Generation of switchable singular beams with dynamic metasurfaces

Singular beams have attracted great attention due to their optical properties and broad applications from light manipulation to optical communications. However, there has been a lack of practical schemes with which to achieve switchable singular beams with sub-wavelength resolution using ultrathin and flat optical devices. In this work, we demonstrate the generation of switchable vector and vortex beams utilizing dynamic metasurfaces at visible frequencies. The dynamic functionality of the metasurface pixels is enabled by the utilization of magnesium nanorods, which possess plasmonic reconfigurability upon hydrogenation and dehydrogenation. We show that switchable vector beams of different polarization states and switchable vortex beams of different topological charges can be implemented through simple hydrogenation and dehydrogenation of the same metasurfaces. Furthermore, we demonstrate a two-cascade metasurface scheme for holographic pattern switching, taking inspiration from orbital angular momentum-shift keying. Our work provides an additional degree of freedom to develop high-security optical elements for anti-counterfeiting applications

In situ visualization of flow and fouling layer formation in ceramic hollow fiber membranes by magnetic resonance imaging (MRI)

Within membrane processes, fouling is one of the critical issues affecting the productivity, plant operation and maintenance costs. Focusing on wastewater treatment processes, it has been reported that extracellular polymeric substances (EPS) are one of the main causes of membrane fouling. In membrane filtration research, sodium alginate often serves as a model compound for EPS. Sodium alginate is a hydrophilic unbranched binary copolymer. In the presence of divalent cations, e.g. Ca2+, alginates form complexes, which lead to a significant change in filtration mechanisms in dead-end filtration and also to a change in filtration performance during cross-flow filtration experiments. Filtration conditions (e.g. transmembrane pressure or cross flow velocity), feed composition as well as membrane material have a major influence on the fouling behavior of the system. In this study ceramic hollow fiber membranes were used due to their high chemical and thermal stability coupled with a high specific membrane surface. In addition to the evaluation of the filtration data using conventional cake filtration model, nuclear magnetic resonance imaging was used to elucidate the influence of Ca2+ on the fouling layer structure for alginate filtration with ceramic hollow fiber membranes. In order to visualize the alginate layers inside the opaque ceramic hollow fiber membranes by means of MRI, specific contrast agents were applied. Supplementary to multi slice multi echo imaging, flow velocity measurements were performed to gain more insight into the hydrodynamics in the fouled membranes. MRI reveals the structure of the alginate layers and confirms the assumption obtained from the evaluation of filtration data, that the addition of Ca2+ is leading to the formation of an alginate gel layer on the membrane, whereas in the absence of Ca2+, the structure of the alginate layer is rather of concentration polarization manner, hence more fluid and hydrodynamically better controllable

Magnetic switching of nanoscale antidot lattices

We investigate the rich magnetic switching properties of nanoscale antidot lattices in the 200 nm regime. In-plane magnetized Fe, Co, and Permalloy (Py) as well as out-of-plane magnetized GdFe antidot films are prepared by a modified nanosphere lithography allowing for non-close packed voids in a magnetic film. We present a magnetometry protocol based on magneto-optical Kerr microscopy elucidating the switching modes using first-order reversal curves. The combination of various magnetometry and magnetic microscopy techniques as well as micromagnetic simulations delivers a thorough understanding of the switching modes. While part of the investigations has been published before, we summarize these results and add significant new insights in the magnetism of exchange-coupled antidot lattices.Web of Science775073

Ferromagnetic behaviour of ZnO: The role of grain boundaries

The possibility to attain ferromagnetic properties in transparent semiconductor oxides such as ZnO is very promising for future spintronic applications. We demonstrate in this review that ferromagnetism is not an intrinsic property of the ZnO crystalline lattice but is that of ZnO/ZnO grain boundaries. If a ZnO polycrystal contains enough grain boundaries, it can transform into the ferromagnetic state even without doping with “magnetic atoms” such as Mn, Co, Fe or Ni. However, such doping facilitates the appearance of ferromagnetism in ZnO. It increases the saturation magnetisation and decreases the critical amount of grain boundaries needed for FM. A drastic increase of the total solubility of dopants in ZnO with decreasing grain size has been also observed. It is explained by the multilayer grain boundary segregation