Universität Stuttgart
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Item Open Access Ultrafast near- and mid-infrared laser sources for linear and nonlinear spectroscopy(2016) Steinle, Tobias; Giessen, Harald (Prof. Dr.)Item Open Access Functional complex plasmonics : understanding and realizing chiral and active plasmonic systems(2016) Yin, Xinghui; Giessen, Harald (Prof. Dr.)The present thesis concerns itself with the theoretical study and experimental realization of complex plasmonic systems for highly integrated nanophotonic devices and enhanced chiroptical spectroscopy. In particular, the two broad topics of active metasurfaces and chiral plasmonic systems are investigated to this end. In this context, the chalcogenide phase change material GeSbTe is utilized to demonstrate, for the first time, metasurface based beam steering and varifocal lensing devices. The versatility of this approach to lending active functionality to plasmonic systems is further evidenced through our realization of a chiral plasmonic system that both exhibits a wavelength tunable and handedness switchable chiroptical response. Furthermore, in order to enable a systematic study of plasmon- enhanced chiroptical spectroscopy, we rst establish and analyze canonical chiral plasmonic building blocks, in particular, the loop wire and chiral dimer structure. The results from this undertaking lead to fundamental insights for understanding complex chiral plas- monic systems. Finally, we implement chiral media in the commercial electromagnetic full- field solver Comsol Multiphysics to carry out rigorous numerical studies of the macroscopic electrodynamic processes involved in plasmon-enhanced circular dichroism spectroscopy revealing both substantial enhancement due to near-field effects as well as upper boundaries to the magnitude of such enhancements.Item Open Access Spectroscopic investigations of the magnetic anisotropy of lanthanide- and cobalt-based molecular nanomagnets(2016) Rechkemmer, Yvonne; Slageren, Joris van (Prof. Dr.)Single-molecule magnets are metal complexes exhibiting an energy barrier for spin reversal, leading to magnetic bistability and slow relaxation of the magnetization. Their potential for practical applications such as high-density magnetic data storage was recognized early on and with the goal of achieving high energy barriers, different kinds of single-molecule magnets have been synthesized. The quadratic dependence of the barrier height on the spin motivated chemists to synthesize metal complexes with very high total spins; however, with limited success. It was shown that high spins come along with low anisotropies and increased interest thus focused on the synthesis and investigation of (mononuclear) complexes of highly anisotropic metal centers, e.g. lanthanide or cobalt complexes. Although rather high energy barriers can be achieved in such systems, practical application remains problematic and has not been realized yet. Reasons are for example the lack of rational design criteria and the complex interplay of different magnetic relaxation pathways. The aim of this work was therefore the comprehensive magnetic and spectroscopic investigation of selected molecular lanthanide and cobalt compounds in order to obtain a deeper insight into the correlation of molecular and electronic structures as well as the corresponding magnetic properties. The applied spectroscopic methods included electron paramagnetic resonance spectroscopy, far-infrared spectroscopy and optical methods. Special emphasis was placed on magnetic circular dichroism (MCD) spectroscopy, which served as a main tool for electronic structure determination. However, since the MCD-spectrometer was not part of the available experimental equipment at the University of Stuttgart, its design, setup and characterization were the first part of this work. In the further course of this work MCD-spectroscopy was employed for the electronic structure determination of selected lanthanide and cobalt compounds. The studied lanthanide compounds were literature-known molecular tetra-carbonates of erbium (1-Er) and dysprosium (1-Dy). Detailed magnetometric studies showed that both 1-Er and 1-Dy are field-induced single-molecule magnets; however, 1-Er and 1-Dy show significant differences in their magnetic relaxation behavior. The magnetic studies were complemented by detailed spectroscopic investigations.The combination of far-infrared-, luminescence- and MCD-spectroscopy allowed for the experimental determination of 48 energy levels for 1-Er and 55 levels for 1-Dy, which built the foundation for the subsequent crystal field analysis and electronic structure determination. In addition, the results of EPR-spectroscopic studies were used for fine-tuning and verifying the respectively determined crystal field parameters. Calculating the magnetic dipole strengths for transitions between the relevant states led to a quantitative understanding of the magnetic relaxation pathways. Besides the investigation of lanthanide compounds, this thesis deals with two classes of cobalt complexes. The first class comprises mononuclear complexes in which one Co(II) ion is ligated by the nitrogen donors of two doubly deprotonated 1,2-bis(methanesulfonamido)-benzene-ligands. Rather acute N-Co-N bite angles indicate strong deviations from ideal tetrahedral symmetry. The static magnetic properties hint at very high energy barriers for spin reversal and with the help of far-infrared spectroscopy, largely negative axial zero-field splitting parameters were determined. The corresponding energy barriers belong to the highest ever reported for 3d-transition metal complexes and investigating the dynamic magnetic properties confirmed single-molecule magnet behavior. The unique magnetic properties were fully explained by analyzing spectroscopic results. The MCD-spectra showed intense signals that were assigned to spin-allowed d-d-transitions. Subsequent crystal field analysis revealed that the strong axial crystal field generated by the ligands leads to a large splitting of the electronic terms and thus in turn to a relatively small energy gap between the electronic ground state and the first excited state. The resulting increase in second-order spin-orbit coupling explains the high energy barriers observed in the studied complexes. The second class of cobalt compounds studied in this work included dimers of distorted octahedrally coordinated Co(II) ions bridged by symmetrical or asymmetrical quinone based bridging ligands. The main focus of investigation lay on the impact of the bridging ligand on the magnetic coupling between the cobalt centers. Thus, the magnetic properties of the complexes were studied with the help of static susceptibility and magnetization measurements and analyzed by means of different models. Depending on the bridging ligand, different signs for the exchange coupling constants were found. The varying signs can be explained by different relative contributions of possible exchange paths, influenced by the different substituents at the bridging ligands or slight geometry differences. The observations indicate that electron withdrawing substituents favor ferromagnetic couplings, which are preferred in the context of molecular magnetism. All in all, it can be concluded that this work provides a contribution to the deeper understanding of the features relevant for single-molecule magnets. The electronic structure determination for selected lanthanide and cobalt complexes applying advanced magnetometric and spectroscopic techniques not only led to an understanding of the static and dynamic magnetic properties but also allowed for the development of design criteria and new approaches for improved single-molecule magnets in the future.Item Open Access Realisierung von Balanced Gain and Loss in einem Bose-Hubbard-Modell mit zeitabhängigen Potentialen(2016) Dizdarevic, DanielItem Open Access Spectroscopic investigation of metallic nanostructures towards percolation(2016) De Zuani, Stefano; Dressel, Martin (Prof. Dr.)In this work, we focus on the investigation of the geometrical parameters leading to the tuning of the electrodynamic properties of ultra-thin metallic films at and around the percolation threshold and of metal-dielectric nanostructures with effective optical properties.Item Open Access Excitonic Fano resonances in Ta2NiSe5 and Ta2NiS5(2016) Larkin, Timofei I.; Keimer, Bernhard (Prof. Dr.)Item Open Access Chiral plasmonic near-field sources : control of chiral electromagnetic fields for chiroptical spectroscopies(2016) Schäferling, Martin; Giessen, Harald (Prof. Dr.)This thesis investigates the chiral near-field response of plasmonic nanostructures. The chiral properties of electromagnetic fields can be quantified by the so-called optical chirality, which is a figure that can be directly calculated from basic field properties. The larger the optical chirality is, the stronger the respective field will interact with chiral molecules. In principle, electromagnetic fields with high optical chirality enable the detection of the handedness of chiral molecules with enhanced sensitivity. This is of major importance in biochemistry and pharmaceutics because biological processes essentially depend on the handedness of the involved molecules. We introduce the concept of chiral plasmonic near-field sources to aid the respective chiroptical spectroscopy techniques. We cover two main topics in our systematic analysis: Firstly, what are the conditions and mechanisms to generate and enhance chiral near-fields? And secondly, which requirements for chiral plasmonic near-field sources exist and how can they be fulfilled? For this, we group the near-field sources regarding the chiral symmetry properties of both the nanostructure as well as the incident light. Both of these constituents influence the properties of the resulting chiral plasmonic near-field sources. Chiral nanostructures offer the possibility to enhance the optical chirality of the incident light. We show that planar chirality can lead to regions with chiral near-fields of uniform handedness. Regions with opposite handedness are clearly separated by the structure plane. Three-dimensionally chiral structures can exhibit chiral hot-spots where particularly strong optical chirality can be found. Based on our investigations of chiral structures, we introduce the concept of plasmonic racemates, which are mixtures of both handednesses of the chiral nanostructure. The local interaction with the chiral building blocks allows for the generation of chiral near-fields although the achiral superstructure exhibits no chiroptical far-field response. This enables chiroptical spectroscopy without additional contributions to the signal due to the presence of a chiral structure. Furthermore, we present a concept for metasurfaces that facilitate plasmonic racemates with particularly high integration density. The combination of an achiral linear plasmonic nanoantenna with linearly polarized light demonstrates that chiral near-fields can be formed locally in systems without structural chirality. This can be attributed to interference between incident and scattered light, as shown in our analysis. Based on this finding, we propose a chiroptical spectroscopy method that utilizes linearly polarized light instead of the circular polarization that is commonly used. Furthermore, we demonstrate that the eigenmodes of chiral systems can lead to particularly strong and extended chiral near-fields. The most efficient way to excite these modes is linearly polarized light. We obtained the best results from a design consisting of four intertwined helices. Chiral near-fields have been found in the whole volume surrounded by the structure. In addition, we discuss a configuration of slanted slits on top of a mirror. This design is easy to fabricate and enables chiroptical spectroscopy via reflection measurements. In summary, we provide fundamental insights into the functioning as well as the properties of chiral plasmonic near-field sources. We show that this concept can be used for highly sensitive enantiomer discrimination and how this can be accomplished. Furthermore, we provide a theoretical basis to optimize chiral plasmonic near-field sources.Item Open Access Chiral metamaterials(2016) Eslami, Sahand; Fischer, Peer (Prof. Dr.)Item Open Access General properties of ionic complex fluids(2016) Bier, Markus; Dietrich, Siegfried (Prof. Dr.)Item Open Access Structural and electronic properties of nickelate heterostructures(2016) Wrobel, Friederike; Keimer, Bernhard (Prof. Dr.)The fabrication of thin films and multilayers has led to the discovery of novel functional properties which are widely used in electronic devices nowadays. The limit of such a material design is atomic layer-by-layer deposition which was made possible through shuttered molecular beam epitaxy (MBE) growth. In the course of this thesis project a newly developed oxide MBE system was used to grow two different types of nickelate heterostructures, namely superlattices (SLs) consisting of metallic and paramagnetic LaNiO3 sandwiched between a large band-gap insulator and a combination of lanthanum nickelate and cuprate layers into a single hybrid structure. The former type was intensively studied in the last years and a transition to a weakly insulating, antiferromagnetically ordered state was observed in samples where the LaNiO3 thickness had been reduced to only two unit cells. So far little was known about the influence of the growth method on the defects in the samples and consequently on their physical properties. The use of oxide MBE enabled us to improve the overall sample quality of nickelate SLs and to design a novel material. We first optimized the growth of LaNiO3 and thoroughly analyzed the heterostructures by synchrotron-based x-ray diffraction, transmission electron microscopy, and temperature-dependent electrical resistivity. Furthermore we conducted in-depth studies, including x-ray absorption and magneto-transport measurements. The knowledge gained thereby was used grow new, layered nickelate-cuprate hybrid structures with novel electronic and magnetic properties.