Spectroscopic imaging of laser-induced plasma

Spectroscopic imaging provides 2D images with full spectral resolution at each pixel. Thus, chemical imaging of an object, as well as other useful information, can be obtained. An imaging spectroscopy method in the visible range is presented and applied to laser plasma. This is a powerful research tool with numerous possible applications. This study is focused on spectroscopic imaging of laser-produced plasmas, and such spectral images (full spectrum at each pixel) are presented for the first time. Detailed information on optical and geometrical effects are obtained, and an insight to the optimization of the laser plasma spectroscopy method is achieved. The size and the spatial shape of the plasma, which can be used for matrix effect compensation, are measured. Similarity maps and classification maps of laser-induced plasma are obtained for the first time. These maps are used for allocation of chemical components in the plasma. The signal to noise ratio maps of the spectra obtained from laser-induced plasmas are provided. These surfaces possess a clear maximum, indicating that there is a preferred site in the plasma, where the emitted light provides the best signal to noise ratio. The performance of the current method is limited by the lack of temporal resolution, although it can be extended by a proper temporal gating.     

A New Reactive Atom Plasma Technology (RAPT) for Precision Machining: the Etching of ULE® Surfaces

Results on reactive atom plasma etching performed on ULE® (Corning Ultra Low Expansion) glass samples at atmospheric pressure are presented for the first time. A reactive atomic plasma technology (RAPT®), has been developed by RAPT Industries and employed for the finishing of optical surfaces. An atmospheric pressure argon inductively coupled plasma (ICP) excites a reactive gas injected through its centre. The plume of hot neutral excited species reacts at the substrate yielding controlled and repeatable trenches. In the case of ULE a material removal (up to 0.55 mm³/s) is obtained without pre‐heating the samples. Among the factors influencing the results, an increase in gas concentration at the same power does not change the sample temperature, indicating that thermo‐chemical effects do not influence the removal rates. Due to the plasma constructive constrains, increasing the gas concentration is more practical and of wider effect than increasing the power. The benefits of the process are illustrated and the extension of the technology to large optical surfaces discussed.     

Biomimetic Omnidirectional Antireflective Glass via Direct Ultrafast Laser Nanostructuring

Here, a single‐step, biomimetic approach for the realization of omnidirectional transparent antireflective glass is reported. In particular, it is shown that circularly polarized ultrashort laser pulses produce self‐organized nanopillar structures on fused silica (SiO₂). The laser‐induced nanostructures are selectively textured on the glass surface in order to mimic the spatial randomness, pillar‐like morphology, as well as the remarkable antireflection properties found on the wings of the glasswing butterfly, Greta oto, and various Cicada species. The artificial structures exhibit impressive antireflective properties, both in the visible and infrared frequency ranges, which are remarkably stable over time. Accordingly, the laser‐processed glass surfaces show reflectivity smaller than 1% for various angles of incidence in the visible spectrum for s–p linearly polarized configurations. However, in the near‐infrared spectrum, the laser‐textured glass shows higher transmittance compared to the pristine. It is envisaged that the current results will revolutionize the technology of antireflective transparent surfaces and impact numerous applications from glass displays to optoelectronic devices.     

Low stress abrasion of laser irradiated GFRP composites: an experimental and microstructural study

The present paper reports, the mechanism of material removal during low stress abrasive wear of high weight percent glass fibre reinforced polymer (GFRP) composites. Two different geometries of glass fibre reinforcement namely woven roving (WR) and chopped strand mat (CSM) were used. Unsaturated isophthalic polyester and bisphenol based epoxy resins were used as matrix for the reinforcement. Rubber Wheel Abrasion Tester (RWAT) was used for evaluating the abrasive wear behaviour of the composites. The composite samples were irradiated using a low power He-Ne laser for different time periods, having intensity of 5 mW. The abrasive wear performance of the composites has been determined as a function of applied load, sliding distance and laser irradiation time. The microstructural features of the abraded surfaces of both the laser irradiated and unirradiated composites have been observed by using a scanning electron microscope. Unsaturated polyester based glass fibre woven roving (WR) composite had a higher wear volume as compared to the epoxy based composite. The trend reversed in the case of chopped strand mat (CSM) composites, in which epoxy-based composite showed higher wear volume. The abrasive wear volume of all the composites decreased on irradiating it with laser. These results have been discussed, based on experimental wear data and observed microstructural features of the abraded surfaces.     

Kalte Normaldruck‐Jetplasmen zur lokalen Oberflächenbehandlung

Cold non‐thermal plasma jets for local surface treatment under normal pressure Plasmas at normal pressure are of considerable interest for surface technology because the industrial application requires no vacuum devices. Among other approaches, cold non‐thermal plasma jets represent an emerging technique to generate plasmas at normal pressure with attractive advantages. They allow ambient process temperatures and require only moderate operating voltages (1.5‐2.5 kV). They offer the advantage that the treated surfaces are not placed between the electrodes thus favoring local treatment of non flat, structured 3D surfaces. Moreover, the dimension of the sources is scalable and their integration into automated processes is simple. A capacitively coupled version (27.12 MHz) of a cold plasma jet suitable for surface treatment at atmospheric pressure is presented along with its plasma physical and technical properties and a series of successful applications, including plasma activation of surfaces for increasing printability, adhesion control, surface cleaning, microfluidics, decontamination, its use in plasmamedicine and for deposition of thin SiO₂ films as protective coatings. The device allows the operation with rare gases (e.g. Ar) and reactive gases as N₂, air or admixtures of silicon‐containing compounds.     

Nonlinear effects and nonthermal plasmas

In view of the unique approach to solve the problems of nonradioactive nuclear energy production by the nonthermal migma concept, some critical remarks to the established plasma theory are given. The success of the many particle simulation, of the kinetic theory, and of the hydrodynamic theory is enormous but mostly limited to homogeneous and collisionless plasmas. Especially at laser interaction, the neglection of collisions close to the poles of the functions involved leads to drastic differences. Nonlinear effects enforced an extension of the hydrodynamic theory and the inhomogeneity required the generalization to non-space-charge-neutral plasmas. Together with collisions, numerous properties of plasma were derived indicating new resonances and the nonthermalised state of plasma at laser irradiation. These examples were to demonstrate the need for developing more basic plasma theory and why migma requires a very unconventional approach.     

Überwachung der Plasmahomogenität

Monitoring plasma homogeneity A new imaging system has been built up for the fast qualitative monitoring of plasma processes. It is designed to analyze the spatial distribution of the plasma’s emission intensity at different wavelengths. The system consists of a liquid crystal tunable filter for wavelength selection and a CCD‐camera. Its applicability is presented for non‐polymerizing argon plasmas and polymerizing acetylene plasmas. Our investigations were carried out with focus on the effect of the process pressure on the homogeneity of the plasma emission and the resulting layer deposition. Both the emission’s uniformity as well as the homogeneity of the deposition are affected by the process pressure. Low process pressures are favorable for both of the named parameters.     

Laser plasma spectroscopy of Al-Cu-Fe quasicrystal an d alloy

Electron number density and temperature were determined from laser-induced plasmas produced by irradiating Al-Cu-Fe targets of a quasicrystal and of an alloy of similar composition. The Al(I) atomic emission spectra of the two systems were measured as a function of the distance from the target and of the time delay after laser irradiation. Differences of plasma characteristics were observed for laser ablation of quasicrystal and alloy targets, and the results were interpreted on the basis of different plasma formation mechanisms for the two systems.     

Picosecond-resolution soft-x-ray laser plasma interferometry

We describe a soft-x-ray laser interferometry technique that allows two-dimensional diagnosis of plasma electron density with picosecond time resolution. It consists of the combination of a robust high-throughput amplitude-division interferometer and a 14.7-nm transient-inversion soft-x-ray laser that produces approximately 5-ps pulses. Because of its picosecond resolution and short-wavelength scalability, this technique has the potential for extending the high inherent precision of soft-x-ray laser interferometry to the study of very dense plasmas of significant fundamental and practical interest, such as those investigated for inertial confinement fusion. Results of its use in the diagnostics of dense large-scale laser-created plasmas are presented.     

Mechanisms of pulsed laser microbeam release of SU-8 polymer "micropallets" for the collection and separation of adherent cells

The release of individual polymer micropallets from glass substrates using highly focused laser pulses has been demonstrated for the efficient separation, collection, and expansion of single, adherent cells from a heterogeneous cell population. Here, we use fast-frame photography to examine the mechanism and dynamics of micropallet release produced by pulsed laser microbeam irradiation at lambda = 532 nm using pulse durations ranging between 240 ps and 6 ns. The time-resolved images show the laser microbeam irradiation to result in plasma formation at the interface between the glass coverslip and the polymer micropallet. The plasma formation results in the emission of a shock wave and the ablation of material within the focal volume. Ablation products are generated at high pressure due to the confinement offered by the polymer adhesion to the glass substrate. The ablation products expand underneath the micropallet on a time scale of several hundred nanoseconds. This expansion disrupts the polymer-glass interface and accomplishes the release of the pallet from its glass substrate on the microsecond time scale (approximately 1.5 micros). Our experimental investigation demonstrates that the threshold energy for pallet release is constant (approximately 2 microJ) over a 25-fold range of pulse duration spanning the picosecond to nanosecond domain. Taken together, these results implicate that pallet release accomplished via pulsed laser microbeam irradiation is an energy-driven plasma-mediated ablation process.     

Wetting Properties of Steel Surfaces Modified by Laser Interference Metallurgy

The wetting properties of 100Cr6 bearing steel surfaces modified using laser interference metallurgy (LIMET) are analyzed. The steel surfaces are structured with line‐like patterns with line‐spacing. The topography of the ridged surface is analyzed by means of white light interferometry and scanning electron microscopy and surface chemistry of the different topographic regions by Raman spectroscopy. Contact angle (CA) measurements are performed on modified and non‐irradiated surfaces, using bi‐distilled water and FVA2 industrial oil. The angles are measured parallel and perpendicular to the line‐pattern orientation. The topographical analysis shows steep line‐pattern produced by laser. Raman analysis indicates that the laser irradiation does not significantly change the chemical species of the modified surfaces. The CA measurements elucidates that the parallel orientation provides a better wetting of the surface, because the laser line‐pattern acts as capillary flow channels, whereas the perpendicular orientation imposes energy barrier thus preventing wetting. As expected, the wetting coverage is more effective for larger than for smaller periodic structures, due to the larger area of flat contact. These novel results highlight the relevant use of LIMET to tailor the wetting properties of steel surfaces.     

The characteristic of the ZnO nanowire morphology grown by the hydrothermal method on various surface-treated seed layers

ZnO nanowires with various density and diameters were obtained by altering the wetting properties of aluminum doped ZnO (AZO) seed layers. AZO seed layers deposited on glass substrates were treated by inductor coupled plasmas (ICP) or atmospheric pressure plasma (APP) systems in order to control wettability before ZnO nanowire growth. Morphology of ZnO nanowires grown by the hydrothermal method was influenced by wettability of AZO seed layers. Our investigation shows that the diameter of ZnO nanowire increases and the length of ZnO nanowire decreases when the contact angle on the AZO seed layer is decreased. As a result, we confirmed that the morphology of ZnO nanowires could be easily controlled by surface treatment of seed layers with the plasma system.     

Low-temperature atmospheric pressure argon plasma treatment and hybrid laser-plasma ablation of barite crown and heavy flint glass

We report on atmospheric pressure argon plasma-based surface treatment and hybrid laser-plasma ablation of barite crown glass N-BaK4 and heavy flint glass SF5. By pure plasma treatment, a significant surface smoothing, as well as an increase in both the surface energy and the strength of the investigated glass surfaces, was achieved. It was shown that for both glasses, hybrid laser plasma ablation allows an increase in the ablation depth by a factor of 2.1 with respect to pure laser ablation. The ablated volume was increased by an averaged factor of 1.5 for N-BaK4 and 3.7 for SF5.     

Capillary‐Assisted Fabrication of Biconcave Polymeric Microlenses from Microfluidic Ternary Emulsion Droplets

In this study, a simple capillary‐based approach for producing biconcave polymeric microlenses with uniform size and shape from ternary emulsion droplets is presented. Monodisperse ternary emulsion droplets (0.6–4.0 nL) are produced which contain a photocurable segment of an acrylate monomer and two non‐curable segments of silicone oil (SO) by using a microfluidic sheath‐flowing droplet generator on a glass chip. The curvature radius of the interfaces separating the droplet segments, as well as the droplet size, and production rate can be flexibly varied by changing the flow conditions of the organic and aqueous phases. Subsequently, off‐chip suspension photopolymerization yields non‐spherical polymeric microparticles with two spherical concave surfaces templated by two SO segments at random positions. By ultraviolet light irradiation of ternary droplets with two SO segments trapped by the interior wall of a cylindrical microcapillary (internal diameter: 130 μm), biconcave microlenses can be produced with two spherical concave surfaces with a common lens axis. The produced lenses are suitable for use as optical diverging lenses.     

Atomic hydrogen emission induced by TEA CO(2) laser bombardment on solid samples at low pressure and its analytical application

Hydrogen emission has been studied in laser plasmas by focusing a TEA CO(2) laser (10.6 microm, 500 mJ, 200 ns) on various types of samples, such as glass, quartz, black plastic sheet, and oil on copper plate sub-target. It was found that H(alpha) emission with a narrow spectral width occurs with high efficiency when the laser plasma is produced in the low-pressure region. On the contrary, the conventional well-known laser-induced breakdown spectroscopy (LIBS), which is usually carried out at atmospheric air pressure, cannot be applied to the analysis of hydrogen as an impurity. By combining low-pressure laser-induced plasma spectroscopy with laser surface cleaning, a preliminary quantitative analysis was made on zircaloy pipe samples intentionally doped with hydrogen. As a result, a good linear relationship was obtained between H(alpha) emission intensity and its concentration.     

Laser-induced breakdown spectrometry of titanium dioxide antireflection coatings in photovoltaic cells

Silicon photovoltaic cells have design and material requirements different from those of most other silicon electronic devices. Not only are nearly ideal silicon surfaces required, but also the bulk properties must be of uniform high quality for high-energy conversion efficiency. In this paper, emission spectra of laser-generated plasmas from titanium dioxide antireflection coatings in solar cells are reported. A pulsed nitrogen laser at 337.1 nm was used with a pulse width of 10 ns and a laser fluence of 8.6 J/cm(2) onto the sample. The plasmas were detected using a charge-coupled device. Depth profilings from several samples with different thicknesses of titanium dioxide have been studied. A method for measuring thin TiO(2) films based on the observation of such profiles was developed. The effect of the laser fluence on the sensitivity of the present method has been examined. Depth resolutions are fluence-dependent but are on the order of 40 nm. The dependence between titanium dioxide plasma intensity and material reflectivity at the laser wavelength is discussed. Another valuable aspect of the technique is the ability to perform measurements in a contactless manner at room temperature and atmospheric pressure on wafer-sized samples.     

Laser effects based optimal laser parameter identifications for paint removal from metal substrate at 1064 nm: a multi-pulse model

Paint removal by laser ablation is favoured among cleaning techniques due to its high efficiency. How to predict the optimal laser parameters without producing damage to substrate still remains challenging for accurate paint stripping. On the basis of ablation morphologies and combining experiments with numerical modelling, the underlying mechanisms and the optimal conditions for paint removal by laser ablation are thoroughly investigated. Our studies suggest that laser paint removal is dominated by the laser vaporization effect, thermal stress effect and laser plasma effect, in which thermal stress effect is the most favoured while laser plasma effect should be avoided during removal operations. Based on the thermodynamic equations, we numerically evaluated the spatial distribution of the temperature as well as thermal stress in the paint and substrate under the irradiation of laser pulse at 1064 nm. The obtained curves of the paint thickness vs. threshold fluences can provide the reference standard of laser parameter selection in view of the paint layer with different thickness. A multi-pulse model is proposed and validated under a constant laser fluence to perfectly remove a thicker paint layer. The investigations and the methods proposed here might give hints to the efficient operations on the paint removal and lowering the risk of substrate damages.     

Morphological and animal study of titanium dental implant surface induced by blasting and high intensity pulsed Nd-glass laser

Machined dental implants of titanium were blasted with Al₂O₃ powder of 250 μm particle size. The surface was irradiated in vacuum with a Nd-glass pulsed laser at 1–3 J pulse energies. The morphology of these surfaces was investigated by optical and scanning electron microscopy. The low intensity laser treatment resulted in some new irregularities but we can observe the blasted elements and caves from the original blasted surface too. The blasted elements were washed out and a new surface morphology was induced by the high intensity laser treatment.The osseointegration was determined by measuring the removal torque in the rabbit experiments. The results were referred to the as machined surface. The blasting slightly increased the removal torque. The laser irradiation increased the removal torque significantly, more by a factor of 1.5 compared to the reference at high laser intensity. This shows the influence of the surface morphology on the osseointegration.The combination of the blasting with the laser irradiation is considered a method to determine the morphology optimal for the osseointegration because the pulsed laser irradiation caused modifications of the micrometer sized surface elements and decreases possible surface contamination.     

Ortselektive laserchemische Oberflächenfunktionalisierung von Polymeren im Mikrometerbereich. Selective laser‐chemical micro surface modification of polymers

Polymeric substrate materials like polystyrene (PS), polycarbonate (PC) or cyclic olefins (COC) are getting more attention besides silica, glass and ceramic for the preparation of reaction vessels, optical slides, microfluidic components or microtiterplates in applications like medical diagnostics and pharmaceutical drug screening. Actually, the market of transparent polymeric chips is demanding the availability of modified surfaces with well defined arrays of wettable areas or special chemical functionalities. The modified areas are starting point to graft bioactive molecules, for instance proteins or DNA‐oligomers. Another application is pretreatment of adhesive bonded joints. Presently surface modifications are performed chemically and physically by plasma surface interaction. IWS has developed a new technique for dry chemical structuring of polymeric surfaces based on the principle of excimer laser irradiation in reactive gas atmosphere. This technique is characterized by a high resolution and a negligible amount of chemicals. The procedure consists of only a few processing steps, in contrast to conventional lithographic structuring methods. Also wetting problems do not play a role as they do in printing techniques. The technology provides the possibility of tailoring the chemical and topographical surface properties from ultrahydrophobic to hydrophilic or to functionalize areas of choice in the μm range with a chemical group of defined density. Using an excimer‐laser to induce reactions, mask imaging can be applied for microstructuring the surface with new properties, e. g. for microarrays. In addition, the laser allows defined amount of energy into the elementary reaction, according to the wavelength applied, which opens the possibility of replacing atoms in the polymer molecules by other atoms or molecules taken from an agent in the environment, in a selective way. The precondition is that both the polymer and the agent absorb the same wavelength. For instance 193 nm radiation (ArF excimer laser) is absorbed by polyolefines and by ammonia allowing an exchange of H atoms for amino groups by which the surface is changed to starting point to graft bioactive molecules. Chemical microstructures for instance amino group arrays have been realized on a variety of polymeric materials like cyclic olefin foil. This array of monofunctionality is the starting point for the preparation of parallel microreactors. These samples of topographical and chemical microstructures are the first step for biochemical preparations in medical diagnostic kits, DNA‐, protein‐ or cell biochips. The upscaling of the laser modification process in a multi‐chamber reactor offers the semicontinuous functionalization of polymers in pilot scale or in batch processing. On this basis the surface modification step can be adapted into a mass production line of “Lab‐on‐a‐chip” systems.     

Argon plasma treatment of glass surfaces

The effect of argon plasma treatment of glass surfaces is studied by FTIR and SEM. The argon plasma on cleaned glass surfaces resulted in increased surface area due to microetching and surface rearrangement of the silicate network as indicated by the observed changes in the Si-O stretching infrared absorption. The result was a relative increase in surface hydrophilicity which could be optimized by the plasma reaction conditions. The etching action of the argon plasma on the substrate surfaces facilitated the removal of the micrometre thick sizing from the commercial fibres accompanied by little loss in tensile strength. Plasma was also used to graft selected monomers to the surface of glass fibres for enhancement of bond compatibility in a composite system. This grafting treatment was followed by an argon etching step. The argon plasma action on the coated surfaces improved the wettability further and increased the sur face area. Changes in surface chemistry that accompanied the argon etching treatment were very subtle in the case of the plasma polymer of allylamine, but proved significant in the case of the plasma polymer of hexamethyldisiloxane. On the latter surfaces, rearrangement of the siloxane (Si-O-Si) bonds to silylmethylene (Si-(CH₂) _n -Si) groups is suggested.