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Dissertation
Energy landscapes and dynamics of magnetic nanostructures: disorder, structure, and magnetic field effects
(2020)
The static and dynamic magnetic properties of two-dimensional ensembles of dipole-coupled magnetic nanoparticles (NPs) are investigated from a theoretical perspective. Particular emphasis is put on the correlations between the geometrical arrangements of the NPs, their inherent structural disorder, and the cooperative magnetic dynamics of the nanostructures as a whole. To this aim a significant number of representative two-dimensional NP ensembles with different structural geometries and degrees of disorder are ...
Dissertation
Investigation of diamond nanostructures with incorporated color centers
(2018)
Diamond is a material with excellent properties due to its structure and composition. Its crystal lattice is a perfect host of different defect centers, so-called color centers, whose properties, especially the capability of emitting single photons, can serve in quantum information technologies (QIT). As a high refractive index material only a small part of the photons emitted by these color centers in diamond can be collected. To enhance the photon collection efficiency photonic structures are necessary. They can ...
Dissertation
Efficient Characterisation and Optimal Control of Open Quantum Systems - Mathematical Foundations and Physical Applications
(2015-06-19)
Since no physical system can ever be completely isolated from its environment, the study of open quantum systems is pivotal to reliably and accurately control complex quantum systems. In practice, reliability of the control field needs to be confirmed via certification of the target evolution while accuracy requires the derivation of high-fidelity control schemes in the presence of decoherence.
In the first part of this thesis an algebraic framework is presented that allows to determine the minimal requirements on ...
Dissertation
High-Speed Semiconductor Lasers based on Low-Dimensional Active Materials for Optical Telecommunication
(2012-07-10)
The scope of this work is the fundamental growth, tailoring and characterization of self-organized indium arsenide quantum dots (QDs) and their exploitation as active region for diode lasers emitting in the 1.55 µm range. This wavelength regime is especially interesting for long-haul telecommunications as optical fibers made from silica glass have the lowest optical absorption. Molecular Beam Epitaxy is utilized as fabrication technique for the quantum dots and laser structures. The results presented in this thesis ...
Dissertation
Theory of laser-induced ultrafast structural changes in solids
(2010-04-21)
The present thesis is a contribution to the study of laser-solid interaction.
Despite the numerous applications resulting from the recent use of laser technology, there is still a lack of satisfactory answers to theoretical questions regarding the mechanism leading to the structural changes induced by femtosecond lasers in materials.
We provide here theoretical approaches for the description of the structural response of different solids (cerium, samarium sulfide, bismuth and germanium) to femtosecond laser ...
Dissertation
Laser amplification in excited dielectrics
(2018-02-12)
Today, laser processing of dielectric materials with ultrashort, femtosecond laser pulses finds a great variety of applications. While ablation, cutting or welding of different types of glasses are widespread and popular examples, nanostructuring of transparent crystals or optoporating cells is part of more recent scientific studies. Although the material systems seem to be very different, their interaction with visible and infrared wavelengths is very similar: the light is simply transmitted. This changes drastically ...
Dissertation
Untersuchung der Adsorptionskinetik photoschaltbarer azobenzolfunktionalisierter selbstorganisierter Monolagen mit optischer Frequenzverdopplung
(2011-02-11)
In dieser Arbeit wurde das Adsorptionsverhalten zweier azobenzolfunktionalisierter Liganden auf Goldoberflächen untersucht. Diese Liganden zeichnen sich dadurch aus, dass sie mithilfe von Licht bestimmter Wellenlängen zwischen zwei Isomerisierungszuständen – sowohl in Lösung als auch in den auf der Oberfläche resultierenden Monolage – hin und her geschaltet werden können. Somit ist es möglich, Oberflächen herzustellen, deren physikalische und chemische Eigenschaften zwischen zwei Zuständen variiert werden können. Die ...
Dissertation
Erzeugung remanent stabiler Domänenmuster in austauschverschobenen Dünnschichtsystemen mittels Heliumionenmikroskopie
(2018-04-11)
Künstliche magnetische Domänenmuster in austauschverschobenen Dünnschichtsystemen (EB-Systeme) haben in den letzten Jahren immer größere Bedeutung als Quelle magnetischer Streufeldlandschaften erlangt. Solche Strukturen dienen als Grundlage für den Transport superparamagnetischer Partikel zur Anwendung in mikrofluidischen und biomedizinischen Analysesystemen. EB-Schichtsysteme besitzen aufgrund der Kopplung zwischen einer antiferromagnetischen und einer ferromagnetischen Schicht besondere magnetische Eigenschaften. ...
Dissertation
Domänenwandwiderstandseffekt in künstlich erstellten Domänenwänden nach Ionenbeschuss von magnetischen Zweischichtsystemen
(2014-02-25)
Im Rahmen dieser Arbeit wurden magnetische Zweischichtsysteme mit Exchange-Bias-Effekt und künstlichen lateralen Domänenstrukturen auf den Domänenwandwiderstand (DWR) untersucht. Die Erzeugung dieser Domänenmuster erfolgte über Ion Bombardment induced Magnetic Patterning (IBMP), d.h. 10 keV Helium-Ionenbeschuss durch Schattenmasken auf der Probenoberfläche. Mithilfe eines externen Magnetfelds können anschließend die Domänenwände gezielt ein- und ausgeschaltet werden. Zur Untersuchung des DWR-Effekts müssen einige ...
Dissertation
Metallische Nanopartikel auf Quarz
(2014-08-20)
This work presents the developement of an chemically stable and easy to produce in situ sensor for fast and reliable detection of polycyclic aromatic hydrocarbons (PAH) in low nanomolar concentrations. Metallic nanoparticles on dielectric substrates werde used for the rst time with surface enhanced Raman spectroscopy (SERS) in combination with shifted excitation Raman difference spectroscopy (SERDS).
The preparation of the metallic nanoparticle ensembles with Volmer-Webergrowth is described first. The nanoparticles ...