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We report on an atmospheric pressure plasma (APP) treatment of fused silica and its related surface and near-surface effects. Such treatment was performed in order to improve laser micro-structuring of fused silica by a plasma-induced modification of the glass boundary layer. In this context, an APP jet applying a hydrogenous process gas was used. By the plasma treatment, the transmission of the investigated glass samples was significantly decreased. Further, a decrease in the superficial index of refraction of approx. 3.66 % at a wavelength of 636.7 nm was detected ellipsometrically. By surface energy measurements, a decrease of the surface polarity of 30.23 % was identified. These determined modifications confirm a reduction of silicon dioxide to UV-absorbing silicon suboxide as already reported in previous work. Further, a change in reflexion by maximum 0.26 % was detected which is explained by the superposition of constructive and destructive interferences due to a surface wrinkling. With the aid of atomic force microscopy, an increase of the surface root mean squared roughness by a factor of approx. 19 was determined. It was found that both the surface energy and the strength of the fused silica surface were reduced by the plasma treatment. Even though such treatment led to a clustering of carbonaceous contaminants, a surface-cleaning effect was confirmed by secondary ion mass spectroscopy and energy-dispersive X-ray spectroscopy. The increase in UV-absorption allows enhanced laser ablation results as shown in previous work.

In this work, a common-path interferometer including integrated single-prism beam shaping is presented. Both theoretical considerations and experimental results on single-prism shaping of elliptical and circular laser beams were performed. Based on a single-prism beam shaper, a common-path interferometer without any classical beam splitter device was assembled and characterised. It is shown that the presented setup provides well-evaluable interference patterns with a Michelson contrast of 0.42–0.45.

In this contribution, we report on a novel hybrid laser-plasma method for material processing applications. This method is based on the combination of both an ArF excimer laser (lambda = 193 nm) and a low-temperature atmospheric pressure plasma jet source for the chemical reduction of glass surfaces. Here, a hydrogen-containing plasma gas was applied. Due to the layer of silicon suboxide that is generated in this vein, the absorption of the incoming machining laser beam is significantly increased after 15 minutes of plasma-treatment. Several machining experiments in terms of front-side ablation were performed on fused silica. Here, both pure and plasma-treated surfaces were ab-lated using single laser pulses with a pulse duration of 20 ns. By introducing the presented hybrid technique, the ablation threshold for micro-structuring was reduced significantly by a factor of 4.6 whereas the peak-to-valley height Rz of the machined area was decreased by a factor of 2.3. Further, back-side ablation using the presented method was considered. By a terminal tempering process, the initial transmission characteristics of fused silica can be reconstituted.

Kurzfassung Im Rahmen des Niedersächsischen Innovationsverbunds Plasmatechnik (NIP) werden neue Anwendungen der Plasmatechnologie in den Bereichen Fertigungstechnik und Materialanalyse bis hin zur Medizintechnik erschlossen sowie ihr ökonomischer und ökologischer Nutzen evaluiert. Im Bereich der Materialanalyse zählt dazu die Entwicklung eines spektroskopischen Verfahrens unter Nutzung eines ökoeffizienten Atmosphärendruckplasmas zur Emissionsanregung. Zum Einsatz kommt hierbei eine im Rahmen des Projekts entwickelte neuartige Plasmaquelle, die für die Analyse gasförmiger, flüssiger und fester Proben in unterschiedlichen Modi betrieben werden kann. Der Energieeintrag in das Probenmaterial durch diese Plasmaquelle beträgt hierbei wenige Mikrojoule. An verschiedenen Probenmaterialien wurde gezeigt, dass sich dieses Verfahren zur Erzeugung und Detektion elementspezifischer Spektrallinien eignet.

In this contribution, we report on the impact of direct dielectric barrier discharge argon plasma at atmospheric pressure on femtosecond laser engraving of aluminium. It is shown that the assisting plasma strongly affects the surface geometry and formation of spikes of both laser-engraved single lines and patterns of adjacent lines with an appropriate overlap. Further, it was observed that the overall ablation depth is significantly increased in case of large-scale patterning whereas no notable differences in ablation depth are found for single lines. Several possible mechanisms and underlying effects of this behaviour are suggested. The increase in ablation depth is supposed to be due to a plasma-induced removal of debris particles from the cutting point via charging and oxidation as supported by EDX analysis of the re-solidified debris. Furthermore, the impact of a higher degree of surface wrinkling as well as direct interactions of plasma species with the aluminium surface on the ablation process are discussed.

The interaction of a dense plasma with short laser pulses (ns) is investigated. The dense plasma is created using a low-current hollow-cathode-triggered discharge. This configuration generates a dense plasma with an electron density of ne � 10 17 cm � 3 . Spectra are taken in the extreme ultraviolet (EUV) spectral range from11-18 nm to estimate the temperature. The interaction between plasma and laser beam as a longitudinally pumped light source is discussed. The required properties of the plasma, according to the radial profiles of the temperature and the density, are exemplified. It is shown, that the generation of double pulses in the extrem ultraviolet spectral range is possible by reheating the residuals of an expanding dense plasma with a short laser pulse.

In this study the electron density of z-pinch plasmas driven at relatively low currents (ca. 2-5 kA) was determined using only emission spectroscopy. The suitability of a hollow-cathode-triggered z-pinch plasma as an absorption medium for laser radiation was investigated. The temporal and spatial behaviors of electron temperature and density profiles were estimated using magnetohydrodynamic (MHD) simulations to evaluate the experimental results. Temperature measurements were performed according to the Boltzmann plot method in the visible spectrum range, using the fact that, in low-current z-pinch plasma, a local thermodynamic equilibrium prevails for states at high principal quantum numbers (partial local thermodynamic equilibrium). In this case, the Saha equation can be used to determine the electron density. The results demonstrate that this method of determining the electron temperature and density of z-pinch plasmas is only applicable during the pinching phase. However, in this case the experimentally determined values are in fairly good agreement with the values determined using the MHD model. A user-oriented 1-D radiation MHD code was used to simulate the dynamic evolution of the plasma. The experimentally determined maximum electron temperature of approximately 12 eV is in fairly good agreement with the simulated value. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The longitudinal heating of a z-pinch plasma with short laser pulses is simulated using COMSOL Multiphysics. The heating of an existing pinch plasma for different electron densities with a short laser pulse is shown. It is illustrated that the initial electron density of the plasma has a significant impact on penetration of the pulse and subsequent heating of the plasma. With a Nd:YAG laser pulse the most effective heating occurs when the initial electron density is about 1/15 of the critical electron density of the plasma. It is shown that by increasing the initial electron density by even a factor of two above this level causes the absorption to increase dramatically, preventing penetration of the laser pulse and greatly limiting the heating of the plasma.

Extreme ultraviolet (EUV) spectroscopy has been carried out on a xenon plasma generated using a hollow-cathode-triggered Z-pinch discharge. The temporal behavior of the electron temperature was estimated from the analysis of spectral data. The electron temperature measurements have been performed by the Boltzmann plot method using Xe I and Xe II lines. Furthermore, an analytical collisional-radiative model was used to calculate the EUV spectra in the wavelength range from 13 to 16 nm for different electron temperatures. The measured relative line intensities from EUV spectra of Xe9+ and Xe10+ ions were compared to the results of the calculation. Fairly good agreement has been found between the experiments and the model calculations. These results indicate that a high effective charge number can be achieved by a low current and low-pressure pinch.

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.

A low-temperature atmospheric pressure plasma jet was used for chemical reduction of fused silica. For this purpose, a hydrogen-containing plasma was applied. A silicon suboxide layer was generated and hydrogen was implanted into the bulk material. Changes in stoichiometry, concentration of hydrogen and optical transmission were determined. An ArF excimer laser was used for the ablation of untreated and plasma-treated fused silica. The ablation threshold was significantly decreased by a factor of 4.6 in case of plasma treated substrates. Furthermore, in multi-pulse experiments, the ablation rate remained constant at least up to a depth of 10.5 μm.

In this chapter, the interaction mechanisms between short laser pulses and gases, respectively pre-ionized gases are described in detail. Ordinarily, gases are transparent for electro-magnetic radiation. This is not valid any more when the intensity of radiation goes beyond a certain value. This barrier depends on the pressure, the gas type, the pulse length, the pulse repetition frequency, the beam diameter and the laser wavelength used. In order to understand the interaction mechanisms, knowledge about the propagation of electromagnetic radiation as well as knowledge about the absorption processes (for example: inverse bremsstrahlung) is necessary. Further, the mathematic models for the absorption processes and the following effects (as, for example, heating) of short laser pulses in gases and pre-ionized gases are explained. The appearing effects such as breakdown intensity or heating play an important role in the generation of new radiation sources. With specific laser pulses, individual radiation sources can be created (such as UV, EUV, soft X-ray).

The combination of laser and low-temperature plasmas is of growing interest for micro-structuring purposes for a wide range of materials. This combination can be used for a reduction of the ablation threshold as well as an improvement of the machining quality in various laser material processing applications. The plasma involved in such combination, however, leads to a thermally generated influence on the laser beam quality. In this paper, a DBD-based argon plasma as typically used in low-temperature laser plasma hybrid arrangements was investigated by interferometric and beam profile measurements. The radial temperature profile as well as the beam propagation characteristics in terms of focal shift and effective pointing stability was determined. Due to the argon plasma, a temperature increase ΔT by up to 25 K, and thus a thermal lens was observed within the light path of the laser beam featuring an increase in refractive index Δn by maximum 2.86 × 10−5. In the given setup, the plasma-induced thermal lens caused a focal shift by up to 4 mm. Further, the lateral focus position was deviated by a maximum of about 30 µm, which is in the order of magnitude of the beam waist radius.

In this Letter, we report on the near-surface modification of fused silica by applying a hydrogenous atmospheric pressure plasma jet at ambient temperature. A significant decrease in UV-transmission due to this plasma treatment was observed. By the use of secondary ion mass spectroscopy, the composition of the plasma-modified glass surface was investigated. It was found that the plasma treatment led to a reduction of a 100 nm thick SiO2 layer to SiOx of gradual depth-dependent composition. For this plasma-induced layer, depth-resolved characteristic optical parameters, such as index of refraction and dispersion, were determined. Further, a significant plasma-induced increase of the concentration of hydrogen in the bulk material was measured. The decrease in transmission is explained by the plasma-induced near-surface formation of SiOx on the one hand and the diffusion of hydrogen into the bulk material on the other hand.

A low-current and low-compression z-pinch nitrogen plasma is heated up by means of a short laser pulse. Spectra are taken in the extreme ultraviolet spectral range from 11 to 18 nm to estimate the heating effect. Without additional laser heating the initial plasma conditions are ne≈1017 cm−3 and Te≈11 eV. The additional laser heating causes an increase in the electron temperature of about a factor of 5 to Te≈57 eV, whereas the electron density nearly remains at the initial value. The experimentally determined values are compared to the results obtained by means of simulations and are in fairly good agreement.