Proceedings Volume 5121

Laser Processing of Advanced Materials and Laser Microtechnologies

Friedrich H. Dausinger, Vitali I. Konov, Vladimir Yu. Baranov, et al.
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Proceedings Volume 5121

Laser Processing of Advanced Materials and Laser Microtechnologies

Friedrich H. Dausinger, Vitali I. Konov, Vladimir Yu. Baranov, et al.
View the digital version of this volume at SPIE Digital Libarary.

Volume Details

Date Published: 2 September 2003
Contents: 5 Sessions, 49 Papers, 0 Presentations
Conference: Laser Processing of Advanced Materials and Laser Microtechnologies 2002
Volume Number: 5121

Table of Contents

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Table of Contents

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  • Fundamentals of Laser Interaction with Solids
  • Laser Microprocessing of Materials
  • Pulsed Laser Deposition of Thin Films
  • High-Power Laser Technologies
  • Laser Isotope Separation
Fundamentals of Laser Interaction with Solids
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Laser welding and cutting: recent insights into fluid-dynamics mechanisms
Vladimir S. Golubev
The paper presents a brief survey of non-stationary hydrodynamic motions of liquid at deep penetration of laser beam into materials or heterogeneous condensed matter in laser materials processing or in laser destruction of biotissues. The physical models are considered of such phenomena as formation of moving liquid “shelves”, mass transfer by the way of droplets formation in evolution of liquid surface hydrodynamic instabilities, turbulent transfer of mass and heat in laser action area. The main goal of the presentation is to outline the “bridges” between the outcomes of fundamental hydrodynamics and the results of investigations in the field of practical laser applications.
Laser-induced transparency during nanosecond laser ablation
S. N. Andreev, A. A. Samokhin, I. N. Kartashov, et al.
Steady state laser vaporization regime of condensed matter is investigated in the case of laser-induced transparency in irradiated target. Vaporization front stability problem is also considered taking into account bulk absorption in the target and different Mach number in evaporation plume.
Plasma effects during ablation and drilling using pulsed solid-state lasers
Detlef Breitling, Andreas Ruf, Peter W. Berger, et al.
Plasma and vapor plumes generated by ultrashort laser pulses have been studied by various optical methods for both single pulse ablation as well as high-repetition rate drilling. Time-resolved shadow and resonance absorption photographs enable to determine the plume and vapor expansion behavior and, by means of an analytical shock wave model, allow to estimate an energy balance that can be refined by plasma transmission measurements. The results furthermore suggest that several types of laser-induced plasmas can be distinguished according to their origin: the material vapor plasma originating at the ablated surface even at moderate intensities, a breakdown plasma at increased power densities occurring in cold vapor or dust particles left from previous ablations during repetitively-pulsed processing and, finally, the optical breakdown in the pure atmosphere at high intensities. The latter also gives rise to nonlinear scattering phenomena resulting in a strong redistribution of the energy density in the beam profile.
UV-laser-induced photolysis and desorption of molecules adsorbed on transparent substrates
Vladimir N. Varakin, Alexander P. Simonov
Dissociation and desorption of physisorbed on a transparent fused quartz surface halogenomethane molecules induced by their absorption of UV excimer laser radiation have been investigated. The desorption has occurred to be a nonlinear photoprocess. It required from 5 to 7 UV photons, which were not absorbed simultaneously. A desorption mechanism based on excitation of adsorption bond in course of several cycles of one-photon absorption followed by electron-vibrational relaxation has been proposed. Then the one-photon absorption spectra of these adsorbed molecules should be spread to much longer wavelengths compared with that of the gaseous species. A one-photon dissociation has taken place only for halogenomethane molecules that could be photolyzed by such excimer lasers in gaseous phase. A multiple photon dissociation of some molecules has been observed as a result of direct multiphoton absorption, successive fragmentation or photolysis of vibrationally photoexcited species.
Laser studies of chiral molecules
Anna Giardini Guidoni, Susanna Piccirillo, D. Scuderi, et al.
Laser ablated chiral molecules and their clusters are studied by mass resolved R2PI technique. Attractive (electrostatic and polarization) and repulsive (steric) interaction influence both the shift and the binding energy of the complexes. In monofuctional systems, the homochiral complexes are found to be more stable than the heterochiral ones. The formation of stable L-Tyrosine-Aluminum (Al-Tyr) cluster in a supersonic beam expansion of a laser ablated Al-Tyr target has been observed.
Transfer of electronic excitation energy and secondary processes in the media used for optical limiting
M. V. Gryaznova, O. V. Chistyakova, Vladimir V. Danilov, et al.
Influence of electron excitation energy transfer and secondary effects, occurred under action of laser radiation, on dynamics and efficiency of optical limiting is discussed. Two different systems, operating on effects of two-photon absorption and reverse saturable absorption, are considered as an example.
Laser beam propagation in opaque liquids
Sergei I. Dolgaev, A. V. Simakin, George A. Shafeev
We experimentally studied the transmission of the Er3+:YAG laser radiation (λ = 2.94 µm) by strongly absorbing liquids in the mode of free running. The radiation was shown to pass through liquids due to the development of a channel, produced by the laser pulse itself. The transmittances of water, ethanol, and glycerin were studied depending on the laser pulse energy and liquid layer thickness. It was also shown that in the case of oblique incidence of the laser beam, the channel propagates in liquid, slightly deflecting from the beam axis, which is caused by nearly equal refractive indices of air and vapor in the channel. It was found that the beam-liquid interaction is accompanied by a liquid jet directed outwards from its free surface.
Laser-induced phase transformations in GaAs
Sergei P. Zhvavyi, Gennadii D. Ivlev, Elena I. Gatskevich, et al.
Solid-liquid phase transitions induced in monocrystalline GaAs by nanopulsed radiation of a ruby laser were studied by means of time-resolved reflectivity and pyrometric measurements. GaAs samples were irradiated in transparent liquid media (alcohol’s and toluene) or in air ambient. Dependencies of the peak temperature and melt duration on laser energy density have been determined. Experimental data obtained for a case of irradiation of GaAs samples in air are compared with the results of computer simulation of the laser-induced phase transitions.
Optical guiding using Gaussian and Bessel light beams
A. E. Carruthers, Svetlana A. Tatarkova, V. Garces-Chavez, et al.
Optical guiding of micron-sized particles is shown using both Gaussian and zeroth-order Bessel light beams. Axial and transverse forces for guiding in both beams are calculated. Experiments show that the Bessel beam allows for extended guiding distances compared to a Gaussian beam, at the expense of guiding velocity.
Effect of nonlinear scattering of radiation in air on material ablation by femtosecond laser pulses
Sergei M. Klimentov, Taras V. Kononenko, P. A. Pivovarov, et al.
This work investigates the role of ambient atmosphere in material ablation by ultra-short intense laser pulses. It is shown, that ablative action of femtosecond pulses reveals limitations imposed by nonlinear optical response of gases resulting in significant modification or the incident laser beam. This phenomenon called conical emission (CE) manifests itself as strong scattering or emission of radiation in the forward direction developed at focusing of intense pulses of Ti:Sa laser (π=110÷1500 fs) in air. Transformation of the nearly Gaussian spatial profile into a wide angle cone is followed by spectral conversion of the fundamental laser frequency into a broad spectrum with relatively shorter wavelengths extending up to the visible range. Thresholds, converted energy, spectra and profiles of scattered radiation were measured at variable laser pulse duration and the ambient pressure. It was found, that more than 70% of the incident pulse energy can be scattered at conventional focusing of the beam by a long focal length lens. Effect of CE on material ablation in air was investigated, and the data obtained allowed to explain paradox morphology of steel channels drilled by high power femtosecond pulses.
Modeling of formation of deep 2D channels in metal targets via laser irradiation
Vladimir I. Mazhukin, O. N. Koroleva, M. M. Chuiko
The method of dynamic adaptation is applied for the solution of a multifront Stephan problem in an arbitrary 2-D domain with explicit tracking of interphase fronts. Method of solution is based on the idea of dynamic adaptation of the computational grid by means of the transition to an arbitrary non-stationary coordinate system. The results of computational experiments of the modeling of the processes of metal treatment by intense energy flows are presented.
Continuous motion of interference patterns using the angular Doppler effect
We demonstrate the use of the angular Doppler effect to obtain continuous motion of interference patterns. A small frequency shift between two beams can create such a moving pattern. By rotating a half wave plate in one arm of an interferometer, frequency shifts in the optical domain from less than 1 Hertz to kHz are achieved. We apply moving interference patterns in an optical tweezers set-up to enable controlled and continuous motion of optically trapped particles and structures.
Self-organized 3D structures under laser evaporation of solids: formation and properties
A. V. Simakin, V. V. Voronov, George A. Shafeev
The review of recent results is presented on laser-assisted formation of self-organized 3D structures under laser irradiation solids (Si, Ge, Ti, W, Nb, Mo, stainless steel, etc.) by sufficiently long sequence of laser pulses of order of 104. At laser fluence close to the melting threshold this leads to appearance of an array of micro-cones on the solid surface that grow towards the laser beam axis. The spatial period of the micro-cones is determined by that of the capillary waves on the melt surface and is typically of 10-20 μm. The formation of 3D structures is interpreted as the non-linear stage of the instability of the melt on laser-exposed solid surface, where the feedback is due to the variation of surface reflectivity with the angle of incidence of laser radiation and concentration of incident radiation into the bottom of the sinusoidal relief. The formation of 3D structures is assigned to displacement of the melt under gradient of the surface tension (Marangoni effect). The reflectivity of the 3D array of micro-columns is studied in a wide range of wavelengths from 0.2 to 25 μm. The low-threshold field electron emission of 3D structures on Si is demonstrated.
Study on the interaction of Xe and XeCl-laser radiation with metals and ceramics
Andrei V. Fedenev, Sergey B. Alekseev, Igor M. Goncharenko, et al.
The interaction of Xe- (λ~1.73 µm) and XeCl- (0.308 μm) lasers radiation with surfaces of metal and TiN-ceramic coatings on glass and steel substrates has been studied. Correlation between parameters of surface erosion (area of crater and amount of evaporated material versus laser focus position and number of pulses) was investigated. Monitoring of laser induced erosion on smooth polished surfaces was performed using optical microscopy. The correlation has been revealed between characteristic zones of thin coatings damaged by irradiation and energy distribution over laser beam cross section allowing evaluation of defects and adhesion of coatings.
Study on the interaction of pulse-periodical CO2 and Xe atomic laser radiation with teflon and vinypros
The interaction of pulsed periodical CO2-(λ~10.6 µm), and Xe-(λ~1.73 μm) lasers radiation with surfaces of polymers has been studied. Correlation between parameters of surface erosion (area of crater and amount of evaporated material versus laser focus position and number of pulses) was investigated using teflon (polytetrafluoroethylene-PTFE) and vinypros (plasticized semi-transparent polyvinylchloride) sample. Monitoring of erosion track on surfaces was performed through optical microscopy. It has been shown that at pulsed periodical CO2-radiation interaction with teflon and vinypros the sputtering of polymers with formation of submicron-size particles occurs. Dependencies of particle sizes, form and sputtering velocity on laser pulse duration and target temperature have been obtained.
Self-channeling of femtosecond visible laser pulse with microjoule energy and micromodification in transparent target
E. A. Chutko, Vyacheslav M. Gordienko, B. A. Kirillov, et al.
We have observed self-guiding of a single femtosecond visible laser pulse in the bulk of fused silica. The filament length and diameter versus pulse energy have been measured. Velocity of ions flying out from the plasma channel was determined with the use of time-of-flight measurements. The laser-induced micromodification in the bulk of the fused silica has discrete structure and is close to the filament form.
Discrete velocities of slow soliton-like excitations in copper: comparison of two independent experimetal results
Eugene M. Kudriavtsev, Sergey D. Zotov
Single exciting pulse with a threshold about (10-100)kW/cm2 produces in solids (and even possible more generally in condense matter) the whole series from more than i=30 of solitary waves - soliton-like excitations as it was shown in our group since first findings in 1992. They are spread on a stuff with constant velocity Ui, the values by which one "are quantized" - decrease twice for each subsequent (i+1)-th component, and all - on nine orders, starting from a longitudinal speed of sound vl. With the purpose of confirmation and study of this fundamental property there were analyzed results for poly-crystal copper samples obtained by different methods and in two different labs. Many components of laser-induced solitary wave structure were registered in copper by IR-detector and thermocouple. Their velocities Ui(with i= 10-17, 29-31) less than longitudinal speed of sound vl are in agreement with expression (expression available in paper) The first part of results was measured in the P.N. Lebedev Physical Institute. We obtained another part of results from measurements of heat conduction, made in Prokhorov General Physics Institute. This work was sponsored as project 00-02-17249 by RFBR.
Collisional effects after selective laser excitation of polyatomic molecules
G. A. Zalesskaya, D. L. Yakovlev, E. G. Sambor
Application of multistep schemes for excitation of polyatomic molecules makes it possible to improve bond-selected excitation with light. Fast collisional energy transfer is one of the most important processes that hinder bond-selected reactions. Collisional effects after multistep laser excitation of molecules is an active area of research at present time because of both the little studied characteristics of relaxation processes for polyatomic molecules in vibrational quasi-continuum and possible practical applications. In this report, the intensities and decay rates of the time-resolved delayed fluorescence (DF) activated by several ways of multistep laser excitation of complex organic molecules (acetophenone, benzophenone, anthraquinone, fluorenone) were used to study collisional processes after nonequilbirium vibrational excitation of triplet molecules mixed with bath gases N2, CO2, NH3, H2O, C2H2, CCl4, C6H6, C5H12, many of which participate in important chemical and photochemical organic molecules transformations that occur in nature. The quantitative characteristics of collisional processes in vibrational quasicontinuum were obtained. Analysis was made of rate constant dependences for near-resonant vibration-vibration (V-V) and vibration-translation (V-T) energy transfer processes on such factors as: properties of excited molecules and bath gases; vibrational energy of excited molecules; temperature, etc. Conclusions were made that collisional efficiencies of V-V process in mixture with polyatomic bath gases were governed by long-range attractive interactions. Upper levels, initially populated following laser excitation relaxed to vibrational distribution after several collisions. Majority of the collision took place only in V-T transfer of relatively small energies. The regularities of this process reflected the dominant role of short-range repulsive forces.
Excited singlet and triplet states and of the new photoionization electron-vibrational mechanism in the laser-active molecules
Alexandr Evganyevic Obukhov
Physical principles of the simulating of structure, spectral-luminescence, and lasing properties of complex N, O, S heteroaromatic molecules by means of the quantum-chemical one-electron approximation models LCAO-MO SCF CI CNDO/S and INDO/S (complete and intermediate neglect of differential overlap, sp-valence basis), PPP/S (Pariser-Parr-Pople σπ*-approximation) approaches are considered. The principles of control of the molecular structure; the properties of the excited singlet (Si*) and triplet (Ti) states; and the parameters of the S1*S0,S0Sn*,S1*Sn*,T1Tn and T1S0 transitions aimed at the selection of organic luminophores and UV-laser-active molecules with preset properties for specific scientific and technical applications are demonstrated. In LCAO-MO CSF CI methods, the wave function of the quantum systems contains the information on each atom and spectral parameters of atoms. The ionization potential and the electron affinity are taken from the gas-phase experiments. A complex multiatomic molecule can be represented as a system of excited electronic states with different spin and orbital structures where the energy of a quantum evolves under the action of intramolecular mechanisms with determine photophysical and photochemical properties of organic molecules.
Laser Microprocessing of Materials
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Marking of materials by CO2 laser beam scanning
Constantin Blanaru, Ramona C. Cernat, L. Chitu, et al.
Using a CO2 laser beam steered by a X-Y galvanometric scanner, alphanumeric characters, geometric figures or complex pictograms were marked on different target materials, with a special emphasis given to leather samples. Laser and scanner operating parameters are evaluated regarding the laser interaction with biological tissues. The developed set-up and associated software structure for generating and marking images in different scanning modes are presented. The experimental results present samples of leather of various qualities marked by a low-power CO2 laser beam, swept with marking speeds in the range of (100-300)Bit/ms.
Laser technology of SNOM-tips fabrication: process diagnostics, processing, and testing
Vadim P. Veiko, Alexey N. Kalachev, Lev N. Kaporsky, et al.
Basic principles of laser assisted process of fiber etching for scanning near-field optical (SNO) probes formation and control technique are presented. The thermal and temporal regimes are considered in order to provide stable reproducibility and high quality of a tapered end of the optical fiber. Problems of adequate definition of the scanning imaging properties of a SNO probe are discussed. Thus an optical method of far-field registration and processing together with a new autoelectric emission method are considered for solution of the task of a subwavelength SNO probe aperture measurement and estimation of its apparatus function.
Photochemical laser technology for integrated optical components of polymer basis
Carsten Wochnowski, Simeon Metev
In this paper a technology is presented for photochemical modification of polymers by UV laser irradiation. By this UV photon-induced modification process the optical properties of the irradiated polymer surface can be changed in a controllable way. For the UV laser treatment with different UV wavelengths (193 nm and 248 nm) PMMA was used as an UV-modifiable model polymer with good optical properties. The exact modification mechanism has been clarified by various spectroscopic methods like QMS, FTIR and XPS. The local modification of the refractive index by UV laser irradiation permits the fabrication of a wide range of integrated-optical components like strip waveguides, power splitters and Bragg-gratings. These basic integrated-optical elements are crucial for the realization of Dispersive components like WDM, which have a wide application in the optical sensor and information technology. Some of the optical and functional properties like loss rate or mode propagation of some as prepared integrated-optical components have been investigated and are discussed in this paper.
Fabrication and operation of Er-Yb glass waveguide laser arrays at 1.5 um
Gin Jose, Gino Sorbello, Stefano Taccheo, et al.
Waveguide laser arrays are fabricated on Er:Yb-doped phosphate glasses by a two-step ion exchange technique. The channel fabrication based on Ag-Na thermal diffusion followed by field assisted burial step is described. Single mode as well as multimode behavior of the laser is studied at four wavelengths representative of the telecom C-band between 1530-1565 nm. Each laser cavity is made by two fiber Bragg gratings butt-coupled to the waveguide. Fiber-coupled single-mode output powers > 0.8 mW and slope efficiency > 2% are obtained for all wavelengths.
Laser distillation synthesis of crystalline MoO3
V. M. Marchenko, V. V. Koltashev, S. V. Lavrishchev, et al.
Continuous laser distillation synthesis of MoO3, was realized by irradiation of molybdenum foil in atmosphere by cw Co2-laser. Monocrystalline crystallites of up to 500x200x5 μm3 were evidenced by x-ray microanalysis, scanning electron microscopy and Raman spectroscopy.
Intracavity laser processing of reflecting surfaces
Vladimir P. Osipov, Vasily V. Valyavko, Andreas Feld, et al.
By observing the TEA CO2 laser generation dynamics during the intracavity processing of reflecting surfaces, physical mechanisms responsible for the fabrication of periodic sub-micrometer structures are studied.
Laser surface microstructuring to improve tribological systems
Valerio Romano, Heinz P. Weber, Gabriel Dumitru, et al.
The controlled laser patterning of solid surfaces improves their wear properties: laser generated microcraters of defined dimensions and morphology can act as lubricant reservoirs and as traps for wear particles. For generating such microstructures different techniques are possible; however laser ablation has the advantage of a great versatility, since it can be adapted to produce a wide range of structures crater arrangements. We have generated microstructures with ps- and ns- pulses on TiN coated steel, uncoated steel and on uncoated steel with was subsequently coated with TiCN. Laser patterning of metals with pulses in the 100ns range is very effective however the process is governed by local melting and vaporization. The melt ejection yields the formation of resolidified droplets and rims on the target surface. This undesired artifacts can be removed by gentle polishing. Furthermore, a recast layer of increased hardness and brittleness of about 1µm thickness which homogeneously covers the crater walls is observed. Tribological tests were performed using the ball-on-disk method. The lifetime of the structured samples, defined as the sliding distance after which the friction coefficient showed an abrupt increase, was found to be significantly enhanced for all structured surfaces. The enhancement ranged from a 30% lifetime increase when TiN coatings were directly structured to an increase of over 10 times for structured hard metal which was subsequently coated with TiCN. To evaluate the possibility of circumventing some of the drawbacks of laser ablation in the ns regime, mainly the melt rims on the surface and the recast layer film on the crater walls of the single microholes the quality of microstructures produced in various metals with pulses of femtosecond duration was studied. To that end the evolution of the ablated depth over a large number of incident femtosecond laser pulses and the occurrence of structure and hardness changes in the immediate vicinity of the laser induced craters was analyzed for two sorts of steel and for hard metal. The analysis evidenced changes in the crystalline structure of the target materials in the fluence regime above 2J/cm2 but only minor alterations up to this fluence. Hardness measurements were performed on the cross-section surfaces in points situated in the immediate vicinity of the laser induced pores. Affected zones in the material surrounding few pores induced in the fs- regime were found and in such zones a significant hardness increasing was evidenced.
Nanoparticles produced by laser ablation of solids in liquid environment
A. V. Simakin, V. V. Voronov, N. A. Kirichenko, et al.
The review of results on nanoparticles formation is presented under laser ablation of Ti, Ag, Au, CdS, and ZnSe solids targets in liquid environments (H2O, C2H5OH, C2H4Cl2, etc.). The use of a high-repetition-rate Cu vapor laser (wavelengths of 510.5 nm, 20 ns pulse duration, repetition rate of 15 kHz) allows high rate of nanoparticles formation as the suspension in the liquid. X-ray diffractometry (XRD), UV-Vis optical transmission spectrometry, and High Resolution Transmission Electron Microscopy (HRTEM) characterize the nanoparticles. The size of nanoparticles is studied as the function of both laser fluence and nature of the liquid. A theoretical model is derived for evolution of the distribution function of particles size under laser ablation. The influence of laser parameters as well as the nature on the liquid on the properties of nanoparticles is elucidated.
Surface alloying of metals by nanosecond laser pulses under transparent overlays
Igor Smurov, A. Smirnov, N. Martsenyuk, et al.
A thorough experimental study was made of the processes determining formation of metastable surface alloys in a film-substrate system (Au-Ni, Sn-Cr) irradiated by laser pulses of nanosecond duration. The irradiation was performed in conventional conditions (on air) and, following the new method, through transparent overlay. A physical model of laser induced processes has been developed. The model predicts an increases of the depth of the melt, elevation of the liquid-phase temperature, rise of the diffusion speed of atoms in the liquid phase, and decrease of the cooling rate of the alloy during irradiation through the transparent overlay.
Advanced laser processing of glass materials
Three kinds of advanced technologies using lasers for glass microprocessing are reviewed. Simultaneous irradiation of vacuum ultraviolet (VUV) laser beam, which possesses extremely small laser fluence, with ultraviolet (UV) laser achieves enhanced high surface and edge quality ablation in fused silica and other hard materials with little debris deposition as well as high-speed and high-efficiency refractive index modification of fused silica (VUV-UV multiwavelength excitation processing). Metal plasma generated by the laser beam effectively assists high-quality ablation of transparent materials, resulting in surface microstructuring, high-speed holes drilling, crack-free marking, color marking, painting and metal interconnection for the various kinds of glass materials (laser-induced plasma-assisted ablation (LIPAA)). In the meanwhile, a nature of multiphoton absorption of femtosecond laser by transparent materials realizes fabrication of true three-dimensional microstructures embedded in photosensitive glass.
Laser-assisted direct manufacturing of functionally graded 3D objects
A. Iakovlev, E. Trunova, Dominique Grevey, et al.
Coaxial powder injection into a laser beam was applied for the laser-assisted direct manufacturing of 3D functionally graded (FG) objects. The powders of Stainless Steel 316L and Stellite grade 12 were applied. The following laser sources were used: (1) quasi-cw CO2 Rofin Sinar laser with 120 μm focal spot diameter and (2) pulsed-periodic Nd:YAG (HAAS HL 304P) with 200 μm focal spot diameter. The objects were fabricated layer-by-layer in the form of "walls", having the thickness of about 200 μm for CO2 laser and 300 μm for Nd:YAG laser. SEM analysis was applied for the FG objects fabricated by CO2 laser, yielding wall elements distribution in vertical direction. It was found that microhardness distribution is fully correlated with the components distribution. The compositional gradient can be smooth or sharp. Periodic multi-layered structures can be obtained as well. Minimal thickness of a layer with the fixed composition (for cw CO2 laser) is about 50 μm. Minimal thickness of a graded material zone, i.e. zone with composition variation from pure stainless steel to pure stellite is about 30 μm.
Laser-induced transient gratings application for measurement of thermal conductivity of CVD diamond
E. V. Ivakin, A. V. Sukhadolau, Victor G. Ralchenko, et al.
In-plane DII (kII) and normal D⊥ (k⊥) thermal diffusivity (thermal conductivity) of CVD diamond plates have been measured by method of transient gratings and laser flash technique. Samples of insulating and semiconducting boron-doped polycrystalline diamond were synthesized in microwave plasma maintained in methane-hydrogen mixtures. Averaged over the film thickness kII values at room temperature are determined to be in the range of 15-20 W/cmK approaching to those known for single crystal diamonds. From comparison of kII and k⊥ values a thermal anisotropy of 10-20% for in-plane and normal directions (k⊥ > kII) is found, this being ascribed to specific columnar growth of diamond grains. A strong variation (up to 2.7 times) in thermal conductivity across film thickness is revealed for selected specimens using a quasi-surface thermal grating excitation in VUV spectral range. In this case an additional fast-decay diffraction is observed due to nonthermal, owing to generated free charge carriers, recording of the grating.
Laser-induced structure transformations of diamonds
V. V. Kononenko, T. V. Kononenko, S. M. Pimenov, et al.
Results are reported on the study of phase transformations in diamonds induced by nano- and picosecond pulses of KrF excimer laser (λ=248 nm) and second harmonic of a YAP:Nd laser (λ=539 nm). Main attention in the research was paid to i) laser-induced graphitization of high-quality CVD diamond plates and ii) laser-induced structure transitions in ion-implanted diamond single crystals. For CVD diamond, the thickness of the laser-graphitized surface layers was measured and the accumulation period for graphitization to occur was found to be longer for lower laser fluences. In the experiments with ion-implanted diamonds, multipulse laser irradiation at fluences lower than the graphitization thresholds resulted in progressive annealing, i.e., in an increase of the optical transmission and surface contraction. Under certain low-intensity irradiation conditions, it was also found that, competing with the annealing process, laser etching of the ion-implanted diamond occurred at extremely low rates of 10-4-10-3 nm/pulse. A correlation between the defect concentration distribution and graphitization thresholds in partially annealed ion-implanted diamonds is discussed.
Precise drilling of steel with ultrashort pulsed solid state lasers
The accuracy of laser drilled holes in metals is limited by a relatively large amount of molten material which is produced when lasers with pulse durations in the range of nanoseconds or longer are used. In general, shortening the pulse duration down to the picosecond or femtosecond regime promises to overcome these problems. In this contribution different influences on hole quality such as energy density, beam quality, and polarization as well as processing strategies for high precision drilling of steel with ultra-short pulses are presented and discussed. A new method of polarization control is demonstrated by which the hole geometry can significantly be improved and ripples in the surface of the hole walls can be avoided during helical drilling. Furthermore, results of investigations on the influence of the ambient pressure will be presented.
Pulsed Laser Deposition of Thin Films
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Laser-induced heterogeneous processes at deposition of elements from vapors of transition-metal carbonyles
Sergay A. Mulenko
Two carbonyls of transition-metal Mo(CO)6 and Fe(CO)5 were used for laser chemical vapor deposition (LCVD) of elements under the action of KrF-laser radiation (λL= 248 nm) and Ar+-laser radiation (λL= 448 nm) on SiO2 and Si substrate surface. The rate constant of heterogeneous reaction of Mo atoms deposition was K(T)≅1.1x106 cm/s at Mo(CO)6 vapor pressure of 0.1 Torr and KrF-laser power density of 1.9x107 W/cm2 while irradiating SiO2 substrate surface. The same rate constant of Fe atoms deposition was K(T)≅3.4x106 cm/s at Fe(CO)5 vapor pressure of 5 Torr and KrF-laser power density of 3.0x106 W/cm2 while irradiating SiO2 substrate surface. This rate constant was K(T)≅1.2x10-3cm/s and 2.8x10-3cm/s at Fe(CO)5 vapor pressure of 5 Torr and Ar+-laser power density of 102 W/cm2 while irradiating Si and SiO2 substrate surface accordingly. Kinetic data of atomic concentrations demonstrated that during of the initial stage of element deposition the content of deposited films reflects mainly chemical processes on the substrate surface and as irradiation time is being increased, the composition of films reflects mainly gas phase processes above the substrate surface.
Multilayered films deposition from laser plume in a high-intensity electrostatic field
V. Yu. Fominski, V. N. Nevolin, A. L. Smirnov, et al.
Pulsed laser deposition in a uniform high-intensity electrostatic field was used to grow multilayered 56Fe/MoSx/57Fe films. The ions of laser-generated plume were significantly accelerated by counter flat fine-cell grid electrode, which was positively charged regarding the substrate. The difference of potentials was 50 kV. This method was recognized to be promising for the efficient intermixing processes in the MoSx/57Fe interface thus initiating the new chemical bond formation in the interfacial layers. By penetrating through the interface the ions gave rise to growth of amorphous layer consisting of rather evenly distributed Fe, S and Mo atoms. The characteristic property of the ion-induced chemical reaction was that the sulfur atoms have efficiently made bonds with iron (FeS2 type) while the molybdenum atoms transformed into the metallic state and were localized in the environment of Fe atoms. The influence of thermo-chemical properties of Fe-Mo-S system on the characteristic features of the ion-induced processes was also studied.
Pulsed laser deposition of MoSex(Ni)/a-C tribological coatings
W. Scharff, V. Yu. Fominski, R. Romanov, et al.
The present work was undertaken to study tribo-induced chemical and structural changes occurred in the MoSex(Ni)/a-C antifrictional coatings formed by pulsed laser deposition under varied conditions. The influence of these parameters on the wear resistance of the coatings was investigated.
Formation and decomposition of laser-deposited metastable Fe-Cr phases
Andre A. Gorbunov, Alexander A. Levin, Egbert Wieser, et al.
Contrary to expectations to obtain a continuous series of supersaturated b.c.c. Fe-Cr solid solutions by co-deposition of both metals in cross-beam PLD, there form unusual for this materials system metastable intermetallic phases. The structure of alloys develops from a tetragonally distorted b.c.c. at low Cr-content through a face centered orthorhombic (f.c.o.) to partly ordered primitive orthorhombic (p.o.) crystal structure of the A15-like type at nearly equiatomic composition. It seems to be the first observation of an ordering of restrictedly soluble components under the bombardment of hyperthermal species in PLD. Intriguing fact is that these metastable phases are initially paramagnetic (PM) at room temperature but become ferromagnetic (FM) after annealing in a temperature range 400-500 °C or under ion bombardment. The induced PM-FM transformation in the alloys is demonstrated to be useful for formation of stable lateral magnetic microstructures.
Pulsed laser deposition of ZnO thin films in silicon and sapphire
Alexander I. Khudobenko, Alexander N. Zherikhin, R. T. Williams, et al.
We investigated pulsed laser deposition (PLD) of ZnO films on silicon and sapphire substrates. Photoluminescence (PL), electrical properties and crystal structure of films were investigated. Stimulated emission in region 400 nm was observed.
Pulsed laser deposition of Pr3+- doped chalcogenide thin films for optical applications
A. P. Caricato, Gilberto Leggieri, Maurizio Martino, et al.
Thin films of Praseodymium-doped chalcogenide glasses [GeS2-Ga2S3-CsI] were prepared by pulsed laser deposition (PLD) technique. The targets were ablated using XeCl (308 nm) and KrF (248 nm) excimer lasers. The films were deposited on microscope glass slides, SiO2 plates and lithium niobate (LiNbO3) substrates at room temperature and at 300 °C. Morphological, compositional and structural characteristics of deposited films were investigated by different techniques. (Rutherford backscattering spectrometry, scanning electron microscopy and x-ray diffraction). Optical transmission of films and target, at normal incidence, were recorded in the 200-3500 nm spectral region. The optical constants (refractive index n and extinction coefficient k) vs wavelength, as well as the film thickness, were calculated from these spectra with the aid of a computer code. The presence of praseodymium in the doped chalcogenide thin film was analysed by exciting the electrons to the 1G4 level and collecting the photoluminescence spectrum in the 1.335 µm region. The waveguiding properties of the deposited films were investigated by the prism coupling technique (m-lines spectroscopy).
Erosion plume characteristics determination in ablation of metallic copper, niobium, and tantalum targets
Oleg Alexeyevic Novodvorsky, O. D. Khramova, C. Wenzel, et al.
On irradiation of metallic targets from niobium and tantalum with excimer XeCl laser (308 nm) in vacuum the UV and visible spectra of erosion plume were obtained. The erosion plume after ablation of copper, niobium and tantalum targets was studied using Langmuir probe. The ion and electron probe currents were obtained in the range from 0.2 to 2.2 J/cm-2 of energy densities at the target with probe-to-target separations from 10 to 133 mm. Multimodal distributions of ions by scatter velocities have been revealed. The spatial and temporal dependencies of electron probe currents were obtained in real time. The electron temperature of different plume regions was determined from a series of I-U characteristics taken at different distances between probe and copper or tantalum target. It was established that the plume electron temperature is non-uniform and has a maximum value in front of the plume. The acceleration of tantalum ions on plume irradiation with cw CO2 laser recorded. The increase in ions energy from 25 to 50 eV was observed. The emission optical spectra were used in studying the plume qualitative composition. The velocities of atom and ion scatter were measured by the time-of-flight method from the emission optical spectra.
High-Power Laser Technologies
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Application of near infrared pyrometry for CW Nd:YAG laser welding of stainless steel and laser cladding of stellite
Ph. Bertrand, Igor Smurov, M. Ignatiev, et al.
A bi-dimensional monochromatic and a 1-spot multiwavelengths pyrometers were applied for surface temperature monitoring in Nd:YAG continuous laser welding. The experiments were performed for stainless steel and titanium alloy applying variation of laser parameters: power, welding velocity, focalisation distance, shielding gas flow rate, presence of surface pollution, with fixed or variable gap and misalignment between the dissimilar plates. Surface temperature evolution along and perpendicular to the welding seam and full temperature image versus laser action parameters were measured with the help of the bi-dimensional pyrometer. The dynamics of temperature gradients, transient periods and steady-state temperature distributions are analysed. The same approach is applied for Nd:YAG laser cladding of stellite on steel substrate. A new type of CCD based optical diagnostic tool has been applied for monitoring the particle in flight size and velocity.
Influence of the base preheating on cracking of the laser-cladded coatings
Rafal Jendrzejewski, Gerard Sliwinski, Ana Conde, et al.
Coatings of stellite SF6 are prepared on the chromium steel base by means of a direct laser cladding. A 1.2 kW CO2 laser is applied and the original material is delivered into the processing zone coaxially with the laser beam. The samples are produced with and without a controlled preheating of the substrate and are investigated by means of metallographic techniques. It is found that the observed micro-cracking susceptibility decreases markedly with increase of the base preheating temperature up to 750 K and the crack-free coatings are produced for preheating around 950 K. The substrate-coating interface reveals a metallic bond and the microstructure is characterized by a fine-grained, dendritic structure. The nearly constant concentration dependence of Fe, Co, Cr and Ni on the distance from interface indicates on homogeneous chemical composition of the produced coatings.
Double focus technique: influence of focal distance on the welding process
Andreas Russ, Wolfgang Gref, Markus Leimser, et al.
The main goal of welding aluminium with double focus technique is to improve the quality of the welding result, that means to reduce porosity and to avoid melt ejection. The idea behind this approach, namely utilizing two or more individual focal spots on the workpiece instead of just one, is to shape the keyhole geometry such that the tendency of a keyhole collapse is considerably reduced. With one spot or with a small distance between the two laser spots, the weld bead is irregular and a high number of process pores can be observed. With a larger focal distance the bead is becoming stable and the porosity is reduced dramatically. At the same time, however, the welding depth is decreasing compared to the situation with a small focal distance. The best quality was achieved at the focal distance 0.6 mm. In this case, pores which are larger in diameter than 0.6 mm were avoided completely and the number of smaller pores is strongly reduced. After a further separation of the keyhole the number of pores increases again. Whereas the process quality can be improved by appropriate arrangement of the spots some reduction of the process efficiency might occur.
Interaction model for near infrared pulsed-laser sintering of metal powders
Pascal Fischer, Valerio Romano, Heinz P. Weber
The densification processes occurring in metallic powders upon interaction with pulsed laser radiation have been studied experimentally and compared with results obtained from a numerical simulation model. The analysis of the sintered samples shows consolidation features, which are in very good agreement with the model predictions. The limited amount of molten material due to pulsed interaction and the recoil pressure exerted on the powder by the plasma and the ablation plume allow to achieve a predetermined density (or porosity) from less than 60% to more than 90% of the bulk density, depending on the laser parameters and processing steps. A wider range of porosity could be achieved by mixture of the metallic powder with later removable spacers of the desired size.
Process stabilization by dual focus laser welding of aluminum alloys for car body
Kimihiro Shibata, Takakuni Iwase, Hiroki Sakamoto, et al.
Aluminum alloys were welded using dual focus beams formed with two Nd:YAG lasers with the aim of obtaining a stable welding process. The relationship between the configuration of the spot beams and the quality of the weld beads was investigated using X-ray and high-speed camera observations. The number of pores was clearly related to the ratio of the keyhole depth to the keyhole opening. A larger keyhole opening and/or a shallower keyhole depth resulted in a smaller number of pores caused by instability of the weld pool. Based on the investigation, a car body component was welded with a dual focus beam system. The results show that aluminum car body panels can be welded stably at high speed with little distortion under optimum conditions.
Laser welding of polymers using high-power diode lasers
Laser welding of polymers using high power diode lasers offers specific process advantages over conventional technologies, such as short process times while providing optically and qualitatively valuable weld seams, contactless yielding of the joining energy, absence of process induced vibrations, imposing minimal thermal stress and avoiding particle generation. Furthermore this method exhibits high integration capabilities and automatization potential. Moreover, because of the current favorable cost development within the high power diode laser market laser welding of polymers has become more and more an industrially accepted joining method. This novel technology permits both, reliable high quality joining of mechanically and electronically highly sensitive micro components and hermetic sealing of macro components. There are different welding strategies available, which are adaptable to the current application. Within the frame of this discourse scientific and also application oriented results concerning laser transmission welding of polymers using preferably diode lasers are presented. Besides the used laser systems the fundamental process strategies as well as decisive process parameters are illustrated. The importance of optical, thermal and mechanical properties is discussed. Applications at real technical components will be presented, demonstrating the industrial implementation capability and the advantages of a novel technology.
Rapid prototyping with lasers using metal powder jets
Gerhard Liedl, D. Schuoecker, E. Kny
Rapid Prototyping is an important part of modern development sequences, where a variety of solutions is available nowadays, ranging from ultrafast machining via casting technologies, laminated objects manufacturing, also with lasers, to stereolithography and laser sintering. Most of these processes suffer from certain restrictions, either in terms of raw material, geometry or quality of the finished workpiece. In particular, laser sintering leads to structures with a low density that reduces the strength of the part considerably. Since the individual powder particles are only molten at their surface and adhere thus to each other, only in small regions of the surface leaving the majority of the grain mass unmolten and resulting in a faint lattice that must be filled with an additional material to reach the necessary strength. A different solution offers the Blown Powder Process, where a jet of e.g. metal powder is directed towards the surface of the workpiece in the focus of a laser beam, thus melting each metal particle totally and producing practically molten droplets that settle on the momentary surface of the workpiece, thus being welded to the latter. Since all the material delivered to the workpiece is perfectly molten and resolidified, very dense structures with a high strength comparable to the strength of the initial unpowderized material are obtained, as experiments carried out by the authors with differed powderized materials have shown. It has also been demonstrated that the latter process can be used to generate nearly arbitrary 3D geometry. A lot of effort has been invested by the authors to avoid the influence of the direction of the necessary relative motion between laser, powder jet and workpiece and also to reduce the roughness of the surface generated by the actual process. Further experimental investigations will be devoted to the clarification of the range of materials, where the actual process can be applied and to the generation of practical parts followed by testing of their strength, fatigue properties and wear.
Laser Isotope Separation
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AVLIS of neodymium
I. S. Grigoriev, A. B. Diachkov, S. K. Kovalevich, et al.
A new step forward in AVLIS of neodymium was taken with a narrowband dye-laser on the first stage of a three step photoionization scheme. Enrichment of Nd-150 isotope achieved 67% in the separating installation.
Stabilized single-mode dye laser
I. S. Grigoriev, A. B. Diachkov, V. A. Kuznetzov, et al.
We have designed and tested a pulsed single-mode dye laser for use as the master-oscillator in a laser isotope separation system. The frequency stabilized single-mode laser with grazing-incidence diffraction grating is pumped by a 10 kHz copper vapor laser to produce up to 600 mW average power at over 30% efficiency in diffraction limited beam. A broad single-mode tuning range (without mode-hopping) of 7 nm was obtained. By using active stabilization of the cavity length, a frequency drift for long term operation within ±30 MHz and frequency width of about 100 MHz were obtained.