Michael Schmidt's research while affiliated with Friedrich-Alexander-University of Erlangen-Nürnberg and other places
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Publications (509)
Dynamic phase-only beam shaping with a liquid crystal spatial light modulator is a powerful technique for tailoring the beam's intensity profile or wave front. While shaping and controlling the light field is a highly researched topic, dynamic nonlinear beam shaping has hardly been explored so far. One potential reason is that generating the second...
Laser welding of copper hairpins is required to produce a conductive connection in electric stators. Past manufacturing processes introduce misalignments that lead to poor weld connections with increased electrical resistance. In this work, we discuss correlations between the electrical resistance of the weld connection and possible misalignment ty...
In laser beam fusion cutting of metals, the interaction of the gas jet with the melt determines the dynamics of the melt extrusion and the quality of the resulting cutting kerf. The gas-dynamic phenomena occurring during laser beam cutting are not fully known, especially regarding temporal fluctuations in the gas jet. The observation of gas and mel...
The high demand for electronic products increases the need for high-quality welds of copper. Laser welding can be applied but may result in undesired weld characteristics such as humping or spatter. Process control is needed to identify defective welds in the production line. Surface topographical features can be used to identify different weld cha...
In-process monitoring of weld penetration depth is possible with optical coherence tomography (OCT). The weld depth can be identified with OCT by statistical signal processing of the raw OCT signal and keyhole mapping. This approach is only applicable to stable welding processes and requires a time-consuming keyhole mapping to identify the optimal...
Optical coherence tomography (OCT) is an inline process monitoring technology for laser welding with various applications in the pre-, in-, and post-process. In-process monitoring with OCT focuses on the measurement of weld depth by the placement of a singular measurement beam into the keyhole. A laterally scanned measurement beam gives the opportu...
With the increasing demand for copper connections in the field of renewable energies, e.g., for electric vehicle applications, various approaches were pursued to reduce the challenging spatter and melt ejection susceptibility in laser beam welding of copper materials. One is the use of adjustable intensity profiles of multi-mode beam sources with a...
The topographical information of a weld seam bears information about quality relevant characteristics such as humping or spatter. Optical coherence tomography (OCT) can be used for inline scanning the weld topography coaxially mounted at a laser scanning optic. Feature extraction from this topographical information is challenging due to finding mat...
Light plays the central role in many applications, ranging from telecommunications to cutting metals. The key to unlocking its amazing versatility lies in shaping it into the most appropriate form for the task at hand. Computer generated volume holography is a powerful light shaping approach and lifts the concepts of traditional holography into the...
Biodegradable Polylactide Supraparticle Powders In article number 2205730, Nicolas Vogel and co‐workers show that composite supraparticles form powders of biodegradable polymers and nanoscale bioactive additives with tailored composition. These powders can be applied in the powder bed fusion process to fabricate functional biomaterials by additive...
![Determination of the ablation threshold according to Liu [23]. Each...](publication/363489319/figure/fig4/AS:11431281095378586@1667876010131/Determination-of-the-ablation-threshold-according-to-Liu-23-Each-measurement-was_Q320.jpg)
Phase-only beam shaping with a spatial light modulator (SLM) is a powerful tool in laser materials processing. It enables adapting the shaped profile to the ablation geometry and thereby tailoring the energy deposition. To mitigate speckle noise in tailored beam profiles, methods for uniform beam shaping were proposed. The two main approaches eithe...
Due to their good material properties (e.g., corrosion and wear resistance, biocompatibility), thermoplastic materials like polyamide 12 (PA12) are interesting for functional coatings on metallic components. To ensure a spatially resolved coating and to shorten the process chain, directed energy deposition of polymer powders by means of a laser bea...
Inline monitoring technologies gain increasing importance in industrial laser welding applications. Multiple sensing technologies can be implemented for classification of welding defects. Feature extraction from sensing data is a time-consuming process due to various algorithm possibilities for the identification of relevant features. Feature extra...
Laser remote fusion cutting (RFC) offers a method for laser fusion cutting of metal sheets without need for a cutting gas. It is closely related to laser keyhole welding, which can transition into RFC for suitable parameters, such that the pressures in the process zone lead to a melt ejection on the bottom of the metal sheet. The mechanisms leading...
The idea of laser surgery is nearly as old as the laser itself. From the first trials to modern laser surgery systems, it was and is the aim to selectively cut the tissue in the focus spot without causing harm to surrounding structures. This is only possible when the correct parameters for the surgical laser are chosen. Usually, this is done by par...
Powder bed fusion with laser beam of metals (PBF-LB/M) is a widely used technology to produce parts with a high freedom in design paired with excellent mechanical properties. The casting alloy AlSi10Mg features a wide process window and a microstructure with excellent mechanical properties which are not obtainable through conventional manufacturing...
The coaxial integration of optical coherence tomography (OCT) enables the determination of surface topography and measurement of
object features along the optical path within laser machining. The measurement of surface information from the processed workpiece
allows for the identification of features from the joint configuration and the subsequent...
Additive manufacturing, in particular powder bed‐based fabrication processes hold promise to revolutionize biomedical engineering for the ability to provide customized, functional implants, for example as bone replacement materials. However, providing functional powder particles that unify material requirements for biodegradable and bioactive bioma...
Background
Chronic kidney disease (CKD) is a global burden affecting both children and adults. Novel imaging modalities hold great promise to visualize and quantify structural, functional, and molecular organ damage. The aim of the study was to visualize and quantify murine renal vasculature using label-free raster scanning optoacoustic mesoscopy (...
With its excellent automation capability and localized energy input enabling precise, reproducible welds, laser beam welding represents a preferred industrial joining technology. Electro-mobility drastically increases the need for defect-free and automatable copper joining technologies. However, copper welds that are produced with state-of-the-art...
Fe–Co–B–Si–Nb bulk metallic glasses are prone to the formation of micro-cracks during laser powder bed fusion (LPBF) additive manufacturing. Introducing ductile Cu into Fe–Co–B–Si–Nb bulk metallic glass alloy system rich with Fe and Co could be a promising solution to reduce or even eliminate micro-cracks since there is almost no miscibility of Cu...
Multi- and hyperspectral endoscopy are possibilities to improve the endoscopic detection of neoplastic lesions in the colon and rectum during colonoscopy. However, most studies in this context are performed on histological samples/biopsies or ex vivo. This leads to the question if previous results can be transferred to an in vivo setting. Therefore...
Selective laser melting (SLM), taking advantage of its inherent rapid cooling rates and near-net-shape forming ability, has been employed to fabricate bulk metallic glasses (BMGs). However, crystallization is frequently triggered during the SLM process, which results in a loss of the advantageous properties of BMGs, such as extremely high hardness...
Within powder bed fusion of metals by a laser-beam (PBF-LB/M) spatter particles evolve due to the entrainment of powder particles and their ejection along the vapor plume. Motion properties of the evolving spatter particles are directly linked to the process zone topography, qualifying them as an interesting process signature for in-situ sensing. T...
Quality and reliability are of the utmost importance for manufacturing in the optical and medical industries. Absorber-free laser transmission welding enables the precise joining of identical polymers without additives or adhesives and is well-suited to meet the demands of the aforementioned industries. To attain sufficient absorption of laser ener...
Various clinically applicable scores and indices are available to help identify the state of a microcirculatory disorder in a patient. Several of these methods, however, leave room for interpretation and only provide clues for diagnosis. Thus, a measurement method that allows a reliable detection of impending or manifest circulatory malfunctions wo...
Thermoplastic polymers such as polyamide 12 (PA12) are of great interest for functional coatings in industry due to their good material properties (e.g., chemical and wear resistance and biocompatibility). In order to provide a high local selectivity of polymer deposition and to shorten the process chain, a laser-based coating process represents a...
Joining of copper materials has become a key factor in laser material processing of electric components like electric engines, batteries, or power electronics. A paramount challenge is the design of the laser welding process, which has to fulfil the requirements of high energy efficiency and highest seam quality at the same time. By now, high-power...
The fabrication of complex integrated photonic devices via direct laser writing is a powerful and rapidly developing technology. However, the approach is still facing several challenges. One of them is the reliable quantitative characterization of refractive index (RI) changes induced upon laser exposure. To this end, we develop a tomographic recon...
The great interest, within the fields of research and industry, in enhancing the range and functionality of polymer powders for laser powder bed fusion (LB-PBF-P) increases the need for material modifications. To exploit the full potential of the additivation method of feedstock powders with nanoparticles, the influence of nanoparticles on the LB-P...
For many applications in tissue optics, knowledge of the scattering coefficient or at least the reduced scattering coefficient is essential. In addition, its spectral dependence is also an important feature, as this provides information about scatterer sizes. While the characterisation of the spectral dependence should be a simple fit, experimental...
Protective ceramic‐based coatings are frequently the most suitable solutions for problems like corrosion and wear. It has been shown, that the precursor technology is suitable for the preparation of ceramic coatings by pyrolysis in a furnace. However, the required high temperatures for the preparation of the ceramic coatings limits this approach to...
In recent years, the application field of laser powder bed fusion of metals and polymers extends through an increasing variability of powder compositions in the market. New powder formulations such as nanoparticle (NP) additivated powder feedstocks are available today. Interestingly, they behave differently along with the entire laser powder bed fu...
The determination of the optical properties of a turbid medium is a major topic in the field of optics. Generally, they comprise the parameters µ a , µ s , g and n. There is, however, a lack of techniques for the direct determination of the scattering coefficient µ s . This study, therefore, proposes the random laser (RL) as a tool to directly meas...
Evolution of spectroscopic properties of a soda–lime silicate glass with different thermal history and under applied uniaxial stress was investigated using Raman and Brillouin spectroscopies as well as Nd3+ photoluminescence techniques. Samples of soda–lime silicate with a cooling rate from 6 × 10−4 to 650 K/min were prepared either by controlled c...
This paper contributes to an integrated and systematic methodology of disaster risk assessment for (World) cultural heritage sites. A Cultural Heritage Risk Index is proposed while looking at the risk components of ‘hazard’ (earthquakes), ‘exposure’ (significance of the heritage assets), and ‘vulnerability’ (susceptibility and coping capacity). To...
Particulate materials with well-engineered properties are of key importance for many aspects in our daily life. Polymer powders with high flowability, for example, play a crucial role in the emerging field of powder-based additive manufacturing processes. However, the polymer- and composite material selection for these technologies is still limited...
Knowledge of the correct temperature distribution on the cutting front is essential to understand and quantify the thermo-fluid dynamics during laser cutting, which are directly related to the edge quality. For that purpose, hyperspectral imaging, coupled with trim-cut observations with boundary conditions close to industrial processing, have been...
Phase-only beam shaping with liquid crystal on silicon spatial light modulators (SLM) allows modulating the wavefront dynamically and generating arbitrary intensity patterns with high efficiency. Since this method cannot take control of all degrees of freedom, a speckle pattern appears and drastically impairs the outcome. There are several methods...
In medical applications, hyper-spectral imaging is becoming more and more common. It has been shown to be more effective for classification and segmentation than normal RGB imaging because narrower wavelength bands are used, providing a higher contrast. However, until now, the fact that hyper-spectral images also contain information about the three...
In this work, we investigate single-pulse laser ablation of bulk stainless steel (AISI304), aluminium (Al) and copper (Cu) and its dependence on the pulse duration. We measured the reflectivity, ablation thresholds and volumes under the variation of pulse duration and fluence. The known drop of efficiency with increasing pulse duration is confirmed...
The need for tissue contact makes photoacoustic imaging not applicable for special medical applications like wound imaging, endoscopy, or laser surgery. An easy, stable, and contact-free sensing technique might thus help to broaden the applications of the medical imaging modality. In this work, it is demonstrated for the first time that remote phot...
The optical energy input during laser-based powder bed fusion of polymers (PBF-LB/P) is influenced by a variety of process parameters (e.g., energy density) and powder material properties (e.g. optical properties, additives). Qualification of newly developed and/or modified powder materials still requires extensive, empirical parameter studies to a...
In this paper, the influence of 0.005 vol% and 0.05 vol% of carbon nanoparticles on the surface of polyamide 12 powder particles by dry coating and colloidal additivation is evaluated in great detail concerning thermal and microstructural properties. The dispersion of the nanoparticles on the polymer surface influences the flowability of the feedst...
In powder bed fusion (PBF), a laser based energy source selectively fuses polymer powder particles in a layer-by-layer process building up a three-dimensional structure. For powder bed fusion of polymers the knowledge about which material properties are important for the process and the product quality is insufficient. This causes significant diffi...
The ablation rate in double-pulse material processing is strongly influenced by the pulse separation. For pulse separations exceeding 3 ps a significant decrease in ablation volume has been observed. This was attributed to three mechanisms: rarefaction wave interaction, shielding by ablation plume and material re-deposition. Here we present careful...
In general, the measurement of the main three optical properties (µa, µs and g) in turbid media requires a very precise measurement of the total transmission (TT), the total reflection (TR) and the collimated transmission (CT). Furthermore, an inverse algorithm such as inverse adding doubling or inverse Monte-Carlo-simulations is required for the r...
The resolution of complex parts produced by laser powder bed fusion of polymers (PBF-LB/P) is defined significantly by the shape and size distribution of the feedstock powder. Its analysis is usually performed by optical measurement systems such as laser diffraction or image analysis. In this study, the most relevant particle size parameters will b...
Until today, the electrophotographic transfer of toner powders has been applied successfully in common printing technology for many years. Utilizing the electrophotographic principle for the transfer of polymer powders in the context of additive manufacturing can give a variety of advantages like e.g. the possibility of generating multi-material co...
Remote focusing means to translate the focus position of an imaging system along the optical axis without moving the objective lens. The concept gains increasing importance as it allows for quick 3D focus steering in scanning microscopes, leaves the sample region unperturbed and is compatible with conjugated adaptive optics. Here we present a novel...
During the cooling process of the molten material, residual stresses appear because the reduced volume of the cooled material cannot fully fill the space formerly occupied by the molten material. The morphology in and around the weld is formed by different factors depending on the material and process parameters. Different morphological structures...
Screening colonoscopy is crucial in reducing the mortality of colorectal cancer. However, detecting adenomas against the backdrop of an inflamed mucosa (e.g. in ulcerative colitis) remains exceedingly difficult. Therefore, we aimed to improve neoplastic lesion detection by employing a fluorescence-based endoscopic approach. We used the well-establi...
The development of new feedstock materials is a central prerequisite for advances in Additive Manufacturing (AM). To increase the breadth of potential applications for 3D and 4D printing of polymers, micro- and nano-additives incorporated into the feedstock material play an important role. In this context, magnetic materials are of great interest....
Driven by the rapid development of additive manufacturing technologies and the trend towards mass customization, the development of new feedstock materials has become a key aspect. Additivation of the feedstock with nanoparticles is a possible route for tailoring the feedstock material to the printing process and to modify the properties of the pri...
Photoacoustic endoscopy (PAE) is a method of in-vivo imaging that uses tissue absorption properties. In PAE, the main tools used to detect the acoustic signal are mechanical ultrasound transducers, which require direct contact and which are difficult to miniaturize. All-optic photoacoustic sensors can challenge this issue as they can provide contac...
As Additive Manufacturing (AM) is fast-growing, properties adaption of feedstock materials for AM is becoming more and more relevant due to high quality standards in industrial applications. Compared to traditional manufacturing techniques like injection molding, laser powder bed fusion (PBF-LB) of polymers has a very limited variety of processable...
Additive manufacturing promises high flexibility and customized product design. Powder bed fusion processes use a laser to melt a polymer powder at predefined locations and iterate the scheme to build 3D objects. The design of flowable powders is a critical parameter for a successful fabrication process that currently limits the choice of available...