arXiv subjects

Andreas Heinrich

Publications and source records attributed to Andreas Heinrich.

5 recordsLinked to original sources

arXiv2026

Dynamic analysis of refractive index evolution and diffraction properties during single-photon polymerization of photopolymers for micro-optical applications

Photopolymerization enables the production of micro-optical elements, such as diffractive optical elements or GRIN optics. This process utilizes targeted spatial modulation of the refractive index, which is achieved through additive manufacturing. In this context, a thorough understanding of the dynamic processes during curing is essential in order to be able to accurately predict the optical function of the element. For this reason, this work investigates the kinetics and resulting optical properties of an acrylate-based photopolymer under UV irradiation using a DLP projection system. The experimental approach combines two measurement methods: On the one hand, the absolute change in the refractive index is determined in a time-resolved manner at the interface of a prism using a method based on total reflection. On the other hand, the formation of a phase grating in the volume of the polymer is monitored in real time by analyzing the diffraction orders of a coherent sample laser. The results show a characteristic S-shaped curve of the refractive index change, which reflects the phases of polymerization: oxygen inhibition, autoacceleration, and vitrification. Analysis of the diffraction patterns reveals complex intensity curves and substructures in the diffraction orders. These could be traced back experimentally and through simulations to the discrete pixel structure of the DLP projector and the existing dead zones. Furthermore, the simulation model developed on the basis of Fourier optics reproduces the experimental diffraction patterns and confirms the hypothesis that scattered light and radical diffusion lead to time-delayed polymerization in the theoretically unexposed areas. This results in a reduction of the refractive index contrast over time. This work thus provides parameters for the simulation and optimization of exposure strategies in 3D printing of micro-optics.

physics.optics↗

arXiv2021

Atomic Manipulation of In-gap States on the $β$-Bi$_2$Pd Superconductor

Electronic states in the gap of a superconductor inherit intriguing many-body properties from the superconductor. Here, we create these in-gap states by manipulating Cr atomic chains on the $β$-Bi$_2$Pd superconductor. We find that the topological properties of the in-gap states can greatly vary depending on the crafted spin chain. These systems make an ideal platform for non-trivial topological phases because of the large atom-superconductor interactions and the existence of a large Rashba coupling at the Bi-terminated surface. We study two spin chains, one with atoms two-lattice-parameter apart and one with square-root-of-two lattice parameters. Of these, only the second one is in a topologically non-trivial phase, in correspondence with the spin interactions for this geometry.

cond-mat.supr-con↗

arXiv2014

Pattern formation during the oscillatory photoelectrodissolution of n-type silicon: Turbulence, clusters and chimeras

We report and classify the rich variety of patterns forming spontaneously in the oxide layer during the oscillatory photoelectrodissolution of n-type doped silicon electrodes under limited illumination. Remarkably, these patterns are often comprised of several dynamical states coexisting on the electrode, such as subharmonic phase clusters and spatio-temporal chaos, and include so-called 'chimera states'. The experiments suggest that the subharmonic phase clusters emerge from a period doubling bifurcation which, upon further parameter changes, evolve into classical phase clusters. Experimentally the occurrence of the patterns is controlled via two coupling mechanisms: A linear global coupling by an external resistor and a nonlinear coupling imposed on the system by the limitation of the illumination.

nlin.PS↗

arXiv2008

Increase of the wear resistance of carbide layers deposited by Pulsed Laser Deposition in addition with an auxiliary laser

Carbides stand out because of their high hardness and wear-resistance. Thus these materials are often discussed for coatings of machine tools etc. Within this work Boron Carbide (B4C) and Carbide (C) thin films were deposited on Si (100) substrates by pulsed-laser deposition technique. In order to improve the wear-resistance of the deposited films, we introduced a new working technique including the application of a second excimer laser in a special working mode. Thereby one laser was used to ablate the carbide material from a target and to deposit the material on the substrate. The light of the second laser was directed directly onto the substrate in order to modify the ablated material. We report on details for film deposition and film properties determined by Scanning Electron Microscope, Energy Dispersive X-Ray Spectroscopy, X-Ray Diffraction, Rutherford Backscattering, Raman Spectroscopy and tribological experiments.

cond-mat.mtrl-sci↗

arXiv2005

Pulsed Laser Deposition of La0,67Ca0,33MnO3 thin films on LiNbO3

Surface Acoustic Waves on piezoelectric substrates can be used to investigate the dynamic conductivity of thin films in a non-contact and very sensitive way, especially at low conductivities. Here, we report on such surface acoustic wave studies to characterize thin manganite film like La0.67Ca0.33MnO3, exhibiting a Jan Teller effect with a strong electron phonon interaction and a metal insulator transition at high temperatures. We report on the deposition of La0.67Ca0.33MnO3 on piezoelectric substrates (LiNbO3 in different crystal cuts) employing a pulsed laser deposition technique. The structural qualities of the thin films are examined by X-Ray Diffraction, Scanning Electron Microscope and Energy Dispersive X-ray spectroscopy. For the electrical characterization, we employ the surface acoustic wave technique, accompanied by conventional DC-resistance measurements for comparison.

cond-mat.stat-mech↗