University of Paderborn

About: University of Paderborn is a education organization based out in Paderborn, Nordrhein-Westfalen, Germany. It is known for research contribution in the topics: Control reconfiguration & Software. The organization has 6684 authors who have published 16929 publications receiving 323154 citations.

Single-pixel computational ghost imaging with helicity-dependent metasurface hologram.

Abstract: Different optical imaging techniques are based on different characteristics of light. By controlling the abrupt phase discontinuities with different polarized incident light, a metasurface can host a phase-only and helicity-dependent hologram. In contrast, ghost imaging (GI) is an indirect imaging modality to retrieve the object information from the correlation of the light intensity fluctuations. We report single-pixel computational GI with a high-efficiency reflective metasurface in both simulations and experiments. Playing a fascinating role in switching the GI target with different polarized light, the metasurface hologram generates helicity-dependent reconstructed ghost images and successfully introduces an additional security lock in a proposed optical encryption scheme based on the GI. The robustness of our encryption scheme is further verified with the vulnerability test. Building the first bridge between the metasurface hologram and the GI, our work paves the way to integrate their applications in the fields of optical communications, imaging technology, and security.

Lithium niobate X -cut, Y -cut, and Z -cut surfaces from ab initio theory

Abstract: Density-functional theory calculations of the ${\text{LiNbO}}_{3}$ $(2\overline{1}\overline{1}0)$, $(1\overline{1}00)$, and (0001) surfaces, commonly referred to as $X$, $Y$, and $Z$ cuts, are presented. In case of the $Z$ cut, we find a pronounced dependence of the surface structure and stoichiometry on the direction of the ferroelectric polarization. In contrast, the influence of the chemical potentials of the surface constituents is limited. Rather electrostatics governs the surface stability. Different from the $Z$ cut, the stoichiometry of the $X$ cut and $Y$ cut is clearly dependent on the preparation conditions. The surface charge observed for the nominal nonpolar $Y$ cut is traced back to the formation of a strong surface dipole.

Multiscale structure and properties of cast and deformation processed polycrystalline NiTi shape-memory alloys

Abstract: The objective of this study is to examine fundamental processing-structure-property relationships in polycrystalline NiTi bars. Three different polycrystalline Ti-50.9 at. pct Ni (Ti-55.7 wt pct Ni) materials were examined: (1) cast, (2) cast then hot rolled, and (3) cast, hot rolled, then cold drawn. The structure of the materials was investigated at various scales ranging from nanometers to micrometers. The cast materials contained random crystallographic textures along the loading axis of the extracted samples. The hot-rolled and cold-drawn materials contained a strong 〈111〉 texture parallel to the deformation-processing direction. The high-temperature hot-rolling process facilitated recrystallization and recovery, and curtailed precipitate formation, leaving the hot-rolled and cold-drawn materials in near solutionized states. The cold-drawn material contained a high density of dislocations and martensite. After a mild aging treatment, all three materials contained distributed coherent Ti3Ni4 precipitates on the order of 10 nm in size. The cast material was capable of full shape-memory transformation strain recovery up to approximately 5 pct strain at room temperature under both tension and compression. The hot-rolled and cold-drawn materials demonstrated significant tension-compression stress-strain asymmetry owing to their strong crystallographic texture. Under compression, the deformation-processed materials were only capable of 3 pct transformation strain recovery while under tension they were capable of nearly 7 pct transformation strain recovery. Based on the present results, the presence of small coherent Ti3Ni4 precipitates is determined to be the driving force for the favorable strain transformation strain recovery properties in all three materials, despite drastically different grain sizes and crystallographic textures. The unique dependence of elastic modulus on stress-state, temperature, and structure is also presented and discussed for the deformation-processed materials. In addition, we demonstrate that the appearance of a Luders band transformation under tensile loading can be controlled by material structure. Specifically, the presence of significant martensite and dislocations in the cold-drawn materials was shown to mitigate the Luders band propagation and result in a more gradual transformation.