Multi-dimensional modeling and simulation of semiconductor nanophotonic devices

Self-consistent modeling and multi-dimensional simulation of semiconductor nanophotonic devices is an important tool in the development of future integrated light sources and quantum devices. Simulations can guide important technological decisions by revealing performance bottlenecks in new device concepts, contribute to their understanding and help to theoretically explore their optimization potential. The efficient implementation of multi-dimensional numerical simulations for computer-aided design tasks requires sophisticated numerical methods and modeling techniques. We review recent advances in device-scale modeling of quantum dot based single-photon sources and laser diodes by self-consistently coupling the optical Maxwell equations with semiclassical carrier transport models using semi-classical and fully quantum mechanical descriptions of the optically active region, respectively. For the simulation of realistic devices with complex, multi-dimensional geometries, we have developed a novel hp-adaptive finite element approach for the optical Maxwell equations, using mixed meshes adapted to the multi-scale properties of the photonic structures. For electrically driven devices, we introduced novel discretization and parameter-embedding techniques to solve the drift-diffusion system for strongly degenerate semiconductors at cryogenic temperature. Our methodical advances are demonstrated on various applications, including vertical-cavity surface-emitting lasers, grating couplers and single-photon sources

Diffusive gradients in thin films measurement of sulfur stable isotope variations in labile soil sulfate

A diffusive gradient in thin films (DGT) technique, based on a strongly basic anion exchange resin (Amberlite IRA-400), was successfully tested for ³⁴S/³²S analysis in labile soil sulfate. Separation of matrix elements (Na, K, and Ca) that potentially cause non-spectral interferences in ³⁴S/³²S analysis by MC ICP-MS (multi-collector inductively coupled plasma-mass spectrometry) during sampling of sulfate was demonstrated. No isotopic fractionation caused by diffusion or elution of sulfate was observed below a resin gel disc loading of ≤79 μg S. Above this threshold, fractionation towards ³⁴S was observed. The method was applied to 11 different topsoils and one mineral soil profile (0-100 cm depth) and compared with soil sulfate extraction by water. The S amount and isotopic ratio in DGT-S and water-extractable sulfate correlated significantly (r² = 0.89 and r² = 0.74 for the 11 topsoils, respectively). The systematically lower ³⁴S/³²S isotope ratios of the DGT-S were ascribed to mineralization of organic S.Ondrej Hanousek, Jakob Santner, Sean Mason, Torsten W. Berger, Walter W. Wenzel, Thomas Prohask

Trace elements in the soil-plant interface: Phytoavailability, translocation, and phytoremediation–A review

Trace elements (TEs) are deposited to soils mainly due to anthropogenic activities and pose a significant threat to human health. In this review we aimed at (a) discussing the phytoavailability of TEs as affected by various soil parameters, and by plant defense mechanisms related to uptake and translocation; (b) examining soil and plant indices related to TE phytoavailability; (c) clarifying the challenges and problems related to phytoremediation; and (d) exploring the often encountered discrepancies of lower-than-expected TE toxicity. We particularly discussed the soil-to-plant availability index (transfer coefficient, TC), because it encompasses all soil and plant factors related to TE phytoavailability. As for soil, we explored the effect of pH, redox potential, clay and organic matter contents, as well as aging of added elements. The latter is a key factor in interpreting the observed lower-than-expected toxicity to plants in real field conditions. This is because the discrepancy is very often generated by growth experiments that expose plants to TEs directly from TE-laden solutions or by studies that spike soils with TEs only days or weeks before planting. Also, the behavior of TEs depends on the nature and quantity of TEs. As for plant, TE absorption or exclusion is highly related to species-specific defense mechanisms developed by plants so that they are exposed to TE-induced stress. These mechanisms address TE exposure by operating both outside and inside the plant body; outside with the assistance of root exudates, and the rhizosphere microflora, and inside with selective translocation and storage processes. The absorption/exclusion behavior of plants also depends on root activities and related soil chemical processes which are highly localized within a spatial scale of a few mm from roots. Novel techniques for the imaging of TE biogeochemistry at the root-soil interface are therefore addressed and their explanatory power is demonstrated. Such plant behavior greatly affects phytoremediation, a process which also depends on the maximal TE uptake capacity of plants, especially of hyperaccumulators. However, phytoremediation also greatly depends on plant biomass yield, an important factor in determining the time required to complete the procedure. In conclusion, soil factors, as well as plant- and TE- related issues, may create discrepancies in TE phytoavailability and phytoremediation that need to be thoroughly understood and addressed. © 2017 Elsevier B.V

Biogeochemistry of trace elements

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