621 Angewandte Physik
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We present a camera-based photoacoustic tomograph that uses planar Fabry-Perot polymer sensors with uniform optical thickness. By using a photopolymer spacer, local refractive index adjustments effectively compensate for fabrication-induced inhomogeneities, resulting in highly uniform sensors suitable for parallel, time-resolved acoustic signal acquisition. The tomograph utilizes a green-wavelength interrogation laser and a high-speed, intensified sCMOS camera. This system enables high-resolution, three-dimensional imaging of tissue phantoms in backward mode, achieving both lateral and vertical resolutions of tens of microns. While the acoustic sensitivity is currently restricted by the specifications of the sensor, laser, and camera, this method allows for rapid, wide-area, and high-resolution photoacoustic imaging without the need for mechanical and optical scanning.
Xolography is a volumetric 3D printing technology utilizing two different wavelengths of light to produce objects with micrometer resolution and excellent surface quality within minutes. Dual-color photoinitiators (DCPIs) are crucial for spatial control of polymerization. However, their performance has mainly been assessed indirectly by evaluating printed objects. In this study, we present a methodology using in situ Raman spectroscopy to characterize the dual-color curing response of a representative spiropyran-based DCPI in a methacrylate-based resin composition. A dual-wavelength irradiation setup has been developed to monitor the degree of curing (DoC) as a function of the applied light doses, providing quantitative, real-time insights into bond conversion and polymerization kinetics, as well as enabling the Xolography process window to be derived. The DCPI exhibited a contrast of 89%, showing improved performance compared to first generation systems, while its efficiency was benchmarked against a benzophenone reference. Spatially resolved Raman mapping of volumetric test prints revealed DoC gradients ranging from 40% to 80% directly after printing. We demonstrate that subsequent posttreatments can eliminate these nonuniformities, increasing the DoC to > 90%. This approach provides a framework for evaluating novel dual-color photoinitiator systems, facilitating the rational design of resins, illumination protocols and optimized configurations for volumetric 3D printing.
Indium gallium nitride (InGaN) nanowire structures were investigated as photoelectrochemical transducers for bioanalytical applications. Chopped-light voltammetry in HEPES buffer (pH 7.0) revealed a light intensity-dependent anodic photocurrent that varies with applied potential. In the presence of hypoxanthine unmodified InGaN showed no photocurrent changes when electrode potentials around 0 mV vs Ag/AgCl or below have been applied. The three-dimensional surface of the nanowires was then used to adsorb the enzyme xanthine dehydrogenase (XDH). After this modification, the InGaN electrode exhibited a distinct increase in anodic photocurrent in the presence of hypoxanthine. The photocurrent showed a clear concentration-dependent behavior. This is indicative for a direct electron transfer from XDH to the semiconductor material. These findings highlight the suitability of InGaN nanowires for coupling with redox enzymes and their potential for developing light-driven biosensing platforms.
Das EU/ILB-geförderte Projekt MINT’oVation verfolgt ein XR-gestütztes Lehrkonzept zur Vermittlung von Grundlagen und Prozessen des metallischen 3D-Drucks mit Fokus auf das Selective Laser Melting (SLM). Ziel ist der Aufbau eines fundierten technologischen Verständnisses dieser industriellen Schlüsseltechnologie sowie die anschauliche, sichere und niedrigschwellige Vermittlung komplexer metallphysikalischer Zusammenhänge. Das didaktische Design orientiert sich am Cognitive-Affective Model of Immersive Learning (CAMIL) und nutzt visuelle Immersion, Interaktion und spielerische Elemente zur Förderung von Motivation, Aufmerksamkeit und nachhaltigem Wissenserwerb. In der virtuellen Umgebung können Prozesse, Experimente und Demonstrationen risikofrei und ohne kostenintensive Anlagen durchgeführt werden. Der modular aufgebaute Kurs umfasst unter anderem eine Einführung in XR-Technologien, metallphysikalische Grundlagen, additive Fertigungsverfahren, CAD-Konstruktion sowie den vollständigen Prozess von der Modellierung bis zur Druckdurchführung. Zum Einsatz kommen marktgängige, hardwareunabhängige XR-Systeme, beispielhaft Apple Vision Pro oder Meta Quest 3. Das Angebot richtet sich an Studierende, Auszubildende, Schüler* innen, Arbeitssuchende und weitere Interessierte ohne nötiges Vorwissen und unterstützt gezielt Inklusion und Chancengleichheit. Zwischen April und Dezember 2025 verzeichnete der Kurs 97 Anmeldungen, davon 82 für den vollständigen Kurs; 66 Teilnehmende schlossen diesen bereits erfolgreich ab. Die hohe Nachfrage unterstreicht das Potenzial immersiver XR-Lehrformate zur nachhaltigen Qualifizierung im Bereich der additiven Fertigung und zur Vorbereitung auf einen sich wandelnden Arbeitsmarkt.
Automatic impact modal testing is a technique gaining momentum in recent years thanks to the popularization of Scanning Laser Doppler Vibrometry. These systems allow automatizing the output measurement of thousands of degrees of freedom in a short time. The use of automatic impact modal hammers allows automatizing the excitation input and broadband excitation without loading a structure with an extra mass or other drawbacks. However, the impact force repeatability is a prominent concern among test engineers, especially those who work with materials with non-proportional force/response ratios. Assessing the impact force repeatability of a given automatic modal hammer or test rig is necessary in order to ensure the right response level is measured impact after impact.
The assessment procedure can be misleading if not done right. Studying the automatic modal hammer repeatability under typical modal test conditions invariably leads to impact signals strongly distorted by the so called picket fence effect. This results in impacts sampled by only 3–4 data points; insufficient to accurately describe the actual impact force signals and the short contact times between hammer tip and structure. In the reality, the impacts are of larger magnitudes and shorter contact times than what is shown by the analyzer in typical test conditions.
This work studies the influence of the sampling frequency and the test structures used on the repeatability assessment of automatic impact modal hammers. Impact force signals are acquired in this work with enough resolution to eliminate the picket fence effect and truly evaluate how repeatable and reproducible automatic impacts are. The practicality of the procedure, which involves very large datasets and long testing times, is discussed. Guidelines are offered at the end of the paper for a successful repeatability and reproducibility assessment of automatic impact modal hammers.
Industrielle Robotersysteme werden aufgrund der stetig wachsenden Technik der additiven Fertigung zunehmend zugängliche. Die dadurch verringerte Kosten- und Aufwandschwelle führt zu einer Verbreitung von selbstgebauten Robotersystemen im didaktischen als auch industriellen Kontext, besonders in kleinen- und mittelständischen Unternehmen. Eigenbauroboter im Gegensatz zu kommerziellen Robotern besitzen keine validierten Leistungsmerkmale. Die Charakterisierung von 3D-gedruckten Robotern ist von besonderer Bedeutung, da das Herstellungsverfahren einen direkten Einfluss auf die charakteristischen Merkmale des Robotersystems hat. Womit die Qualität der Fertigung sowie die Genauigkeit des Roboters wesentlich durch die Raumtemperatur, die Bauart des Druckers, den Düsendurchmesser, die Schichthöhe sowie das eingesetzte Material determiniert wird. Die Bewertung der Eignung eines Roboters für präzise Aufgaben ist ohne Kenntnis seiner Leistungsmerkmale nur eingeschränkt möglich. Die Charakterisierung eines Robotersystems erfolgt anhand unterschiedlicher Methoden, darunter SMR (Sphärisch Montierter Retroreflektor)-Verfolgung, Photogrammetrie, Laser-Interferometrie, Teleskop-Ballbar und externe Messuhren sowie Taster. Bei einer Vielzahl dieser Ansätze entstehen bei der Beschaffung der benötigten Mess- oder Kalibrierungsinstrumente hohe Kosten, was die Wirtschaftlichkeit der additiven Fertigung nicht widerspiegelt und für Anwender von Eigenbaurobotern, insbesondere im didaktischen Kontext, eine Herausforderung darstellt. Als repräsentative Leistungscharakteristika stehen hierbei die Positionierung und die Wiederholungsgenauigkeit von Robotersystemen im Fokus und gelten als wesentlich zu ermittelnde Parameter.
We report on the design and performance of two solid-state Raman lasers, pumped by a frequency-doubled Nd:YAG laser at 532 nm (8 ns FWHM, 10 Hz, up to 10 mJ), that generate either the first or second Stokes radiation with wavelengths of 563 nm and 598 nm. Barium nitrate crystals with a 1 cm² face area and lengths of 3 cm and 5 cm were used as the Raman-active medium in stable semi-spherical resonators. The Raman lasers produced first Stokes and second Stokes pulses, achieving pulse energies up to 2.3 mJ and conversion efficiencies up to 35% (first Stokes, 5 cm crystal) and 21% (second Stokes, 5 cm crystal). The output beams exhibit nearly Gaussian profiles with beam quality factors of M² = 1.67 and M² = 1.83, respectively. The proposed design and characterization demonstrated a Raman laser with high efficiency and superior beam quality. After subsequent frequency doubling, the output appears well dedicated for future differential absorption LiDAR (DIAL) applications.
Biomass is an important resource for the utilisation of renewable energy not only in Germany, but in many other countries. Being sufficient to provide base load it is capable to stabilise electricity grids besides contributing to climate change precaution. Additionally it is essential that the applications for heat, power and fuel feature high conversion efficiencies. Therefore the on-site generation of electricity is a crucial contribution to renewable energy supply, especially in real small-scale applications. The project “HTBioStir” - development of a high temperature heat exchanger for the coupling of biomass boiler and Stirling engine - is designed and realized as a fundamental research project focusing on important mechanisms of heat transfer. The basic approach to generate electricity and heat implies the transfer of enthalpy on high temperature levels from the flue gases of a wood chip operated fire tube boiler to the working fluid helium through the heater head of a Stirling engine generator set. Using an indirect heat transfer mechanism, air as the carrier, an innovative heat exchanger with surface structured industrial power tube bundle and upstream a similar recuperative air-preheater by which an α-type SOLO 2V is operated to gene rate electricity. The leaving air at a much lower temperature level serves then as preheated oxidant in the furnace, while the flue gases produce hot water in the boiler. At present measurement campaigns were carried out to establish energy and mass balances of the fire tube boiler as well as the Stirling engine.
At present and in future the necessity of generating electricity using renewable resources globally is most important and clearly evident. Depending on how the scenarios are designed and disregarding fossil resources the contributions of renewable energy sources have to be increased by a factor of four to six as far as it concerns the installed capacity and the amount of electricity generated. The corresponding annual investment costs amount some hundred billion US-$ and the CO₂ emission reduction equals one Gt. In any case heat to electricity conversion systems based on biomass, geothermal and process heat recovery are expected to provide significant contributions to this objective. Besides several others the conversion technology of a Rankine Cycle Process is most suitable over a wide range of capacities notwithstanding the possibilities to applicate working fluids of non-organic or organic character. Particularly for the heat recovery from the exhaust gas of biogas operated internal combustion engines the type of a newly developed friction expander was designed, constructed, built and evaluated. Based on preceding theoretical and numerical calculations as well as constructional and experimental studies using hexane and hexamethoxydisiloxane in the project reported here saturated water steam was used as working fluid. During the experiments carried out in a special test facility the electric power of the system amounted 1.4 kW representing two thirds of the conceptual design.
