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Item Open Access Tailored nanocomposites for 3D printed micro-optics(2020) Weber, Ksenia; Werdehausen, Daniel; König, Peter; Thiele, Simon; Schmid, Michael; Decker, Manuel; Oliveira, Peter William de; Herkommer, Alois; Giessen, HaraldItem Open Access Towards an underdamped thermodynamic uncertainty relation(2020) Fischer, Lukas P.; Seifert, Udo (Prof. Dr.)A recent result of stochastic thermodynamics is the so-called thermodynamic uncertainty relation (TUR). This relation, appearing in the form of an inequality, bounds the precision of fluctuating currents by the entropic costs that are required to drive the non-vanishing mean of the observable. As a consequence, the relation enables the access to parameters that are not accessible in an experimental setting via the precision of a experimentally accessible observable. For instance, it was possible to bound the efficiency of molecular machines by means of their measurable moments of motion. Albeit being generalized and modified to more general terms and dynamics, the putative generalization of the thermodynamic uncertainty relation to underdamped dynamics where the inertia is not negligible remains a puzzling problem. Although there are convincing indications for the overdamped TUR being valid for underdamped dynamics as well in some systems, a straightforward application can also lead to violations of the bound. This thesis summarizes the efforts towards an underdamped generalization of the thermodynamic uncertainty relation and shows challenges and chances that come along by generalization of the TUR. To this end, the intriguing limitations of the TUR in the underdamped domain are explored and discussed. For instance, the TUR is inherently broken for finite times where the evolution is governed by ballistic dynamics due to the inertia being present. Furthermore, it is possible to improve the precision beyond the overdamped bound in presence of velocity dependent forces such as the Lorentz force induced by a magnetic field. Beyond the limitations of the TUR in the underdamped regime, this thesis gives a thorough analysis of the proof that leads to the TUR in the overdamped regime and discusses the obstacles which have to be overcome to find the sought-after proof that is valid for underdamped dynamics. The method is illustrated by deriving thermodynamic bounds that are, however, not as transparent and often not as tight as the original TUR. Finally, a conjecture for a generalized TUR is presented which is based on the precision of free diffusion and holds for all times. The corresponding bound converges to the overdamped TUR in the appropriate limit and tightly bounds the precision, even in the ballistic regime. Being based on free diffusion this conjecture also puts the interpretation of the original TUR in a different perspective.Item Open Access Fusionsforschung : eine Einführung(2020) Köhn-Seemann, AlfIn diesem Vortrag wird ein Überblick und eine Einleitung in das Gebiet der Fusionsforschung gegeben.Item Open Access Perturbation and manipulation of leaky modes in photonic crystal fibers(2020) Upendar, Swaathi; Weiss, Thomas (Apl. Prof. Dr.)Optical fibers guide light in a central core surrounded by a cladding. The most common fibers are step-index fibers, which guide light using total internal reflection in the fiber core. Recently, a new class of fibers, with a microstructured cladding, which also include photonic crystal fibers have been developed. The photonic crystal fibers have a periodic refractive index profile in the cladding and guide light using a bandgap effect or modified total internal reflection. Photonic crystal fibers promise to surpass the guiding properties of the traditional step-index fiber and are being studied extensively. However, these new fibers support leaky modes in contrast to the perfectly guided or bound modes of the conventional step-index fiber. Leaky modes are solutions to Maxwell’s equations that radiate energy in the transverse direction of the fiber. This energy leakage leads to growing fields in the homogeneous exterior. Due to these growing fields in the exterior, the normalization of leaky modes has been a long standing challenge. The normalization for bound modes, which have exponentially decaying fields as we move away from the fiber core, is achieved using an integral of the time-averaged Poynting vector over the xy plane. However, this expression diverges for the case of leaky modes. In this thesis, we derive a general analytical normalization for leaky and bound modes in fiber structures that is independent of the region of integration as long as it encloses all spatial inhomogeneities. Using this analytical normalization, which is an essential factor in any perturbation theory, we develop perturbation theories for interior and exterior perturbations in fiber geometries supporting leaky modes. The perturbations are considered to be changes in the permittivity and permeability tensors of the fiber, which also extend to the axial, i.e., the translationally invariant direction. We formulate the exterior perturbation theory to also treat wavelength as a perturbation. This is highly useful to obtain important fiber quantites such as group velocity as a simple post processing step instead of repeatedly solving Maxwell’s equations for different wavelengths. We demonstrate the accuracy of both perturbation theories on analytically solvable capillary fibers and the more complicated photonic crystal fibers. We also demonstrate the usefulness of a perturbation theory in studying disorder, which involves averaging over many realizations. Furthermore, we present a theoretical study of a novel design to reduce the confinement loss of the fundamental core mode in photonic bandgap fibers with high index strands. This is done by modifying the radius of specific strands, which we call “corner strands”, in the core surround. We demonstrate the usefulness of the analytical normalization in optimizing the fiber design by providing a physically meaningful way of comparing field confinement for different fiber structures. As fundamental working principle, we show that varying the radius of the corner strands leads to backscattering of light back to the core. By using an optimal radius for these corner strands in each transmission window, the losses are decreased by orders of magnitude in comparison to the unmodified cladding structure. We do a parametric analysis of this phenomenon by varying different structural properties such as radius, pitch and the radius-to-pitch ratios to find the optimal design. Thus, we generalize the previously studied case of missing corner strands which only works for certain radius-to-pitch ratios in the first bandgap. This design can be adapted to any photonic bandgap fiber including hollow core photonic crystal fibers and light cage structures.Item Open Access Cavity QED based on room temperature atoms interacting with a photonic crystal cavity : a feasibility study(2020) Alaeian, Hadiseh; Ritter, Ralf; Basic, Muamera; Löw, Robert; Pfau, TilmanThe paradigm of cavity QED is a two-level emitter interacting with a high-quality factor single-mode optical resonator. The hybridization of the emitter and photon wave functions mandates large vacuum Rabi frequencies and long coherence times; features that so far have been successfully realized with trapped cold atoms and ions, and localized solid-state quantum emitters such as superconducting circuits, quantum dots, and color centers Reiserer and Rempe (Rev Modern Phys 87:1379, 2015), Faraon et al. (Phys Rev 81:033838, 2010). Thermal atoms, on the other hand, provide us with a dense emitter ensemble and in comparison to the cold systems are more compatible with integration, hence enabling large-scale quantum systems. However, their thermal motion and large transit-time broadening is a major bottleneck that has to be circumvented. A promising remedy could benefit from the highly controllable and tunable electromagnetic fields of a nano-photonic cavity with strong local electric-field enhancements. Utilizing this feature, here we investigate the interaction between fast moving thermal atoms and a nano-beam photonic crystal cavity (PCC) with large quality factor and small mode volume. Through fully quantum mechanical calculations, including Casimir-Polder potential (i.e. the effect of the surface on radiation properties of an atom), we show, when designed properly, the achievable coupling between the flying atom and the cavity photon would be strong enough to lead to quantum interference effects in spite of short interaction times. In addition, the time-resolved detection of different trajectories can be used to identify single and multiple atom counts. This probabilistic approach will find applications in cavity QED studies in dense atomic media and paves the way towards realizing large-scale, room-temperature macroscopic quantum systems aimed at out of the lab quantum devices.Item Open Access Mass-producible micro-optical elements by injection compression molding and focused ion beam structured titanium molding tools(2020) Ristok, Simon; Roeder, Marcel; Thiele, Simon; Hentschel, Mario; Guenther, Thomas; Zimmermann, André; Herkommer, Alois; Giessen, HaraldItem Open Access Imaging microspectroscopy of functional nanoplasmonic systems(2020) Sterl, Florian; Giessen, Harald (Prof. Dr.)Item Open Access From ground state properties to high energy spectroscopy : extending the application of DMFT for correlated quantum materials(2020) Schmid, Michael; Daghofer, Maria (Prof. Dr.)Strongly correlated electron systems exhibit rich physical phenomena reaching from superconductivity, Kondo- and, Mott physics to novel magnetic phases, which lie beyond most single-particle approaches such as density functional theory (DFT) or static mean-field theory. For many transition metal oxides (TMOs) such as Ca2RuO4 or LiV2O4 this is often a result of the partially filled d shells, leading to many-body wave functions, which cannot expressed as a single-slater determinant. Moreover, within this compounds there is often no clear hierarchy of energy scales, e.g. strong spin-orbit coupling, Hund’s coupling, and crystal-field splitting, making the description with minimal models difficult. The breakdown of the single-particle picture triggered the development of numerous numerical methods (DMFT, DMRG, VCA, . . . ) within the last decades, all aimed at tackling the aforementioned phenomena with complementary approximations. One of the most prominent methods for describing real compounds has become dynamical mean-field theory (DMFT), which in many cases has proven to describe local electronic phenomena in good agreement with experimental results. In this thesis we perform state of the art DFT+DMFT calculations in its single shot approach to complement theoretical k-resolved one-particle spectral functions to neutron and x-ray diffraction experiments on Ca2RuO4 . In the experiment small DC currents were applied to a Ca2RuO4 single-crystal resulting in the stabilization of new nonequilibrium phases. Based on experimentally refined structures, DFT calculations are performed to extract a tight binding model by projecting the correlated t2g -subspace onto maximally localized Wannier orbitals. Within our DMFT calculations spin-orbit coupling (SOC) and the spherical invariant Coulomb interaction are added to calculate spectral functions. The results indicate a semimetalic state with partially gapped Fermi surface in the nonequilibrium phases with elongated RuO6 octahedra. Additionally, we extend the DFT+DMFT scheme by a discretization scheme to obtain core-level spectroscopy data, such as XAS or RIXS spectra. This concept is based on the discretization of the DMFT hybridization function to construct an Anderson impurity model of finite bath sites. The discretized model is then extended by the core levels and core-valence interaction. To include sufficiently large amounts of bath sites, despite using an exact diagonalization (ED) solver, we choose the natural orbital basis as the single particle basis of choice to compute RIXS and XAS spectra.Item Open Access X-ray and Raman scattering studies of novel phases in 3d and 4d transition metal oxides(2020) Fürsich, Katrin; Keimer, Bernhard (Prof. Dr.)Item Open Access Vibrational quenching of weakly bound cold molecular ions immersed in their parent gas(2020) Jachymski, Krzysztof; Meinert, FlorianHybrid ion–atom systems provide an excellent platform for studies of state-resolved quantum chemistry at low temperatures, where quantum effects may be prevalent. Here we study theoretically the process of vibrational relaxation of an initially weakly bound molecular ion due to collisions with the background gas atoms. We show that this inelastic process is governed by the universal long-range part of the interaction potential, which allows for using simplified model potentials applicable to multiple atomic species. The product distribution after the collision can be estimated by making use of the distorted wave Born approximation. We find that the inelastic collisions lead predominantly to small changes in the binding energy of the molecular ion.