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Classen C.,University of Paderborn | Classen C.,Center for Optoelectronics and Photonics Paderborn | Forstner J.,University of Paderborn | Forstner J.,Center for Optoelectronics and Photonics Paderborn | And 4 more authors.
2010 IEEE International Symposium on Antennas and Propagation and CNC-USNC/URSI Radio Science Meeting - Leading the Wave, AP-S/URSI 2010 | Year: 2010

The electromagnetic field in the vicinity of sharp edges needs a special treatment in numeric calculation whenever accurate, fast converging results are necessary. One of the fundamental works concerning field singularities has been proposed in 1972 [1] and states that the electromagnetic energy density must be integrable over any finite domain, even if this domain contains singularities. It is shown, that the magnetic field H(ρ, φ) and electric field E(ρ, φ) are proportional to ∝ ρ(t - 1) for ρ → 0. The variable ρ is the distance to the edge and t has to fulfill the integrability condition and thus is restricted to 0 < t < 1. This result is often used to reduce the error corresponding to the singularity without increasing the numerical effort [2 - 5]. For this purpose, a correction factor K is estimated by inserting the proportionality into the wave equation. It is shown, that this method improves the accuracy of the result significantly, however the order of convergence is often not studied. In [4] a method to modify the material parameters in order to use analytic results to improve the numeric calculation is presented. In this contribution we will - inspired by the scheme given in [4] - develop a new method to estimate a correction factor for perfect conducting materials (PEC) and demonstrate the improvement of the results compared to the standard edge correction. Therefore analytic results (comparable to [1]) are consequently merged with the scheme in [4]. The main goal of this work is the calculation of the second harmonic generation (SHG) in the wave response of so-called metamaterials [6]. Frequently these structures contain sharp metallic edges with field singularities at the interfaces which have a strong impact on the SHG signals. Thus, an accurate simulation of singularities is highly important. However, the following approach can also be applied to many other setups, and one of them is shown in the example below. © 2010 IEEE.


Riedl T.,University of Paderborn | Riedl T.,Center for Optoelectronics and Photonics Paderborn | Lindner J.K.N.,University of Paderborn | Lindner J.K.N.,Center for Optoelectronics and Photonics Paderborn
Physica Status Solidi (A) Applications and Materials Science | Year: 2014

This paper presents a low-cost procedure that allows for self-organized fabrication of periodically arranged, sub-50 nm diameter, vertical-sidewall GaAs nanopillars on GaAs surfaces based on nanosphere lithography, and reactive ion etching (RIE). Monodispersed polystyrene (PS) sphere double layers are deposited from a colloidal suspension on pre-treated, hydrophilized GaAs substrates. Ni is thermally evaporated to act as a hard mask for subsequent RIE. Scanning electron microscopy reveals that the Ni nanoparticles left on the substrate after PS sphere removal have polygon-shaped in-plane cross-sections corresponding to the shape of the double layer mask openings. RIE using SiCl4 at low pressure and high power density leads to the formation of vertical nanopillars with circular to oval crosssections, whose diameters are reduced by ∼1/3 compared to those of the Ni nanoparticles. These findings can be explained by plasma-enhanced surface diffusion and sputtering processes during RIE. The obtained GaAs nanopillars have an average diameter of only ∼23 nm, exhibit an excellent verticality with a sidewall angle of 88.9 ± 0.4° and planar top faces, as visible in transmission electron microscopy images. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.


Mackwitz P.,University of Paderborn | Mackwitz P.,Center for Optoelectronics and Photonics Paderborn | Rusing M.,University of Paderborn | Rusing M.,Center for Optoelectronics and Photonics Paderborn | And 6 more authors.
Applied Physics Letters | Year: 2016

We report the fabrication of periodically poled domain patterns in x-cut lithium niobate thin-film. Here, thin films on insulator have drawn particular attention due to their intrinsic waveguiding properties offering high mode confinement and smaller devices compared to in-diffused waveguides in bulk material. In contrast to z-cut thin film lithium niobate, the x-cut geometry does not require back electrodes for poling. Further, the x-cut geometry grants direct access to the largest nonlinear and electro-optical tensor element, which overall promises smaller devices. The domain inversion was realized via electric field poling utilizing deposited aluminum top electrodes on a stack of LN thin film/SiO2 layer/Bulk LN, which were patterned by optical lithography. The periodic domain inversion was verified by non-invasive confocal second harmonic microscopy. Our results show domain patterns in accordance to the electrode mask layout. The second harmonic signatures can be interpreted in terms of spatially, overlapping domain filaments which start their growth on the +z side. © 2016 Author(s).


Riedl T.,University of Paderborn | Riedl T.,Center for Optoelectronics and Photonics Paderborn | Lindner J.K.N.,University of Paderborn | Lindner J.K.N.,Center for Optoelectronics and Photonics Paderborn
Materials Research Society Symposium Proceedings | Year: 2014

The formation of misfit dislocations is an important issue for the performance of heteroepitaxial micro- and optoelectronic devices. We analyze three approaches that quantify the stability of misfit dislocations in axial-heteroepitaxial nanowires with respect to applicability and predictions of critical nanowire dimensions. The nanoheteroepitaxy approach of Zubia and Hersee proves suitable for determination of strain partitioning in the presence of an elastic mismatch. Concerning the critical thickness and diameter however the descriptions of Ertekin et al. and Glas respectively yield more reliable results, owing to the consideration of the total coherent and dislocation related energies plus the residual strain energy. In contrast to the model of Ertekin et al., which refers to infinitely long nanowires, the other two mentioned approaches allow predictions of the critical thickness of mismatched deposits on the nanowire axial face. Copyright © 2014 Materials Research Society.


Brassat K.,University of Paderborn | Brassat K.,Center for Optoelectronics and Photonics Paderborn | Lindner J.K.N.,University of Paderborn | Lindner J.K.N.,Center for Optoelectronics and Photonics Paderborn
Materials Research Society Symposium Proceedings | Year: 2014

A novel process for the formation of pairs of opposing metallic nanotips within linear trenches on a silicon wafer is investigated in detail. The process is based on a spreading knife technique typically used in nanosphere lithography to generate monolayers of colloidal polystyrene beads. Here it is applied to initiate self-assembly of spheres in long linear trenches acting as a template for the sphere arrangement. The optimum blade velocity to deposit the spheres selectively and densely packed in the trench depends on the trench surface fraction and can be described by a modified Dimitrov model. It is demonstrated that the spheres can be used as a shadow mask to deposit metallic nanotips in a channel, which are electrically interconnected on each side of the trench, possibly enabling the control and manipulation of nanoobjects in the channel. Copyright © 2014 Materials Research Society.


Riedl T.,University of Paderborn | Riedl T.,Center for Optoelectronics and Photonics Paderborn | Strake M.,University of Paderborn | Sievers W.,University of Paderborn | And 3 more authors.
Materials Research Society Symposium Proceedings | Year: 2014

An experimental analysis of the morphology changes of hexagonally close packed polystyrene sphere monolayers induced by annealing in air is presented. The triangular interstices between each triple of spheres, which are frequently used as nanoscale mask openings in colloidal lithography, are observed to gradually shrink in size and change in shape upon annealing. Top view scanning electron microscopy images reveal that different stages are involved in the closure of monolayer interstices at annealing temperatures in the range between 110°C and 120°C. In the early stages shrinkage of the triangular interstices is dominated by material transport to and thus shortening of their corners, in the late stages interstice area reduction via displacement of the triangle edges becomes significant. At intermediate annealing times the rate of interstice area reduction displays a maximum before a stabilized state characterized by a rounded isosceles triangular shape forms. Copyright © 2014 Materials Research Society.


Brassat K.,University of Paderborn | Brassat K.,Center for Optoelectronics and Photonics Paderborn | Assion F.,Center for Optoelectronics and Photonics Paderborn | Assion F.,University of Paderborn | And 4 more authors.
Physica Status Solidi (A) Applications and Materials Science | Year: 2013

The selective deposition and self-assembly of nanospheres from a colloidal suspension in trenches on silicon surfaces is investigated using conventional light, confocal laser scanning and scanning electron microscopy. Trenches with widths of one to several nanosphere diameters are formed on silicon surfaces by photolithography and reactive ion etching. The spreading knife convective self-assembly technique is employed to distribute the nanosphere suspension on the pre-patterned surface. It is shown that this technique is particularly useful in combination with a functionalized surface where a self-assembled molecular monolayer changes the contact angle such that sphere deposition takes place almost exclusively in the trenches. By this, lines selectively filled with a chain of beads with a length of 0.5 mm have been achieved. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.


Wahle M.,University of Paderborn | Wahle M.,Center for Optoelectronics and Photonics Paderborn | Kitzerow H.-S.,University of Paderborn | Kitzerow H.-S.,Center for Optoelectronics and Photonics Paderborn
Applied Physics Letters | Year: 2015

We present a liquid crystal (LC) infiltrated photonic crystal fiber, which enables the electrical tuning of the position of zero dispersion wavelengths (ZDWs). A dual frequency addressable liquid crystal is aligned perpendicular on the inclusion walls of a photonic crystal fiber, which results in an escaped radial director field. The orientation of the LC is controlled by applying an external electric field. Due to the high index of the liquid crystal the fiber guides light by the photonic band gap effect. Multiple ZDWs exist in the visible and near infrared. The positions of the ZDWs can be either blue or red shifted depending on the frequency of the applied voltage. © 2015 AIP Publishing LLC.

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