Investigation of temperature and stress fields in laser cladded coatings

Temporal and spatial distributions of temperature and strain-stress have been modelled and investigated experimentally for the laser cladding process. The model corresponded to experimental conditions where the multilayer protective coatings were prepared by direct laser cladding of stellite SF6 powder on X10Cr13 chromium steel by means of a 1.2 kW CO₂ laser. For calculations the effect of base preheating, temperature dependent material properties, and also influence of time-break between cladding of the consecutive layers were taken into account. The calculated temperature fields indicated good bonding of the substrate and coating, which was in agreement with the micro-analytical test results. A decrease of the number of microcracks in the coating with an increase of substrate preheating temperature was concluded from stress calculations and confirmed in the experiment. Moreover, an increase of the cracking susceptibility with an increase of the time delay between cladding of the consecutive layers was evidenced by modelling. The best technological results were obtained for the case of single-layer coatings prepared on a preheated substrate and for higher coating thickness required the processing of consecutive layers with a possibly short time delay is advisable due to effective usage of laser beam energy for preheating and lower temperature gradients.     

Formation of MnAs and MnP layers by reactive laser sputtering

We show that the technique of pulsed laser sputtering of metallic manganese in a flow of hydrides (arsine or phosphine) allows the deposition of half-metallic MnB ⁵ compound layers on GaAs (100) substrates. The crystal structure, magneto-optical, and galvanomagnetic properties of the layers are determined by synthesis conditions, mainly by substrate temperature. It is established that the MnAs and MnP layers are ferromagnetic at temperatures up to 300 K.     

Luminescence and structural properties of thermally evaporated benzanthrone dyes thin films

We report optical and luminescence properties of 3-N, N-diacetylaminobenzanthrone thin films deposited on glass substrate by thermal evaporation. The structural and optical properties of organic thin films were studied by means of the confocal microscope with an input of femtosecond laser radiation, X-ray diffractometer, and scanning electron microscope (SEM). Intense luminescence with the maximum at 530 nm was observed when excited by laser radiation with the wavelengths 458, 476, 488, 496, 514 nm. In addition, the luminescence caused by two-photon absorption of femtosecond (fs) laser radiation has been investigated. Semi empirical calculations by AM1 and ZINDO/S methods and ab initio calculations using Gaussian software were carried out to estimate the electron system of structure. The calculations show planar configurations for the aromatic core and diacetylamino fragment of this compound. The study of the structure of benzanthrone derivative thin films with X-ray diffraction (XRD) methods, indicates the distance between molecular layers and ordered molecular fragments.     

The chemisorption of hydrogen on silicon and silicon carbide (1 0 0) surfaces

The chemisorption of hydrogen onto semiconductor surfaces is examined. The hydrogen bonds to the dangling bond of a surface atom. These dangling bonds also dictate the reconstruction of the crystal surface. The chemisorbed hydrogen therefore modifies the reconstructed surface topology. In this work theoretical calculations of the surface structures of both covalent elemental silicon and polar silicon carbide are presented. The periodic MINDO method is employed to determine the topologies for the 2 × 1 reconstructed (1 0 0) surfaces. These topologies are obtained from the minimisation of the total energy, for silicon and silicon carbide films of 14 layer thickness, with respect to the atomic co-ordinates of the hydrogen adsorbate and the first four layers of the substrate. The results show that the formation of the hydrogen bond to the substrate leads to a general lengthening of the surface dimer bond. In addition, the buckling of the silicon dimer determined for silicon terminated SiC is removed by hydrogen chemisorption.     

Experiment and modeling of a small-scale,supersonic chemical oxygen-iodine laser

We report on detailed experiment and modeling of a small-scale, supersonic chemical oxygen-iodine laser. The laser has a 5 cm long active medium and utilizes a simple sparger-type O₂(¹ ) chemical generator and a medium-size pumping system. A grid nozzle is used for iodine injection and supersonic expansion. 25 W of cw laser emission at 1.315 µm are obtained in the present experiments. The small size and the simple structure of the laser system and its stable operation for long times make it a convenient tool for studying parameters important for high-power supersonic iodine lasers and for comparison to model calculations. The lasing power is studied as a function of the molar flow rates of the various reagents, and conditions are found for optimal operation. Good agreement is found between the experimental results and calculations based on a simple one-dimensional semi-empirical model, previously developed in our laboratory and modified in the present work. The model is used to predict optimal values for parameters affecting the laser performance that are difficult to examine in the present experimental system.     

Laser-induced fabrication of randomly distributed nanostructures in fused silica surfaces

The nanostructuring of dielectrics is a big challenge for laser patterning methods. In this study a novel laser structuring method for the fabrication of randomly distributed nanostructures, called laser-induced front side etching using in situ pre-structured metal layers (IPSM-LIFE), is presented. The pulsed laser irradiation of a thin metal film deposited onto a dielectric substrate with fluences below the ablation threshold results in the formation of randomly distributed metal structures by self-assembly processes. Further pulsed laser irradiation of these metal structures with higher or equal laser fluences causes the formation of complex patterns at the surface of the dielectric due to localized ablation and melting processes of the dielectric surface induced by the absorption of the laser energy by the metal structures and the local energy transfer into the dielectric surface. The pattern formation observed in the film and the dielectrics substrate after irradiation of 10 nm chromium layers on fused silica, with laser pulses (Δt _p =25 ns, λ=248 nm), was studied by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Different features with a lateral size down to a few tens of nanometers, like concentric ring patterns, donut-like structures, and bar patterns were observed at the dielectric.     

Towards controlled flyer acceleration by a laser-driven mini flyer

A laser driven flyer (LDF) system is designed to blast off a very small, thin flyer plate for impact on a target. When a Nd:YAG laser beam is focused through a transparent substrate onto thin metal, a fraction of the metal is ablated. The blow-off products being contained between the substrate and the flyer make the remaining thin film launch as a separate flyer. Some energy of the laser beam is lost by reflection at the boundary between substrate and metal because of the high reflectivity. By using a proper metal of high absorptance at 1.064 μm wavelength, the laser coupling to the flyer would define the system efficiency of a launch system. An effort is presented here to improve the coupling results in the enhancement of the flyer velocity for a given pulse energy. An optimum energy conversion between laser energy and kinetic energy of the flyer is achieved through a black paint coating technique as opposed to a more conventional means of a multi-layered approach requiring electron beaming or magnetron sputtering that are rather expensive and time consuming. The mini flyer flown under 1.4 km/s showed a controlled flight trajectory without fragmentation, suggesting that performance of this simple system is competitive to if not better than other attempts by the multi-layered LDF systems.     

Vibrational spectroscopy of a compound with a CS7 ring

The product of the Asinger reaction between elemental sulfur, n‐butylamine and acetophenone is 8‐(n‐butylaminophenylmethyliden)‐1,2,3,4,5,6,7‐heptathiocane which contains a CS₇ ring. A combination of infrared, Raman and inelastic neutron scattering spectroscopies with periodic density functional theory calculations is used to provide a complete assignment of the vibrational spectra of this unusual species. The similarity between the Raman spectra of the compound and that of elemental sulfur is particularly striking. Copyright © 2009 John Wiley & Sons, Ltd.     

Influence of oxygen on the ion-assisted regrowth of deposited Si layers

Summary The ion-assisted regrowth of chemical-vapour deposited Si films onto (100) Si substrates is reported. The regrowth was induced by a 600 keV Kr⁺⁺ beam at doses in the range (2·10¹⁵÷6·10¹⁵)/cm² and at a dose rate of 1·10¹²/cm²s. The target temperature was set at 450°C. During irradiations the crystal-amorphous interface velocity was measuredin situ by monitoring the reflectivity of a He-Ne laser light focused onto the sample surface. After irradiation some samples were also analysed by Rutherford backscattering in combination with the channelling effect and by transmission electron microscopy. The growth rate of deposited layers depends on the cleaning procedure performed prior to deposition,i.e on the total amount of oxygen present at the deposited layer/substrate interface. Moreover, twinned material is observed in the recrystallized layers and its concentration is strongly dependent on substrate cleaning. These phenomena are explained in terms of a decrease in the ion-assisted growth rate in the presence of high oxygen concentrations. The data are discussed and compared with those obtained during pure thermal annealing. To speed up publication, the authors of this paper has agreed to not receive the proofs for correction.     

Anchoring transitions with polar and non-polar nematic liquid crystals on incompletely oxidized silane substrates

The active oxygen gas arising from a plasma reactor is used to realize progressive chemical modifications onto silane coatings that could be particularly interesting as alignment layers for liquid crystal display applications. Depending on the oxygen density grafted onto the substrate, these alignment layers provide different zenithal anchoring angles, or pretilt angles, with anchoring transitions, for polar and non-polar nematic liquid crystals as 5CB and MBBA, respectively. The anchoring transitions are found to be smoother with the polar nematics. Such a behavior is discussed in terms of the differential wetting model by adding a cosine term to the interaction energy between the nematic and the substrate. A local justification is proposed for this symmetry breaking term. Received: 18 May 1998     

Hyperfine fields in ZnO studied under uni- and biaxial pressure

The II-VI semiconductor ZnO has many potential applications in optoelectronic and sensor devices. When used as a transparent conducting contact it is often grown epitaxially onto a different substrate with the consequence that the layers are biaxially strained due to lattice mismatch. Similarly, impurity-implanted layers can lead to the development of local strain fields. Strain usually changes the electronic properties of layers and/ or implanted crystal regions. Detailed knowledge about local strain and its influence on the crystal fields is therefore helpful in predicting changes in crystal properties. The perturbed angular correlation technique was applied to study the electric field gradient (EFG) at the site of implanted In dopants in ZnO under uniaxial and biaxial strain. The observed linear change of the EFG with pressure and a change in symmetry due to compression perpendicular to the c-axis could be well reproduced by theoretical calculations using the point charge model.     

Plasmonic laser nanoablation of silicon by the scattering of femtosecond pulses near gold nanospheres

We present the fabrication of nanostructures ablated on silicon(100) by the plasmonic scattering of 780 nm, 220 fs laser pulses in the near-field of gold nanospheres. We take advantage of the enhanced plasmonic scattering of ultrashort laser light in the particle near-field to ablate well-defined nanocraters. Gold nanospheres of 150 nm diameter are deposited onto a silicon surface and irradiated with a single laser pulse. We studied the effect of laser polarization on the morphology of ablated nanostructures and estimated the minimum fluence for plasmonic nanoablation. When the polarization of the incident radiation is directed at a 45° angle into the substrate surface, a near-field enhancement of 23.1±7.6 is measured, reducing the required silicon ablation fluence from 191±14 mJ/cm² to 8.2±2.9 mJ/cm². Enhancements are also measured for laser polarizations parallel to the substrate surface when the substrate is angled 0° and 45° to the incident irradiation, giving enhancements of 6.9±0.6 and 4.1±1.3, respectively. Generated nanocrater morphologies show a direct imprint of the particle dipolar scattering region, as predicted in our theoretical calculations. The measured near-field enhancement values agree well with the maximum field enhancements obtained in our calculations. The agreement between theory and measurements supports that the nanocraters are indeed formed by the enhanced plasmonic scattering in the near-field of the nanoparticles. PACS 42.62.-b; 52.38.Mf; 81.65.Cf; 81.16.-c; 78.67.Bf     

Carrier Dynamics and Saturation Effect in (113)B InAs/InP Quantum Dot Lasers

Quantum dot (QD) lasers exhibit many interesting and useful properties such as low threshold current, temperature insensitivity or chirpless behavior. In order to reach the standards of long-haul optical transmissions, 1.55 μm InAs QD lasers on InP substrate have been developed. Based on time resolved photoluminescence (PL) measurements, carrier dynamics behavior is at first investigated. Electroluminescence (EL) results are then shown at room temperature exhibiting a laser emission centered at 1.61 μm associated to a threshold current density as low as 820 A/cm² for a six InAs QD stacked layers. Finally, a rate equation model based on the reservoir theory is used to model both time-resolved photoluminescence (TRPL) and electroluminescence results. It is shown that carrier dynamic calculations are in a good agreement with measurements since the saturation effect occurring at high injected power is clearly predicted. P. Miska: Previously at Laboratoire d’Etude des Nanostructures à Semiconducteurs.     

Formation of aminosilane-oxide interphases

The aim of this work is to understand the interactions and interphase formation mechanisms between a liquid aminosilane oligomer (γ-APS) and glass, steel or gold surfaces as a function of the pH of the liquid aminosilane. When basic liquid γ-APS (pH 11.4) is applied onto a gold coated substrate, no interphase is detected. Similarly, when liquid γ-APS controlled at pH 8 is applied onto steel or glass substrates and cured, properties are the same as the bulk ones. In contrast, when the liquid γ-APS (pH 11.4) is applied onto steel or glass substrates and cured, an interphase, with chemical, physical and mechanical properties quite different from those of the bulk oligomer, is created between the substrate and the oligomer. Using various analytical techniques (DSC, FTIR, ICP, SEM, AFM, nano-indentation and XPS) it was shown that the amino-silane chemically reacts with and dissolves the oxide or hydroxide layers. Then metallic ions diffuse through the organic layer to form a complex, assumed to be of coordination type with the amine function of the oligomer molecule. These organometallic complexes are insoluble at room temperature and crystallize in the form of sharp needles. The Young's modulus of the resulting crystal is equal to approximately 5 GPa, i.e.over two orders of magnitude higher than that of the silane. In other words, these organometallic complexes act as a short fiber in a matrix.     

Laser transfer of thin mesotetraphenylporphyrin layers

The technique of forming mesotetraphenylporphyrin microstructures on the quartz substrate surface by laser-induced forward transfer from a target was developed. The target was a transparent substrate with a thin titanium coating onto which five mesotetraphenylporphyrin layers were deposited by the Langmuir-Schaefer method. The target was irradiated with single 500-ps pulses through a transparent substrate, which causes nondestructive thermal deformation of a metal film, resulting in efficient porphyrin emission from the target. The effect of the titanium film thickness and the laser emission wavelength on the transfer process was studied. An analysis of the optical absorption and fluorescence spectra of obtained surface microstructures suggests that the material structure is retained during laser transfer.     

Injection level dependence of the gain, refractive index variation, and alpha (α) parameter in broad-area InGaAs deep quantum-well lasers

In this study, a single, simple and an accurate computer-aided design model is developed in order to obtain the injection level dependence of the critical quantities of broad-area (with a width of 50 μm or more) InGaAs deep quantum-well (QW) lasers. Each of these quantities (gain, refractive index variation, and alpha (α) parameter) requires lengthy mathematical calculations with the use of different theories, assumptions, approximations, and estimations of some parameter values. The model is based on artificial neural network (ANN) approach that the total computational time is in the order of microseconds for the whole quantities in order to get their accurate values. The results are in very good agreement with the previously obtained results from an InGaAs deep QW laser sample.     

Surface plasmon resonance spectroscopy on rotated sub-micrometer polymer gratings generated by UV-laser based two-beam interference

Two-beam interference method was applied to generate gratings having periods of 416 nm and 833 nm by the forth harmonic of a Nd:Yag laser on thin poly-carbonate films spin-coated onto silver layer-covered substrates. The dependence of the modulation depth on the fluence and number of laser pulses was investigated by atomic force microscopy. A secondary pattern appeared on very thin polymer layers thanks to the “p” polarized laser beam illumination induced self-organized processes. The conditions of the emergence of grating-coupling caused additional plasmon resonance peak were determined for the sub-micrometer periodic polymer gratings. Surface plasmon resonance measurements were performed in attenuated total reflection arrangement to determine the effect of the angle between the plasmon propagation direction and the polymer groves on the grating-coupling. The effect of the modulation depth on the grating-coupling caused additional resonance minimum was also analyzed. We found coupling phenomena according to our calculations, the differences between the measured and theoretically predicted resonance curves were explained by the scattering on the complex surface structure.     

On the reaction kinetics in laser-induced photochemical gas-phase processing

Model calculations on laser-induced photolytic gas-phase processing have shown that the concentrations of photogenerated species and the reaction rates depend not only on the electronic properties of precursor molecules and on the wavelength and intensity of the laser excitation, but also on the size of the laser focus, the type of the substrate material and, significantly, on the material and geometry of the reaction chamber. In particular, it has been demonstrated that a dependence of the reaction rate,W, on the radius of the laser beam on the substrate surface,w _o, according toWw ₀ ^–n cannot be employed, in general, in a simple way to separate adsorbed-phase and gas-phase contributions to the reaction rate as claimed in previous investigations. Even in pure photolytic gas-phase processing, the exponentn can vary within the range 0<n<=2, depending on the various different parameters and material properties.     

Laser scribing on HOPG for graphene stamp printing on silicon wafer

Highly oriented pyrolytic graphite (HOPG) was scribed by pulsed laser beam to produce square patterns. Patterning of HOPG surface facilitates the detachment of graphene layers during contact printing. Direct HOPG-to-substrate and glue-assisted stamp printing of a few-layers graphene was compared. Printed graphene sheets were visualized by optical and scanning electron microscopy. The number of graphene layers was measured by atomic force microscopy. Glue-assisted stamp printing allows printing relatively large graphene sheets (40×40 μm) onto a silicon wafer. The presented method is easier to implement and is more flexible than the majority of existing ways of placing graphene sheets onto a substrate.     

(InSb/GaAs)-Au hybrid macro-structure prepared by flash evaporation

n-Type indium antimonide-Au hybrid macro-structure was successfully fabricated on p-GaAs substrate by a simple and economic flash evaporation technique. The elemental composition of the prepared InSb film was confirmed by energy dispersive X-ray spectroscopy microanalysis (EDX). The surface topography and crystal structure of the InSb film was characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. The magnetoresistance of the prepared hybrid macro-structure was measured and discussed in terms of the model of current redistribution.