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: Computer science & Context (language use). The organization has 6684 authors who have published 16929 publications receiving 323154 citations.

Nonlinear Imaging with All-Dielectric Metasurfaces.

Abstract: Nonlinear metasurfaces incorporate many of the functionalities of their linear counterparts such as wavefront shaping, but simultaneously they perform nonlinear optical transformations. This dual functionality leads to a rather unintuitive physical behavior which is still widely unexplored for many photonic applications. The nonlinear processes render some basic principles governing the functionality of linear metasurfaces. Exemplarily, the superposition principle and the geometric optics approximation become not directly applicable to nonlinear metasurfaces. On the other hand, nonlinear metasurfaces facilitate new phenomena that are not possible in the linear regime. Here, we study the imaging of objects through a dielectric nonlinear metalens. We illuminate objects by infrared light and record their generated images at the visible third-harmonic wavelengths. We revisit the classical lens theory and suggest a generalized Gaussian lens equation for nonlinear imaging, verified both experimentally and analytically. We also demonstrate experimentally higher-order spatial correlations facilitated by the nonlinear metalens, resulting in additional image features.

Dynamic load balancing in distributed hash tables

Abstract: In Peer-to-Peer networks based on consistent hashing and ring topology, each server is responsible for an interval chosen (pseudo-) randomly on a unit circle. The topology of the network, the communication load, and the amount of data a server stores depend heavily on the length of its interval. Additionally, the nodes are allowed to join the network or to leave it at any time. Such operations can destroy the balance of the network, even if all the intervals had equal lengths in the beginning. This paper deals with the task of keeping such a system balanced, so that the lengths of intervals assigned to the nodes differ at most by a constant factor. We propose a simple fully distributed scheme, which works in a constant number of rounds and achieves optimal balance with high probability. Each round takes time at most $\mathcal{O}(\mathcal{D}+{\rm log n})$, where $\mathcal{D}$ is the diameter of a specific network (e.g. Θ(log n) for Chord [15] and $\Theta(\frac{log n}{log log n})$ for the continous-discrete approach proposed by Naor and Wieder [12,11]). The scheme is a continuous process which does not have to be informed about the possible imbalance or the current size of the network to start working. The total number of migrations is within a constant factor from the number of migrations generated by the optimal centralized algorithm starting with the same initial network state.

[Bis(guanidine)]zinc Complexes and Their Application in Lactide Polymerisation

Abstract: The bis(guanidine)-stabilised zinc complexes [N1,N2-bis(1,3-dimethylimidazolidin-2-ylidene)ethane-1,2-diamine]dichloridozinc(II), [Zn(DMEG2e)Cl2] (C1), diacetato[N1,N2-bis(1,3-dimethylimidazolidin-2-ylidene)ethane-1,2-diamine]zinc(II),[Zn(DMEG2e)(CH3COO)2] (C2), and bis[N1,N2-bis(1,3-dimethylimidazolidin-2-ylidene)ethane-1,2-diamine]zinc(II) bis(trifluoromethanesulfonate), [Zn(DMEG2e)2](CF3SO3)2 (C3), have been synthesised and completely characterised by means of X-ray structure analysis, NMR spectroscopy and mass spectrometry These [bis(guanidine)]zinc complexes were investigated regarding their activity in the bulk polymerisation of D,L-lactide It could be shown that these compounds are able to act as initiators for lactide polymerisation, and polylactides with molecular weights (Mw) of around 18000–59000 g/mol could be obtained Variation of the reaction temperature revealed that the molecular weights decrease with increasing temperature Additionally, the correlation of the intrinsic viscosity with the molecular weight demonstrates that the obtained polymers are linear and structurally homogeneous(© Wiley-VCH Verlag GmbH & Co KGaA, 69451 Weinheim, Germany, 2007)

Convective heat transfer and pressure loss in rectangular ducts with drop-shaped pin fins

Abstract: It has been experimentally researched that convective heat transfer and pressure loss characteristics in rectangular channels with staggered arrays of drop-shaped pin fins in crossflow of air. The effects of arrangements of pin fins on heat transfer and resistance are discussed and the row-by-row variations of the mean Nusselt numbers are presented. By means of the heat/mass transfer analogy and the naphthalene sublimation technique, the heat transfer coefficients on pin fins and on endwall (base plate) of the channel have been achieved respectively. The total mean heat transfer coefficients of pin fin channels are calculated and the resistance coefficients are also investigated. The experimental results show that heat transfer of a channel with drop-shaped pin fins is higher than that with circular pin fins while the resistance of the former is much lower than that of the latter in the Reynolds number range from 900 to 9000.