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Direct laser interference patterning (DLIP) is a well-established technology, which is often used for material surface structuring when both high-throughput and resolution are needed. In this technique, the periodical features of the surface structures are produced simultaneously within a single laser spot. This requires taking into consideration the interaction between the ablation processes of each feature, which includes heat transfer and the interaction between the molten material. In this study, we demonstrate how the molten material dynamics during the four-beam picosecond DLIP of the steel surface affect the topography of the fabricated structure. The detailed analysis of the structures produced with various laser fluences reveals three distinct mechanisms of structure formation, which lead to the structure consisting not only of periodically distributed craters but also of pillars, formed due to the molten material flow dynamics. The topography of the structures formed with different laser fluences and the number of laser pulses is investigated using confocal and scanning electron microscopy techniques.

Direct laser interference patterning (DLIP) has proven to be a fast and, at the same time, high-resolution process for the fabrication of large-area surface structures. In order to provide structures with adequate quality and defined morphology at the fastest possible fabrication speed, the processing parameters have to be carefully selected. In this work, an analytical model was developed and verified by experimental data, which allows calculating the morphological properties of periodic structures as a function of most relevant laser-processing parameters. The developed model permits to improve the process throughput by optimizing the laser spot diameter, as well as pulse energy, and repetition rate. The model was developed for the structures formed by a single scan of the beam in one direction. To validate the model, microstructures with a 5.5 &micro, m spatial period were fabricated on stainless steel by means of picosecond DLIP (10 ps), using a laser source operating at a 1064 nm wavelength. The results showed a difference of only 10% compared to the experimental results.

Metallic samples with unique micro- and nano-scale surface structures can easily be fabricated with Direct Laser Interference Patterning. Like in all laser processes, the material interacts with the laser radiation and as a result, thermal effects occur. These effects have a significant influence on the resulting quality of the surface patterns. In this study, the thermal effects occurring during Direct Laser Interference Patterning of stainless steel and aluminum sheets are investigated. The used experimental setup consisted of a picosecond pulsed laser source operating at 532 nm wavelength, combined with a two-beam interference optical head. An infrared camera in an off-axis position is used to detect the resulting thermal radiation of the laser process varying different process parameters such as laser power and repetition rate. The obtained results reveal a correlation between the recorded signal by the infrared camera and the reached surface quality. They show an impact of the thermal effects on the quality of the surfaces and the amount of solidified material on the resulting line-like pattern. Threshold values of the detected infrared signal detected are determined to classify the obtained surface conditions.

In this study, we report on the optimization of the direct laser interference patterning process by applying the design of experiments approach. The periodic line-like microstructures of a 8.50 &micro, m spatial period were fabricated by a two-beam interference setup with nanosecond laser pulses, varying laser fluence, pulse overlap, and hatch distance. Central composite design with three factors and five levels was implemented to optimize the required number of experiments. The experimental and numerical results show the impact of various structuring process parameters on surface uniformity. The responses measured are the structure height, height error, and waviness of the pattern. An analysis of the microstructures on the patterned surface was conducted by confocal microscopy and scanning electron microscopy. A 3D-characterization method based on morphological filtering, which allows a holistic view of the surface properties, was applied, and a new qualification scheme for surface microstructures was introduced. Empirical models were also developed and validated for establishing relationships between process parameters and performance criteria. Multi-objective optimization was performed to achieve a minimal value of structure height errors and waviness.

The current status of research around the world concurs on the powerful influence of micro- and nano-textured surfaces in terms of surface functionalization. In order to characterize the manufactured topographical morphology with regard to the surface quality or homogeneity, major efforts are still required. In this work, an optical approach for the indirect evaluation of the quality and morphology of surface structures manufactured with Direct Laser Interference Patterning (DLIP) is presented. For testing the designed optical configuration, line-like surface patterns are fabricated at a 1064 nm wavelength on stainless steel with a repetitive distance of 4.9 &micro, m, utilizing a two-beam DLIP configuration. Depending on the pulse to pulse overlap and hatch distance, different single and complex pattern geometries are produced, presenting non-homogenous and homogenous surface patterns. The developed optical system permitted the successfully classification of different pattern geometries, in particular, those showing single-scale morphology (high homogeneity). Additionally, the fabricated structures were measured using confocal microscopy method, and the obtained topographies were correlated with the recorded optical images.

In this work, it is demonstrated that direct laser interference patterning (DLIP) is a method capable of producing microtextured metallic molds for hot embossing processes. Three different metals (Cr, Ni, and Cu), relevant for the mold production used in nanoimprinting systems, are patterned by DLIP using a picosecond laser source emitting at a 532 nm wavelength. The results show that the quality and surface topography of the produced hole-like micropatterns are determined by the laser processing parameters, such as irradiated energy density and the number of pulses. Laser-induced periodic surface structures (LIPSS) are also observed on the treated surfaces, whose shapes, periodicities, and orientations are strongly dependent on the accumulated fluence. Finally, the three structured metals are used as embossing molds to imprint microlenses on polymethyl methacrylate (PMMA) foils using an electrohydraulic press. Topographical profiles demonstrate that the obtained structures are comparable to the masters showing a satisfactory reproduction of the texture. The polymeric microlens arrays that showed the best surface homogeneity and overall quality were those embossed with the Cr molds. Fil: Fu, Yangxi. Technische Universität Dresden; Alemania Fil: Soldera, Marcos Maximiliano. Technische Universität Dresden; Alemania. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte. Instituto de Investigación y Desarrollo en Ingeniería de Procesos, Biotecnología y Energías Alternativas. Universidad Nacional del Comahue. Instituto de Investigación y Desarrollo en Ingeniería de Procesos, Biotecnología y Energías Alternativas; Argentina Fil: Wang, Wei. Technische Universität Dresden; Alemania Fil: Voisiat, Bogdan. Technische Universität Dresden; Alemania Fil: Lasagni, Andrés Fabián. Technische Universität Dresden; Alemania. Institut für Werkstoff-und Strahltechnik IWS; Alemania

This work addresses the fabrication of periodic micro patterns on stainless steel by means of Direct Laser Interference Patterning. A modular optics assembly is introduced which produces two-beam interference patterns within a 5 × 100 mm rectangular shaped laser spot. The optical assembly considers optimal utilization of the available laser power in order to enable high processing rates. Using different optical elements, the spatial period of the periodic structures was set to 8 mm and 19 mm. The effect of the processing parameters on the morphology of the produced patterns is investigated by means of confocal microscopy, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. It is found that the pulse repetition rate shows a strong influence on the structure height accompanied by considerable heat accumulation effects, which is reported for the first time for this method. In addition, aspect ratios slightly higher than one could be realized, which is also reported for the first time when using Direct Laser Interference Patterning with ns-pulsed lasers. Particularly for the smaller periods, the melt flow impairs pattern formation due to the overmelting of the material (humping). Moreover, variations of the surface chemistry are observed, especially for higher accumulated laser fluences.

In this work, a diffractive fundamental beam-mode shaper (FBS) element is combined with a Direct Laser Interference Patterning (DLIP) optical setup to generate a square-shaped top-hat intensity distribution in the interference volume. The interference patterns produced by a symmetrical 4-beam DLIP setup with Gaussian and Flat-top intensity distributions are measured and characterized. The impact of misalignments in the optical setup on the resulting intensity profile is analyzed and supplemented by theoretical calculations of the distorted interference patterns. Finally, top-hat shaped interference patterns can be introduced and utilized in order to generate surface patterns with improved homogeneity which eventually will lead to an improved performance in applications with functionalized surfaces.