temperature model calculations revealing that there is softening on the upper polymer surface and melting down to the silver layer at the lowest and highest applied laser fluences, respectively.

Keywords: Sub-micrometer polymer grating; Two-beam interference; Surface plasmon resonance; Atomic force microscopy

Main characteristics of calcium phosphate coatings obtained by laser cladding by F. Lusqui�os; J. Pou; M. Boutinguiza; F. Quintero; R. Soto; B. Le�n; M. P�rez-Amor (pp. 486-492).

Laser surface cladding has become an extensively used technique in metallurgical applications in order to improve surface properties of materials. We have proposed this technique in the field of biomaterials to coat the surface of titanium alloy substrates used in orthopaedical implants with a calcium phosphate (CaP) bioceramic to promote the growth of the bone when the implant is inserted in the body.An exhaustive study on the influence of the processing parameters on finished surface, microstructure and superficial composition was carried out. Also, the geometrical features of the CaP coatings were correlated to the relevant processing parameters. Simple linear relationships between the studied clad features and processing parameters have been found.Different techniques applied to characterise the coated samples revealed the gradual composition of the coatings starting with a biphasic calcium phosphate on the surface and ending with a compound between the calcium phosphate and the metallic substrate in the coating–substrate interface.

Keywords: Laser cladding; Calcium phosphate; Coatings

Cobalt-doped ZnO – a room temperature dilute magnetic semiconductor by C.B. Fitzgerald; M. Venkatesan; J.G. Lunney; L.S. Dorneles; J.M.D. Coey (pp. 493-496).

Room temperature ferromagnetism is observed in thin films of ZnO doped with 1–25 at.% cobalt. Bulk samples were synthesized by a solid-state reaction technique and (110) textured thin films were prepared by pulsed laser deposition on R-cut sapphire substrates. Films are semiconducting and transparent. Resisitivity increases with increasing cobalt content.Room temperature magnetic moments of 0.5-5μ_B per Co atom were measured in the thin films. Optical spectrometry indicates that cobalt enters the tetrahedral sites of the wurtzite structure as Co²⁺.

Keywords: Cobalt-doped ZnO; Magnetic semiconductor; Ferromagnetism

UV laser-induced ordered surface nanostructures in congruent lithium niobate single crystals by S. Mailis; C.L Sones; J.G. Scott; R.W. Eason (pp. 497-503).

Ultra violet illumination of the –z face of lithium niobate single crystals, under specific conditions, results in an organized arrangement of submicron etch resistant features that reflect the illuminating intensity distribution. Consequently, spatially resolved illumination can produce periodic structures with submicron periodicity. Furthermore, a size self-adjustment of the submicron etch resistant features was observed which is related to characteristic lengths (e.g. grating period) of the overall structure. The effect occurs for a narrow range of illuminating intensities and is attributed to a photo-induced electrostatic charge distribution, which modifies the electrochemical interaction of the acid with the surface. The size and periodicity of the structures, which can be achieved with this method, are suitable for the fabrication of 2D photonic crystal structures in this electro-optically tunable material.

Keywords: Submicron; Lithium niobate; Laser-induced; Nanostructure

Laser interaction in sol–gel based materials—3-D lithography for photonic applications by R. Houbertz (pp. 504-512).

As an example for laser interactions with polymer materials, inorganic–organic hybrid polymers were polymerized with femtosecond laser pulses, making use of two-photon absorption (TPA) processes. The material was synthesized by catalytically controlled polycondensation reactions, resulting in a storage-stable resin which is built up by inorganic (SiOSi) nm-size units which are organically functionalized with typically (oligo-)methacryl and styryl moieties, respectively. This results in materials which can be patterned by single- or two-photon absorption. The latter method allows one to directly write arbitrary structures in three dimensions into the hybrid polymer material with a resolution down to 200nm or even below. The materials and the patterning process will be discussed with respect to applications in photonic devices.

Keywords: PACS; 82.35.−x; 82.50.Hp; 82.50.NdInorganic–organic hybrid polymers; Two-photon absorption; Nanopatterning; Photonic structures

Laser induced local and periodic phase transformations in iron oxide thin films obtained by chemical vapour deposition by Vladimir Sivakov; Christian Petersen; Claus Daniel; Hao Shen; Frank M�cklich; Sanjay Mathur (pp. 513-517).

Iron oxide films have been deposited on Si(100) substrates by chemical vapour deposition (CVD) of iron(III) tert-butoxide ([Fe(O^tBu)₃]₂) in the temperature range 350–450°C. The precursor flux and substrate temperature were varied to control the phase composition, average grain size and film thickness. The nature of substrate and deposition temperature markedly influence the morphology and iron-oxygen stoichiometry in the CVD deposits. Phase transformations in iron oxide films were achieved through precise local and periodic heating of the films by interfering laser beams. The interaction of iron oxide films with short laser pulses (Nd:YAG, 355nm) induced partial transformation of hematite (α-Fe₂O₃) to magnetite (Fe₃O₄) or magnetite to wüstite (Fe1−_xO), respectively. The phase characterization and morphology of the hematite and magnetite films were investigated before and after laser irradiation by X-ray diffractometry, high resolution scanning electron microscopy and white light interferometry.

Keywords: Chemical vapour deposition; Iron oxides; Phase transformation; Laser interference patterning

Properties of ZnO thin films prepared by radio-frequency plasma beam assisted laser ablation by N. Scarisoreanu; D.G. Matei; G. Dinescu; G. Epurescu; C. Ghica; L.C. Nistor; M. Dinescu (pp. 518-525).

Zinc oxide thin films were obtained by laser ablation of a Zn target in oxygen reactive atmosphere, the oxygen being supplied either by a standard gas inlet valve or from a radio-frequency (rf) oxygen plasma. Pt-coated silicon and MgO were used as substrates. The influence of the deposition parameters as laser wavelength (266, 355, 1064nm), laser fluence (1.5–20J/cm²) and oxygen pressure (1–60Pa) was studied. The influence of the rf plasma beam addition on the morphological proprieties of zinc oxide films was particularly investigated, simultaneously with several configurations of the direction of the ablation plasma, the rf plasma beam and the substrate. The obtained films, with thicknesses in the range of 50nm to 1μm have been characterized by atomic force microscopy (AFM), X-ray diffraction (XRD), transmission electron microscopy (TEM).

Keywords: Radio-frequency; Orientation; Piezoelectric

Influence of cerium on the pulsed UV nanosecond laser processing of photostructurable glass ceramic materials by F.E. Livingston; P.M. Adams; H. Helvajian (pp. 526-536).

Photostructurable glass ceramic (PSGC) materials contain a sensitizer that is used to facilitate the optical exposure process. The primary role of the sensitizer is to absorb incident radiation and generate photoelectrons. With thermal treatment, these photoelectrons can then interact with nascent metal ions to induce the formation of metallic clusters and the precipitation of a soluble crystalline phase in the glass matrix. The photo-ionization efficiency of the sensitizer species is strongly dependent on its spectral absorption and oxidation state in the base glass. Stabilizing compounds are typically added to the glass matrix to maintain the photo-active oxidation state and promote efficient exposure. To investigate the effectiveness of the photo-initiator, we have conducted experiments in which sample coupons of a commercial PSGC material (Foturan™, Schott Corp., Germany) were carefully exposed to various photon doses by pulsed UV nanosecond lasers at λ=266nm and 355nm. Foturan is a lithium aluminosilicate glass that contains trace amounts of cerium as the photosensitive agent (0.01–0.04wt.% admixture Ce₂O₃). The photo-initiator efficiency was investigated by using samples with cerium and without cerium. The irradiation wavelengths were selected because they lie above and below the primary absorption band of the cerium photo-initiator. Optical transmission spectroscopy (OTS) was employed to identify and monitor the population density of the photo-induced trapped electron state as a function of incident laser irradiance. The irradiated samples were thermally processed and then analyzed again with OTS to measure the quenching of the trapped electron state and the concurrent growth of a spectral band associated with the formation of nanometer-scale metallic clusters. The growth of metallic clusters signifies the “fixing�? of the exposure and permanent image formation in the glass. The OTS results reveal that for λ=266nm laser irradiation, at least two photoelectron donors (including cerium) contribute to the formation of the trapped electron (defect state) density and fixing of the exposure. For λ=355nm laser irradiation, however, the photo-initiator is critical to the exposure process and nearly all of the trapped electron density is formed via cerium photoelectron donors.

Keywords: Photostructurable glass ceramic; Laser material processing; Photosensitizer; Cerium; Glass spectroscopy

Laser doping for microelectronics and microtechnology by Thierry Sarnet; Gurwan Kerrien; Nourdin Yaakoubi; Alain Bosseboeuf; Elisabeth Dufour-Gergam; Dominique D�barre; Jacques Boulmer; Kuniyuki Kakushima; Cyrille Laviron; Miguel Hernandez; Julien Venturini; Tarik Bourouina (pp. 537-544).

The future CMOS generations for microelectronics will require advanced doping techniques capable to realize ultra-shallow, highly doped junctions with abrupt profiles. Recent experiments have shown the potential capabilities of laser processing of ultra shallow junctions (USJ). According to the International Technology Roadmap for Semiconductors, two laser processes are able to reach the ultimate predictions: laser thermal processing or annealing (LTP or LTA) and gas immersion laser doping (GILD). Both processes are based on the rapid melting/solidification of the substrate. During solidification, the liquid silicon, which contains the dopants, is formed epitaxially from the underlying crystalline silicon. In the case of laser thermal annealing, dopants are implanted before laser processing. GILD skips the ion-implantation step: in this case the dopants are chemisorbed on the Si surface before the laser-shot. The dopants are then incorporated and activated during the laser process. Activation is limited to the liquid layer and this chemisorption/laser-shot cycle can be repeated until the desired concentration is reached. In this paper, we investigate the possibilities and limitations of the GILD technique for two different substrates: silicon bulk and SOI. We also show some laser doping applications for the fabrication of micro and nanoresonators, widely used in the MEMS Industry.

Keywords: Laser doping; Ultra shallow junctions; Implantation; Microresonator; Nanoresonator; MEMS; LTP; GILD

Study of laser crystallization and recording properties of oxygen doped Ge:Sb:Te films by C. Rivera-Rodr�guez; E. Prokhorov; Yu. Kovalenko; E. Morales-S�nchez; J. Gonz�lez-Hern�ndez (pp. 545-549).

The aim of this work is to study the mechanism of the amorphous-to-crystalline phase transformation in Ge₁Sb₂Te₄ films doped with oxygen using nanosecond laser pulses under isothermal annealing in the time scale of minutes.Experimental results show that the nucleation time, t_nucl (minimum laser pulse duration for starting the laser-induced crystallization) depends on the oxygen concentration in the films. For those films with compositions in the range of 2–8at.%, t_nucl is shorter than that observed in films free of oxygen. In contrast, in films with oxygen in the range of 10–28at.%, t_nucl is longer than in the reference sample. Reflection and X-ray measurements on minute time scale have shown that in the films without oxygen annealed under isothermal conditions, the nucleation of the Ge₁Sb₄Te₇ metastable phase is first observed, which is subsequently transformed into the Ge₁Sb₂Te₄ crystalline phase. This effect increases the nucleation time in laser-crystallized materials. For films with 2–8at.% of oxygen the first nucleation phase to be observed is the crystalline Ge₁Sb₂Te₄, where it is assumed that the oxygen acts as the center of nucleation, therefore decreasing the nucleation time. In the films with oxygen concentration above 10at.%, the thermal treatments leads to the formation of stable amorphous GeO₂. This decreases the amount of available Ge and leads to the formation of crystalline Sb₂Te₃. The phase segregation in films with more than 10at.% of oxygen, results in an increase in the nucleation and crystallization times in laser-induced crystallization. This crystallization behavior allows the possibility of having multilevel laser recording.

Keywords: Laser crystallization; Oxygen; Ge:Sb:Te

Combinatorial experiment in Ni–Ti thin films by laser interference structuring by K.W. Liu; C. Gachot; P. Leibenguth; F. M�cklich (pp. 550-555).

Combinatorial experiments are achieved on periodically structured Ni–Ti thin film composition spreads by laser interference irradiation using a Nd:YAG laser. Continuous Ni–Ti compositional spreads covering almost the whole binary system are prepared by combining sputter mask, shutter and movement of substrate. The continuous compositional spread is subsequently micro-structured into a sample library consisting of well-defined lines of individual samples by laser interference irradiation. The composition and microstructure effects in continuous spread and sample libraries after laser structuring are explored by scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and white light interferometry (WLI) microscopy. The sample library consists of individual samples with a distance of about 5μm and a composition resolution as high as 0.1at.% in between. Although, there are certain difficulties so far in obtaining the optimized laser fluence for the spread, the laser interference irradiation provides an effective way to prepare thin film libraries with around 200 sample lines within 1mm.

Keywords: Laser; Ni–Ti; Thin film

Electron–hole relaxation through optical-phonon emission in CdTe/ZnTe quantum dots by S. Cronenberger; Y. Viale; O. Cr�gut; M. Gallart; B. H�nerlage; P. Gilliot (pp. 556-560).

We present a study of ZnCdTe quantum dots (QDs) embedded in Zn-rich ZnCdTe quantum wells (QWs). The Cd-rich ZnCdTe islands are confined along the growth direction by pure ZnTe barriers and laterally by Zn-rich ZnCdTe QWs. Using a time-resolved pump and probe technique, we analyse the phonon emission-cascade in QWs governing, after non-resonant optical excitation in the QWs, the energy relaxation of electron–hole pairs towards self-organized QDs. Since electron–hole pairs thermalize only by emission of an integer number of longitudinal-optical phonons, at the end of the cascade, the QDs become selectively populated. We thus determine the mean value of the optical-phonon emission time. Then, for longer time delays, we observe the population relaxation dynamics of the selected quantum dots.

Keywords: Optical-phonon; Relaxation; Quantum dots

Novel applications of short and ultra-short pulses by M. Stra�l; H. Kopecek; M. Weinrotter; A. B�cker; A.H. Al-Janabi; V. Wieger; E. Wintner (pp. 561-570).

This paper offers recent successful examples for the application of nanosecond (ns) as well as picosecond (ps) and femtosecond (fs) laser pulses to media of gaseous, liquid or solid nature via non-linear interactions as a review, the laser ignition and dental ultra-short pulse interaction being parts of the authors’ own work. Plasma-initiated ignition of combustible gas mixtures represents a potential alternative way for long-lasting operation of gas engines with rather clean exhaust. Ultra-short pulses are useful for materials processing including dental hard tissue. Using miniaturized scanners of different types yields perfect cavity sizes without collateral damage at ablation rates coming close to mechanical drills in dentistry.

Keywords: Ultra-short pulses; Q-switched; Plasma-initiated ignition

Femtosecond laser driven shock synthesis of the high-pressure phase of iron by Tomokazu Sano; Hiroaki Mori; Osami Sakata; Etsuji Ohmura; Isamu Miyamoto; Akio Hirose; Kojiro F. Kobayashi (pp. 571-576).

The synthesis of the high-pressure ɛ phase of iron was attained using a femtosecond laser. The crystalline structure in a recovered iron sample was determined using electron diffraction and synchrotron X-ray diffraction methods. The high-temperature γ phase of iron also existed in the recovered sample. The temperature inside the shock front was calculated using thermodynamic equations. It was found that the ɛ phase was induced by the shock itself but not the γ phase. The γ phase was suggested to be induced as an intermediate structure between the α–ɛ transition. The femtosecond laser driven shock may have the potential to synthesis high-pressure phases, which has not been attained using conventional methods.

Keywords: Femtosecond laser; Shock wave; High pressure; Phase transition; Iron

Current trends in 157nm dry lithography by A.C. Cefalas (pp. 577-583).

Lithography at 157nm using F₂ laser is the next step after 193nm for ULSI fabrication with dimensions below 50nm. However, there are certain issues to be addressed before the acceptance of the technology by the industry. The most important are: (1) design of resists with low absorption and outgassing at 157nm, (2) improved line edge roughness and resolution and (3) defect-free resist surface following illumination at 157nm. In this communication it has been confirmed theoretically and experimentally that outgassing of resists at 157nm is an intrinsic molecular property and does not depend on the laser parameters. Based on a theoretical model, a new method for measuring outgassing of resists has been developed by measuring the thickness loss, following illumination at 157nm, using vacuum ultraviolet absorption spectroscopy and atomic force microscopy.

Keywords: 157; nm lithography; Outgassing; Polymers; AFM

Time resolved schlieren study of sub-pecosecond and nanosecond laser transfer of biomaterials by I. Zergioti; A. Karaiskou; D.G. Papazoglou; C. Fotakis; M. Kapsetaki; D. Kafetzopoulos (pp. 584-589).

A comparative study of the effect of ultrashort (0.5ps) and short (15ns) pulses on the laser forward transfer of DNA molecules is presented in this paper. We use femtosecond laser pulses to directly print a wide range of biomaterials, in complicated patterns and structures. The ultrashort laser pulses reduce the thermal effects, thus allowing the effective deposition of sensitive biomaterials at high spatial resolution for micro-fabricating patterns. This direct laser printing process enables gentle and spatially selective transfer of biomaterials and facilitates application possibilities for the fabrication of biosensors and arrays for multi-analyte assays. Here, we present the direct micro-printing of biomaterials such as enzyme patterns by laser-induced forward transfer method using 500fs laser pulses emitted at 248nm. Furthermore, the dynamics of the process was investigated by stroboscopic schlieren imaging for time delays up to 3μs following the laser irradiation pulse.

Keywords: Schlieren study; Laser pulses; Biomaterials

Laser manipulation of clusters, structural defects and nanoaggregates in barrier structures on silicon and binary semi-conductors by G.I. Vorobets; O.I. Vorobets; V.N. Strebegev (pp. 590-601).

The methods of optical and scanning electron microscopy (SEM) along with electronic probe X-ray spectral microanalysis proved an opportunity to manipulate by means of laser irradiation with clusters, structural impurity defects and nanoaggregates in a space charge region (SCR) of a semiconductor in devices with a Schottky barrier formed on the basis of silicon and chalcogenide semiconductors. The optimal conditions of intensity of the millisecond laser irradiation at which the sizes of clusters diminish considerably are established, as well as the densities of edge dislocations in SCR and of dot defects. The dependences of the surface states density on contact, of the density of the deep levels in SCR and transition nanosize layer on a metal–semiconductor contact on the conditions of a laser exposure are studied. The presence of both the 5/500nm deep transition structured layer and the modulated band gap causes the occurrence of tunnel-resonance charge transfer in contacts with a Schottky barrier, which can result in the appearance of sites with negative differential resistance in a current–voltage characteristic of the investigated structures.

Keywords: Semi-conductors; Laser; Schottky barrier; Structural defects

Manipulation of the osteoblast response to a Ti–6Al–4V titanium alloy using a high power diode laser by L. Hao; J. Lawrence; L. Li (pp. 602-606).

To improve the bone integration of titanium-based implants a high power diode laser (HPDL) was used to modify the material for improved osteoblast cell response. The surface properties of un-treated and HPDL treated samples were characterized. Contact angles for the un-treated and the HPDL modified titanium alloy (Ti–6Al–4V) were determined with selected biological liquids by the sessile drop technique. The analysis revealed that the wettability of the Ti–6Al–4V improved after HPDL laser treatment, indicating that better interaction with the biological liquids occurred. Moreover, an in vitro human fetal osteoblast cells (hFOB 1.19) evaluation revealed a more favourable cell response on the HPDL laser treated Ti–6Al–4V alloy than on either un-treated sample or a mechanically roughened sample. It was consequently determined that the HPDL provides more a controllable and effective technique to improve the biocompatibility of bio-metals.

Keywords: High power diode laser; Wettability; Titanium alloy; Osteoblast cell

Reactive laser synthesis of carbides and nitrides by P. Schaaf; M. Kahle; E. Carpene (pp. 607-615).

Carbides and nitrides play an important role for technological applications. Here, we present a new promising technique for a direct laser synthesis of carbides and nitrides by pulsed laser irradiation in reactive atmospheres, e.g. methane and nitrogen. A number of laser types have been used (excimer laser, Nd:YAG, Ti:sapphire, free electron laser) and a number of different nitrides and carbides have been produced (e.g. TiN, AlN, Fe₃C, SiC). The mechanisms and examples will be presented and it will be shown that also the lateral and in depth structuring can be modified from nanoscale to mesoscale. The dependence of phase formation and resulting properties are given in relation to the laser treatment parameters, such as laser type, pulse duration, material and reactive gas pressure. This reactive laser synthesis can also be used for the improvement of steel surfaces. For example, the irradiation of pure iron in methane atmosphere at low pressures or low number of laser pulses results in the formation of an amorphous carbon surface layer and hexagonalɛ-Fe_1−xC_x, which is more and more transformed into cementite when increasing the number of pulses or the gas pressure.

Keywords: PACS; 81.65.Lp; 52.50.Jm; 61.80.Ba; 76.80.+yLaser surface treatment; Nitrides; Carbides; Laser plasma; Reactive laser treatment

Laser-assisted structuring of ceramic and steel surfaces for improving tribological properties by S. Schreck; K.-H. Zum Gahr (pp. 616-622).

Tribological properties of a system are very important for saving energy or increasing service life as in automotive industry. It is known that wear and friction behaviour is strongly influenced by the topography of the mated surfaces. In case of lubricated sliding contact the tribological properties can be improved by generating a defined surface structure in form of small channels or isolated pits. Depending on the structure it may act as a reservoir for the lubricant and hampers the drain of the lubricant. In addition, wear particles can be removed from the contact area and can be collected inside the structures.In the present paper, a Nd:YAG-laser was used to generate different structures on ceramic (Al₂O₃) and steel (100Cr6) surfaces. Microchannels and micropits were generated onto an otherwise flat surface by a laser ablation process. Due to the high flexibility of the laser system, structural features such as size, density and orientation could be varied easily by changing the laser parameters. The effects of different topographies on friction and wear behaviour were examined using a cylinder/plate tribometer for lubricated and reversed sliding contact.Laser structured 100Cr6 and alumina surfaces mated to 100Cr6 cylinders showed that a reduction of friction could be achieved in comparison to the polished surfaces. Dependent on the kind of the surface structure the coefficient of friction could be reduced up to 30%, especially in the initial period of the tribological experiment. The results show that the laser technique is an effective tool to optimise the topography of tribological surfaces.

Keywords: Laser; Lubricant; Tribological properties; Surface structure

Preliminary study into the effects of YAG laser processing of titanium 6Al–4V alloy for potential aerospace component cleaning application by M.W. Turner; M.J.J. Schmidt; L. Li (pp. 623-630).

This paper outlines some essential gas turbine aeroengine component manufacture cleaning requirements. It describes the preliminary results of work conducted into the effects of laser processing Ti–6A–4V alloy for cleaning of aerospace components, including determination of the melt and surface coupling thresholds for the Nd:YAG ( λ=1.064μm) system in use. The results of laser processing as a preparation for electron beam welding and diffusion bonding are presented. Finally, the direction of future work towards the development of an effective process for laser cleaning titanium aero-engine components is discussed. The results demonstrated the sensitivity of the system in use to gaseous contamination, producing acceptable electron beam welded joints, but unacceptable diffusion bonded joints.

Keywords: Laser; Titanium; Cleaning; Joining

Laser synthesis of amorphous Si–Al oxide nanowires under atmospheric conditions by F. Quintero; J. Pou; F. Lusqui�os; M. Boutinguiza; R. Soto; M. P�rez-Amor (pp. 631-635).

A new technique for the laser synthesis of amorphous aluminium silicate nanowires is described. The method is based in the laser vaporization and melting of a commercial ceramic blank under atmospheric conditions at room temperature. A high-pressure gas jet is employed to assist the process. This technique yields a great quantity of free-standing nanowires and microwires in a short processing time with no previous preparation of the precursor and a straightforward set-up of the processing system. The amorphous nanowires were characterized by field emission scanning electron microscope (FESEM), energy-dispersive X-ray spectroscope (EDX) and transmission electron microscope (TEM) and an explanation for their growth based on the vapor–liquid–solid mechanism is proposed.

Keywords: Nanowires; Laser; Si–Al oxide

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Applied Surface Science (v.247, #1-4)