@article{GerhardLetienCressentetal.2020, author = {Gerhard, Christoph and Letien, Emilie and Cressent, Thomas and Hofmann, Mandy}, title = {Inline monitoring of hydrogenous plasma-induced defect formation within fused silica via plasma emission spectroscopy}, series = {Applied Physics A: Materials Science and Processing}, volume = {126}, journal = {Applied Physics A: Materials Science and Processing}, issn = {1432-0630}, doi = {10.1007/s00339-020-3347-5}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:526-opus4-13118}, year = {2020}, abstract = {In this paper, the indirect monitoring of plasma-induced defect formation within fused silica via plasma emission spectroscopy is presented. It is shown that low-pressure plasma treatment with hydrogen as process gas leads to a decrease in UV transmission of fused silica. This decrease can be directly attributed to oxygen vacancy-related defects and the presence of hydrogen within the silicon dioxide glass network. By the analysis of the plasma composition, it was observed that the amount of oxygen within the plasma increases with increasing treatment duration. Hence, oxygen was continuously released from glass network in the course of the plasma treatment. It was further observed that this release is strongly dependent on the applied plasma power where the lowest process efficiency occurs at the highest plasma power. It is shown that an increase in plasma power leads to a remarkable increase in light emission from the working gas, hydrogen. This observation indicates that the higher the degree of excitation and ionisation of the plasma, the lower the efficiency of plasma-induced formation of oxygen deficiency-related defects. This finding is of mentionable relevance for a better understanding of plasma-induced surface modification and coating processes.}, language = {en} } @article{BauerGutkeHeinrichetal.2020, author = {Bauer, Joachim and Gutke, Marko and Heinrich, Friedhelm and Edling, Matthias and Stoycheva, Vesela and Kaltenbach, Alexander and Burkhardt, Martin and Gruenefeld, Martin and Gamp, Matthias and Gerhard, Christoph and Steglich, Patrick and Steffen, Sebastian and Herzog, Michael and Dreyer, Christian and Schrader, Sigurd}, title = {Novel UV-transparent 2-component polyurethane resin for chip-on-board LED micro lenses}, series = {Optical Materials Express}, volume = {10}, journal = {Optical Materials Express}, number = {9}, issn = {2159-3930}, doi = {10.1364/OME.393844}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:526-opus4-13472}, pages = {2085 -- 2099}, year = {2020}, abstract = {In this work we present a novel optical polymer system based on polyurethane elastomer components, which combines excellent UV transparency with high thermal stability, good hardness, high surface tension and long pot life. The material looks very promising for encapsulation and microlensing applications for chip-on-board (CoB) light-emitting diodes (LED). The extinction coefficient k, refractive index n, and bandgap parameters were derived from transmission and reflection measurements in a wavelength range of 200-890 nm. Thermogravimetry and differential scanning calorimetry were used to provide glass transition and degradation temperatures. The surface tension was determined by means of contact angle measurements. As proof of concept, a commercial InGaN-CoB-LED is used to demonstrate the suitability of the new material for the production of microlenses.}, language = {en} } @article{GerhardDobis2021, author = {Gerhard, Christoph and Dobis, Alexander}, title = {Manufacturing-induced surface contaminations}, series = {Vakuum in Forschung und Praxis}, volume = {33}, journal = {Vakuum in Forschung und Praxis}, number = {1}, publisher = {Wiley}, doi = {https://doi.org/10.1002/vipr.202100752}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:526-opus4-15656}, pages = {33 -- 37}, year = {2021}, abstract = {Summary In the course of classical optics manufacturing glass components are in permanent direct contact with aqueous operating materials. Such contact leads to a certain absorption of water and hydrogenous compounds that may induce severe glass defects. In this contribution, absorption of hydrogen and other contaminants during grinding of glass was observed and qualitatively detected via laser-induced breakdown spectroscopy. It is shown that hydrogen, calcium, magnesium, and carbon are implanted into the glass surface where the contaminant concentration increases over grinding time or contact time of the glass surface with water and the grinding tool, respectively. The contaminants hydrogen, calcium, and magnesium can be attributed to the water used as lubricant. In contrast, carbon most likely originates from wear debris of the used silicon carbide grinding pads. Several possible mechanisms that lead to such surface contamination of glasses during grinding - diffusion, accumulation in micro cracks as well as the formation of hydrated silica - are finally introduced.}, language = {en} } @article{HermannGrojoAxenteetal.2017, author = {Hermann, J{\"o}rg and Grojo, David and Axente, Emanuel and Gerhard, Christoph and Burger, Miloš and Craciun, Valentin}, title = {Ideal radiation source for plasma spectroscopy generated by laser ablation}, series = {Physical Review E}, volume = {96}, journal = {Physical Review E}, number = {5}, publisher = {American Physical Society (APS)}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:526-opus4-16132}, year = {2017}, abstract = {Laboratory plasmas inherently exhibit temperature and density gradients leading to complex investigations. We show that plasmas generated by laser ablation can constitute a robust exception to this. Supported by emission features not observed with other sources, we achieve plasmas of various compositions which are both uniform and in local thermodynamic equilibrium. These properties characterize an ideal radiation source opening multiple perspectives in plasma spectroscopy. The finding also constitutes a breakthrough in the analytical field as fast analyses of complex materials become possible.}, language = {en} } @misc{GerhardLetienCressentetal.2019, author = {Gerhard, Christoph and Letien, Emilie and Cressent, Thomas and Hofmann, Mandy}, title = {Impact of the plasma power on plasma-induced increase in absorption of fused silica}, series = {Wissenschaftliche Beitr{\"a}ge 2019}, volume = {23}, journal = {Wissenschaftliche Beitr{\"a}ge 2019}, issn = {0949-8214}, doi = {10.15771/0949-8214_2019_5}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:526-opus4-10802}, pages = {33 -- 37}, year = {2019}, abstract = {In the last decades, laser machining of fused silica has gained in importance where the main challenge is to overcome the high transmission of this medium in order to achieve surface absorption. One approach is plasma treatment using hydrogenous process gases. In doing so, a near-surface glass layer is chemically modified by a removal of oxygen and an implantation of hydrogen, resulting in the formation of optically active defects and an increase in absorption. Against this background, the impact of the plasma power on plasma-induced increase in absorption of fused silica was investigated in the present work. Here, the highest efficiency of the plasma treatment process was found at the lowest applied plasma power whereas for higher plasma powers, a notably lower increase in absorption was observed. This effect is attributed to the decomposition of plasma species required for initiating the formation of optically active defects within the glass. Such decomposition increases with rising power due to the corresponding increase in electric field strength within the plasma. This finding is of substantial interest for an extensive understanding of plasma-glass interactions.}, language = {en} } @article{GerhardGimpelTascheetal.2018, author = {Gerhard, Christoph and Gimpel, Thomas and Tasche, Daniel and Koch, Jennifer and Br{\"u}ckner, Stephan and Flachenecker, G{\"u}nter and Wieneke, Stephan and Schade, Wolfgang and Vi{\"o}l, Wolfgang}, title = {Atmospheric pressure plasma-assisted femtosecond laser engraving of aluminium}, series = {Journal of Physics D: Applied Physics}, volume = {51}, journal = {Journal of Physics D: Applied Physics}, issn = {1361-6463}, doi = {10.1088/1361-6463/aab6e6}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:526-opus4-10521}, pages = {1 -- 7}, year = {2018}, abstract = {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.}, language = {en} } @article{GerhardStappenbeck2018, author = {Gerhard, Christoph and Stappenbeck, Marco}, title = {Impact of the Polishing Suspension Concentration on Laser Damage of Classically Manufactured and Plasma Post-Processed Zinc Crown Glass Surfaces}, series = {Applied Sciences}, volume = {8}, journal = {Applied Sciences}, number = {9}, publisher = {MDPI}, issn = {2076-3417}, doi = {10.3390/app8091556}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:526-opus4-10556}, pages = {1 -- 15}, year = {2018}, abstract = {The laser-induced damage threshold of optics is an issue of essential importance in high-power laser applications. However, the complex and partially interacting mechanisms as well as the underlying reasons for laser damage of glass surfaces are not yet fully understood. The aim of the present work is to contribute to a better understanding of such damage mechanisms by providing original results on the impact of classical glass surface machining on the laser-induced damage threshold. For this purpose, glass samples were prepared with well-defined process conditions in terms of the used lapping and polishing agents and suspensions. Further, the samples were post-processed by atmospheric pressure plasma for precision cleaning. The laser-induced damage threshold and surface contamination by residues from the manufacturing process were determined before and after plasma post-processing. It is shown that the polishing suspension concentration has a certain impact on the laser-induced damage threshold and surface contamination by residues from used working materials. The highest damage threshold of 15.2 J/cm² is found for the lowest surface contamination by carbon which occurs in the case of the highest polishing suspension concentration. After plasma treatment for merely 60 s, this value was increased to 20.3 J/cm² due to the removal of surface-adherent carbon. The results thus imply that the laser-induced damage threshold can notably be increased by first choosing appropriate process parameters during classical manufacturing and second plasma post-processing for surface finishing.}, language = {en} }