TY - JOUR A1 - Feudel, Fred A1 - Bergemann, Kay A1 - Tuckerman, Laurette S. A1 - Egbers, C. A1 - Futterer, B. A1 - Gellert, Marcus A1 - Hollerbach, Rainer T1 - Convection patterns in a spherical fluid shell JF - Physical review : E, Statistical, nonlinear and soft matter physics N2 - Symmetry-breaking bifurcations have been studied for convection in a nonrotating spherical shell whose outer radius is twice the inner radius, under the influence of an externally applied central force field with a radial dependence proportional to 1/r(5). This work is motivated by the GeoFlow experiment, which is performed under microgravity condition at the International Space Station where this particular central force can be generated. In order to predict the observable patterns, simulations together with path-following techniques and stability computations have been applied. Branches of axisymmetric, octahedral, and seven-cell solutions have been traced. The bifurcations producing them have been identified and their stability ranges determined. At higher Rayleigh numbers, time-periodic states with a complex spatiotemporal symmetry are found, which we call breathing patterns. Y1 - 2011 U6 - https://doi.org/10.1103/PhysRevE.83.046304 SN - 1539-3755 VL - 83 IS - 4 PB - American Physical Society CY - College Park ER - TY - GEN A1 - Federici, S. A1 - Pohl, Martin A1 - Ruppel, J. A1 - Telezhinsky, Igor O. A1 - Hofmann, Werner A1 - Martinez, M. A1 - Knapp, J. T1 - Design concepts for the Cherenkov Telescope Array CTA BT - an advanced facility for ground-based high-energy gamma-ray astronomy T2 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe N2 - Ground-based gamma-ray astronomy has had a major breakthrough with the impressive results obtained using systems of imaging atmospheric Cherenkov telescopes. Ground-based gamma-ray astronomy has a huge potential in astrophysics, particle physics and cosmology. CTA is an international initiative to build the next generation instrument, with a factor of 5-10 improvement in sensitivity in the 100 GeV-10 TeV range and the extension to energies well below 100 GeV and above 100 TeV. CTA will consist of two arrays (one in the north, one in the south) for full sky coverage and will be operated as open observatory. The design of CTA is based on currently available technology. This document reports on the status and presents the major design concepts of CTA. T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 1325 KW - ground based gamma ray astronomy KW - next generation Cherenkov telescopes KW - design concepts Y1 - 2011 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-430149 SN - 1866-8372 VL - 32 IS - 1325 ER - TY - JOUR A1 - Fechner, Cora T1 - Reconstructing the intergalactic UV background with QSO absorption lines JF - Astronomy and astrophysics : an international weekly journal N2 - We present a new approach to observationally constraining the spectral energy distribution of the intergalactic UV background by studying metal absorption systems. We study single-component metal line systems that exhibit various well-measured species. Among the observed transitions, at least two ratios of ionization stages from the same element are required, e. g. C III/C IV and Si III/Si IV. For each system photoionization models are constructed by varying the spectrum of the ionizing radiation. The spectral energy distribution can then be constrained by comparing the models with the observed column density ratios. Extensive tests with artificial absorbers show that the spectrum of the ionizing radiation cannot be reconstructed unambiguously, but it is possible to constrain the main characteristics of the spectrum. Furthermore, the resulting physical parameters of the absorber, such as ionization parameter, metallicity, and relative abundances, may depend strongly on the adopted ionizing spectrum. Even in case of well-fitting models, the uncertainties can be as high as similar to 0.5 dex for the ionization parameter and up to similar to 1.5 dex for the metallicity. Therefore, it is essential to know the hardness of the UV background when estimating the metallicity of the intergalactic medium. Applying the procedure to a small sample of 3 observed single-component metal line systems yields a soft ionizing radiation at z > 2 and a slightly harder spectrum at z < 2. The resulting energy distributions exhibit strong He II Lya re-emission features, suggesting that reprocessing by intergalactic He II is important. Comparing the observed systems to UV background spectra from the literature indicates that a recent model that includes sawtooth modulation due to reprocessing by intergalactic He II with delayed helium reionization fits the investigated systems very well. KW - intergalactic medium KW - quasars: absorption lines KW - diffuse radiation Y1 - 2011 U6 - https://doi.org/10.1051/0004-6361/201117080 SN - 0004-6361 VL - 532 IS - 2 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Fang, Peng A1 - Wang, Feipeng A1 - Wirges, Werner A1 - Gerhard, Reimund A1 - Basso, Heitor Cury T1 - Three-layer piezoelectrets from fluorinated ethylene-propylene (FEP) copolymer films JF - Applied physics : A, Materials science & processing N2 - A process for preparing three-layer piezoelectrets from fluorinated ethylene-propylene (FEP) copolymer films is introduced. Samples are made from commercial FEP films by means of laser cutting, laser bonding, electrode evaporation, and high-field poling. The observed dielectric-resonance spectra demonstrate the piezoelectricity of the FEP sandwiches. Piezoelectric d (33) coefficients up to a few hundred pC/N are achieved. Charging at elevated temperatures can increase the thermal stability of the piezoelectrets. Isothermal experiments for approximately 15 min demonstrate that samples charged at 140A degrees C keep their piezoelectric activity up to at least 120A degrees C and retain 70% of their initial d (33) even at 130A degrees C. Acoustical measurements show a relatively flat frequency response in the range between 300 Hz and 20 kHz. Y1 - 2011 U6 - https://doi.org/10.1007/s00339-010-6008-2 SN - 0947-8396 SN - 1432-0630 VL - 103 IS - 2 SP - 455 EP - 461 PB - Springer CY - New York ER - TY - JOUR A1 - Evans, Nancy Remage A1 - DeGioia-Eastwood, Kathleen A1 - Gagne, Marc A1 - Townsley, Leisa A1 - Broos, Patrick S. A1 - Wolk, Scott J. A1 - Naze, Yael A1 - Corcoran, Michael A1 - Oskinova, Lidia M. A1 - Moffat, Anthony F. J. A1 - Wang, Junfeng A1 - Walborn, Nolan R. T1 - The search for low-mass companions of b stars in the Carina Nebula cluster trumpler 16 JF - The astrophysical journal : an international review of spectroscopy and astronomical physics ; Supplement series N2 - We have developed lists of likely B3-A0 stars (called "late B" stars) in the young cluster Trumpler 16. The following criteria were used: location within 3' of eta Car, an appropriate V and B - V combination, and proper motion (where available). Color and magnitude cuts have been made assuming an E(B - V) = 0.55 mag +/- 0.1, which is a good approximation close to the center of Trumpler 16. These lists have been cross-correlated with X-ray sources found in the Chandra Carina Complex Project. Previous studies have shown that only very rarely (if at all) do late main-sequence B stars produce X-rays. We present evidence that the X-ray-detected sources are binaries with low-mass companions, since stars less massive than 1.4 M-circle dot are strong X-ray sources at the age of the cluster. Both the median X-ray energies and X-ray luminosities of these sources are in good agreement with values for typical low-mass coronal X-ray sources. We find that 39% of the late B stars based on a list with proper motions have low-mass companions. Similarly, 32% of a sample without proper motions have low-mass companions. We discuss the X-ray detection completeness. These results on low-mass companions of intermediate-mass stars are complementary to spectroscopic and interferometric results and probe new parameter space of low-mass companions at all separations. They do not support a steeply rising distribution of mass ratios to low masses for intermediate-mass (5 M-circle dot) primaries, such as would be found by random pairing from the initial mass function. KW - open clusters and associations: individual (Trumpler 16) KW - stars: massive Y1 - 2011 U6 - https://doi.org/10.1088/0067-0049/194/1/13 SN - 0067-0049 VL - 194 IS - 1 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Eichler, David A1 - Pohl, Martin T1 - Can ultrahigh-energy cosmic rays come from Gamma-ray bursts? - cosmic rays below the ankle and galactic gamma-ray bursts JF - The astrophysical journal : an international review of spectroscopy and astronomical physics ; Part 2, Letters N2 - The maximum cosmic-ray energy achievable by acceleration by a relativistic blast wave is derived. It is shown that forward shocks from long gamma-ray bursts (GRBs) in the interstellar medium accelerate protons to large enough energies, and have a sufficient energy budget, to produce the Galactic cosmic-ray component just below the ankle at 4 x 10(18) eV, as per an earlier suggestion. It is further argued that, were extragalactic long GRBs responsible for the component above the ankle as well, the occasional Galactic GRB within the solar circle would contribute more than the observational limits on the outward flux from the solar circle, unless an avoidance scenario, such as intermittency and/or beaming, allows the present-day local flux to be less than 10(-3) of the average. Difficulties with these avoidance scenarios are noted. KW - cosmic rays KW - galaxies: general KW - gamma-ray burst: general Y1 - 2011 U6 - https://doi.org/10.1088/2041-8205/738/2/L21 SN - 2041-8205 VL - 738 IS - 2 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Döring, Sebastian A1 - Kollosche, Matthias A1 - Rabe, Torsten A1 - Stumpe, Joachim A1 - Kofod, Guggi T1 - Electrically tunable polymer DFB laser JF - Advanced materials Y1 - 2011 U6 - https://doi.org/10.1002/adma.201102465 SN - 0935-9648 VL - 23 IS - 37 SP - 4265 EP - 4269 PB - Wiley-VCH CY - Weinheim ER - TY - THES A1 - Döring, Sebastian T1 - Oberflächengitter in azobenzenhaltigen Schichten für organische DFB-Laser T1 - Surface relief gratings in azobenzene containing layers for organic DFB lasers N2 - Ein neuentwickeltes azobenzenhaltiges Material, das auf einem supramolekularen Konzept basiert, wird bezüglich seiner Strukturbildung während einer holografischen Belichtung bei 488 nm untersucht. Im Mittelpunkt stehen dabei eindimensionale, sinusförmige Reliefs mit Periodizitäten kleiner 500 nm. Es wird gezeigt, wie der Grad der Vernetzung der photosensitiven Schicht die Strukturbildung in diesem Größenbereich beeinflusst. Zur Maximierung der Strukturtiefe werden gezielt Prozessparameter der Belichtung sowie Materialparameter variiert. Unter Standardbedingungen und moderaten Belichtungsintensitäten von ca. 200 mW/cm² bilden sich innerhalb weniger Minuten bei einer Periode von 400 nm Strukturtiefen von bis zu 80nm aus. Durch die Beeinflussung von Materialparametern, wie Oberflächenspannung und Viskosität, wird die maximale Strukturtiefe auf 160nm verdoppelt. Durch Mehrfachbelichtungen wird auch die Bildung von zweidimensionalen Gittern untersucht. Die Originalstrukturen werden in einem Abformverfahren kopiert und in Schichten von unter UV-Licht aushärtenden Polymeren übertragen. Durch das Abformen kommt es zu einer geringfügigen Verschlechterung der Oberflächenqualität sowie Abnahme der Strukturtiefe. Dieser Verlust wird durch eine Verringerung der Prozesstemperatur verringert. Mithilfe kopierter Oberflächengitter werden organische Distributed Feedback-(DFB)-Laser zweiter Ordnung hergestellt, um den Einfluss von Gitterparametern auf die Emissionseigenschaften dieser Laser zu untersuchen. Dazu erfolgt zunächst die Charakterisierung der optischen Verstärkungseigenschaften ausgewählter organischer Emittermaterialien mittels der Variablen Strichlängenmethode. Das mit dem Laserfarbstoff Pyrromthen567 (PM567) dotierte Polystyrol (PS) zeigt dabei trotz konzentrationsbedingter geringer Absorption eine vergleichsweise geringe Gewinnschwelle von 50µJ/cm² bei ca. 575 nm. Das aktive Gast-Wirt-System der konjugierten Polymere MEH-PPV und F8BT* weist eine hohe Absorption und eine kleine Gewinnschwelle von 2,5 µJ/cm² bei 630 nm auf. Dieses Verhalten spiegelt sich auch in den Emissionseigenschaften der damit hergestellten DFB-Laser wieder. Die Dicke der aktiven Schichten liegen im Bereich hunderter Nanometer und wird so eingestellt, dass sich nur die transversalen Grundmoden im Wellenleiter ausbreiten können. Die Gitterperiode sind so gewählt, dass ein Lichtmode im Verstärkungsbereich des Emittermaterials liegt. Die Emissionslinien der Laser sind mit FWHM-Werten von bis zu 0,3 nm spektral sehr schmalbandig und weisen auf eine sehr gute Gitterqualität hin. Die Untersuchungen liefern minimale Laserschwellen und maximale differentielle Effizienzen von 4,0µJ/cm² und 8,4% für MEH-PPV in F8BT* (bei ca. 640nm) sowie 80 µJ/cm² und 0,9% für PM567 in PS (bei ca. 575 nm). Die Vergrößerung der Strukturtiefe von 40nm auf 80nm in mit MEH-PPV dotierten F8BT*-Lasern zu einem deutlichen Anstieg der ausgekoppelten Energie sowie der differentiellen Effizienz und einem geringen Absinken der Laserschwelle. Dies ist ein Resultat der erhöhten Kopplung von Lasermode und Gitter. Die Emission von DFB-Lasern mit zweidimensionalen Oberflächengittern zeigen eine Verringerung der Divergenz aber kein Einfluss auf die Laserschwelle. Abschließend erfolgt eine Vermessung der Photostabilität von DFB-Lasern unter verschiedenen Bedingungen. Das Einbringen eines konjugierten Polymers in eine aktive Matrix sowie der Betrieb in einer Stickstoffatmosphäre führen dabei zu einer Erhöhung der Lebensdauer auf über eine Million Pulse. Durch die Kombination von Oberflächengittern in PDMS-Filmen mit elektroaktiven Substraten wird eine elektrisch steuerbare Deformation des Beugungsgitters erreicht und auf einen DFB-Laser übertragen. Die spannungsinduzierte Verformung wird zunächst in Beugungsexperimenten charakterisiert und ein optimaler Arbeitspunkt bestimmt. Mit den beiden Elastomeren SEBS12 und VHB4910 werden in den Gittern maximale Periodenänderungen von 1,3% bzw. 3,4% bei einer Steuerspannung von 2 kV erreicht. Der Unterschied resultiert aus den verschiedenen Elastizitätsmoduln der Materialien. Übertragen auf DFB-Laser resultiert eine Variation der Gitterperiode senkrecht zu den Gitterlinien in einer kontinuierlichen Verschiebung der Emissionswellenlänge. Mit einem Spannungssignal von 3,25 kV wird die schmalbandige Emission eines elastischen DFB-Lasers kontinuierlich um fast 50nm von 604 nm zu 557 nm hin verschoben. Aus dem Deformationsverhalten sowohl der reinen Beugungsgitter als auch der Laser werden Rückschlüsse auf die Elastizität der verwendeten Materialien gezogen und erlauben Verbesserungen der Bauteile. N2 - The photoinduced surface relief formation in a newly developed azobenzene containing material is investigated. A photosensitive film based on a supramolecular material concept is illuminated in a holographic setup with light at 488 nm leading to sinusoidal surface structures. It can be shown how the degree of network formation within the material influences structure formation at periods below 500nm. Different material and process parameters are varied to determine maximum possible structure amplitude. At moderate recording conditions and illumination intensities of 200 mW/cm² maximum structure amplitudes of 80 nm are formed within several minutes at periods of approximately 400 nm. Changing material parameters like surface tension and viscosity leads to an increase of maximum amplitude to 160 nm. Additionally, twodimensional structures are fabricated by multiple recording steps. Original surface structures are copied by soft lithography methods into films of UV cured polymers. This replication leads to a decrease of structure contrast and increase of surface roughness. It is shown that structure contrast is preserved by reduction of process temperature. Organic second order distributed feedback (DFB) lasers are prepared with the help of replicated surface gratings covered with organic active layers with thicknesses of several hundred nanometers. The lasers are investigated regarding the influence of grating amplitude on laser threshold and slope efficiency. For this, the optical gain of two green/red emitter materials, polystyrene (PS) doped with laser dye pyrromethene567 (PM567) and active guest host system of conjugated polymers MEH-PPV and F8BT*, is measured. PS doped with PM567 shows relatively low gain threshold of 50 µJ/cm² at 575 nm caused by the low dye concentration while F8BT* doped with MEH-PPV shows threshold gain of 2.5 µJ/cm² at 630 nm. Similar behavior is observed during laser action. Following Bragg-condition grating periods at approximately 400nm lead to laser emission within the gain spectra of the materials. DFB lasers show single mode narrow line emission with FWHMs of down to 0.3nm. This is a result of high grating quality. Lasers based on MEH-PPV-F8BT* exhibit laser thresholds and slope efficiencies of 4.0µJ/cm² and 8.4%, respectively. PM567-PS lasers exhibit 80 µJ/cm² and 0.9%. An increase of grating amplitude of a MEH-PPV-F8BT*-DFB laser from 40nm to 80nm leads to a considerable increase of energy output and a slight decrease of laser threshold. This is a result of an increase of coupling between light mode and grating which relates to an increase of reflectivity. The use of a two dimensional grating leads to a decrease of divergence angle of laser emission but shows no influence on laser threshold. Finally, laser lifetime is observed under different conditions. Doping the conjugated polymer MEH-PPV in an active matrix in combination with the allocation of nitrogen atmosphere leads to an prolongation of half life time to several million pulses. The combination of elastic surface relief gratings with electro active substrates enables electrical controlled deformation of the surface structure. This deformation is characterized by diffraction measurements. At optimal working conditions electro active substrates based on elastomers SEBS12 and VHB4910 show maximum grating deformation of 1.3% und 3.4% at actuation voltages of 2kV, respectively. The different results are caused by the different elastic modulus of the elastomer materials. The transfer of deformation principle to elastic DFB lasers leads to a continuously shift of emission wavelength. At an actuation voltage of 3.25kV an elastic DFB laser based on polyvinylacetate doped with PM567 shows a shift of nearly 50 nm, from 604 nm to 557 nm. Deformation behavior of grating and laser allow to draw conclusions on elasticity of the material and with this is tool for the improvement of the device. KW - Oberflächengitter KW - Azobenzen KW - Holografie KW - DFB-Laser KW - Wellenlängenverschiebung KW - surface relief grating KW - azobenzene KW - holography KW - DFB laser KW - wavelength shift Y1 - 2011 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus-59211 ER - TY - JOUR A1 - Douce, Laurent A1 - Suisse, Jean-Moise A1 - Guillon, Daniel A1 - Taubert, Andreas T1 - Imidazolium-based liquid crystals a modular platform for versatile new materials with finely tuneable properties and behaviour JF - Liquid crystals : an international journal of science and technology N2 - Ionic liquid Crystals constitute highly versatile materials that have drawn much interest these past few years in the fields of academic research and industrial development. In this respect, the present article is intended as an update of K. Binnemans review published in 2005, but focusing exclusively on the imidazolium cation - the most widely studied. Herein, imidazolium-containing thermotropic liquid crystalline materials will be sorted by molecular structure (mono-, bis-, poly-imidazolium compounds, with symmetrical and non-symmetrical structures) and discussed. Their physico-chemical properties will be exposed in order to adduce the relevancy and potential of the imidazolium platform in various fields of research. KW - imidazolium KW - liquid crystal KW - ionic liquid Y1 - 2011 U6 - https://doi.org/10.1080/02678292.2011.610474 SN - 0267-8292 VL - 38 IS - 11-12 SP - 1653 EP - 1661 PB - Routledge, Taylor & Francis Group CY - Abingdon ER - TY - THES A1 - Dosche, Carsten T1 - Funktionale Farbstoffe und ihre Photophysik und Anwendung in komplexen Matrizes Y1 - 2011 CY - Potsdam ER -