Universität Stuttgart
Permanent URI for this communityhttps://elib.uni-stuttgart.de/handle/11682/1
Browse
171 results
Search Results
Item Open Access Investigating superconductivity by tunneling spectroscopy using oxide heterostructures(2017) Fillis-Tsirakis, Evangelos; Mannhart, Jochen (Prof. Dr.)Item Open Access Nonlinear optical microspectroscopy with few-cycle laser pulses(2017) Wan, Hui; Wrachtrup, Jörg (Prof. Dr.)Nonlinear optical (NLO) microscopy is a powerful tool in physics, chemistry, and material science it probes intrinsic optical properties of the sample without the need of labeling. In order to investigate the ultrafast processes in nonlinear materials with high spatial resolution, we need to combine both ultrashort pulses and techniques focusing them to the diffraction limit. Previously, few-cycle laser pulses have often been tightly focused using conventional microscope objectives. However, the propagation of an ultrashort pulse in optical materials, particularly in the glass of a high numerical aperture (N.A.) microscope objective, results in spatial and temporal distortions of the pulse electric field, which can severely affect its quality in the focus. By purely passive group delay dispersion (GDD) and third-order dispersion (TOD) management, in this thesis, we experimentally demonstrate in-focus diffraction-limited and bandwidth-limited few-cycle pulses by using high N.A. objectives. Based on these achievements, the performance of a novel few-cycle NLO microscope for both second-harmonic generation (SHG) imaging and microspectroscopy in the frequency- and time-domains was characterized. The inverse linear dependence of SHG intensity on the in-focus pulse duration was demonstrated down to 7.1 fs for the first time. The application of shorter in-focus pulses for the enhancement of SHG image contrast was successfully demonstrated on a single collagen (type-I) fibril as a biological model system for studying protein assemblies under physiological conditions. Beyond imaging, a collagen fibril has been found to act as a purely non-resonant χ(2) soft matter under the present excitation conditions, and its ratio of forward- to epi-detected SHG intensities allowed for the estimation of the fibril thickness, which corresponds well with atomic force microscopy (AFM) measurements. The ultrafast dephasing of the localized surface plasmon resonance (LSPR) in the metallic nanoparticles, that only occurs on a time scale of a few femtoseconds, has gained a lot of attraction in the field of nanoplasmonics. This thesis is the first systematic experimental demonstration of time-resolving ultrashort plasmon dephasing in single gold nanoparticles by using interferometric SHG spectroscopy with in-focus 7.3 fs excitation pulses in combination with linear scattering spectroscopy performed on the same nanoparticle. For nanorods, nanodisks, and nanorectangles, strong plasmon resonance enhanced SHG is observed, where the SHG intensity strongly depends on the spectral overlap between the LSPR band and the excitation laser spectrum. For single nanorods and nanorectangles, the polarization dependence of the SHG intensity was found to follow second-order dipole scattering, and the effect of size and shape on the LSPR properties was directly observed in the time-domain. Good agreement between experimental and simulated values of dephasing times and resonance wavelengths is obtained, which confirms that a common driven damped harmonic oscillator model for the LSPR in the nanoparticle can qualitatively explain both the linear scattering spectra in the frequency-domain and the SHG response in the time-domain. Resonance bands in linear transmission and scattering spectra have also been observed for nanoholes with sizes smaller than the wavelength of the incident light in a metal film, which are assigned to LSPR modes of the electric field distribution around the nanohole with qualitatively similar resonance properties as a nanoparticle. The polarization-resolved nonlinear optical properties of the single nanoholes with different shapes and symmetries were also reported. The objective of this thesis has been systematic SHG studies of the size effect in the LSPR of single nanoholes in metal films and of their ultrafast dephasing dynamics. Although, enhancement of both the forward- and epi-detected SHG emissions from single rectangular nanoholes are observed,however,no ultrafast dephasing dynamics of LSPRs in rectangular nanoholes could be time-resolved with our in-focus 7.3 fs excitation laser pulses, which indicates that contributions from LSPR enhanced SHG to the detected SHG signal are negligible. More work needs to be done in order to overcome the current experimental limitations. However, in this thesis, the polarization dependence of the forward- and epi-detected SHG intensity from the single rectangular nanohole was found to follow that of a second-order dipole pattern. While the SHG dipole pattern observed for rectangular nanoparticles is oriented parallel to its long-axis, the SHG dipole pattern of its complementary rectangular nanohole is oriented perpendicular to its long-axis. This observation represents the first experimental demonstration of Babinet’s principle in second-order nonlinear scattering of a single rectangular nanohole in a gold film.Item Open Access From Hermitian to non-Hermitian topological phases of matter(2019) Rui, Wenbin; Metzner, Walter (Prof. Dr.)The focus of this thesis lies on extending the theory of topological phases of matter from the Hermitian to the non-Hermitian regime. This includes not only the extension of conventional concepts such as topological invariants and topological boundary states in the theory of Hermitian topological phases, but also the exploration and characterization of entirely new topological phases unique to non-Hermitian systems.Item Open Access Structure and electronic properties of epitaxial monolayer WSe2(2019) Mohammed, Avaise; Takagi, Hidenori (Prof. Dr.)Item Open Access Die Bedeutung der Kontrolle über mikroskopische Freiheitsgrade für die Effizienz optimierter Maschinen(2017) Bauer, Michael; Seifert, Udo (Prof. Dr.)Item Open Access Bose-Einstein condensates with balanced gain and loss beyond mean-field theory(2017) Dast, Dennis; Wunner, Günter (Prof. Dr.)Most of the work done in the field of Bose-Einstein condensates with balanced gain and loss has been performed in the mean-field approximation using the non-Hermitian PT-symmetric Gross-Pitaevskii equation. However, the exchange of particles with the environment plays a crucial role in such systems which in general leads to deviations from the mean-field behavior. Thus, it is not clear whether a mean-field approach is appropriate. It is the purpose of this work to formulate and study a many-particle description of a Bose-Einstein condensate with balanced gain and loss. This is achieved by using a quantum master equation describing a double well where the incoupling of particles in one well and the outcoupling from the other are implemented with Lindblad superoperators. The in- and outcoupling rates are adjusted in an appropriate manner such that balanced gain and loss is achieved. It is shown that the mean-field limit of this master equation yields a PT-symmetric Gross-Pitaevskii equation. Furthermore the master equation supports the characteristic dynamical properties of PT-symmetric systems. There are, however, fundamental differences compared with the mean-field description revealing a new generic feature of PT-symmetric Bose-Einstein condensates. It is shown that the purity of the condensate periodically drops to small values but then is nearly completely restored, when the particles oscillate in the double well. Since in the mean-field limit a completely pure condensate is assumed, this effect cannot be covered by the Gross-Pitaevskii equation. These purity oscillations have a direct impact on the average contrast in interference experiments. In particular it is found that the extrema of the purity can be precisely measured since the average contrast at these points is not reduced by an imbalance of the particle distribution. To gain a detailed understanding of the purity oscillations, analytic solutions for the dynamics in the non-interacting limit are presented and the Bogoliubov backreaction method is used to discuss the influence of the on-site interaction. A central result is that the strength of the purity revivals does neither depend on the amount of particles in the system nor the interaction strength, but is almost exclusively determined by the strength of the in- and outcoupling processes. However, the strong revivals are shifted towards longer times for larger particle numbers. Without interaction this would make the purity oscillations unobservable for a realistic particle number, but by adjusting the interaction strength the strong revivals again occur earlier.Item Open Access Optical and magneto-optical investigations on 3D Dirac- and Weyl-semimetals(2017) Neubauer, David; Dressel, Martin (Prof. Dr.)This work concentrates on optical investigations on 3D Dirac- and Weyl-semimetals with and without applied magnetic fields. Four compounds are extensively discussed, namely the 3D Dirac semimetal Cd3As2, the Weyl semimetals TaAs and NbP, and finally evidence is found for 2D Dirac states in the iron based superconductor FeSe. For the measurements in magnetic fields a novel magneto-optical installation is designed and implemented in the lab. The design principle and characterization of this setup is presented.Item Open Access High-order methods for computational astrophysics(2015) Núñez-de la Rosa, Jonatan; Munz, Claus-Dieter (Prof. Dr.)In computational fluid dynamics, high-order numerical methods have gained quite popularity in the last years due to the need of high fidelity predictions in the simulations. High-order methods are suitable for unsteady flow problems and long-term simulations because they are more efficient when obtaining higher accuracy than low-order methods, and because of their outstanding dissipation and dispersion properties. In the present work, the development and application of three high-order numerical methods, namely, the conservative finite difference (FD) method, the finite volume (FV) method, and the discontinuous Galerkin spectral element method (DGSEM), is presented. These methods are used here for solving three equations systems arising in computational astrophysics on flat spacetimes, specifically, the ideal magnetohydrodynamics (MHD), relativistic hydrodynamics (SRHD) and relativistic magnetohydrodynamics (SRMHD). Our computational framework has been subject to the standard testbench in computational astrophysics. Numerical results of problems having smooth flows, and problems with shock-dominated flows, are also reported. Finite volume methods are numerical methods based on the weak solution of conservation laws in integral form. Unlike finite volume methods, where cell averages of the solution are evolved in time, in the conservative finite difference schemes only the solution at specific nodal points are considered. This difference offers a high efficiency of finite difference over finite volume methods in two and three dimensional high-order calculations because of the form of the utilized stencils in the reconstruction step. Recently, a lot of effort has been put into the development of efficient high-order accurate reconstruction procedures on structured and unstructured meshes. The most widely used procedure to achieve high-order spatial accuracy in finite volume and conservative finite difference methods is the WENO reconstruction. The basic idea of the WENO schemes is based on an adaptive reconstruction procedure to obtain a higher-order approximation on smooth regions while the scheme remains non-oscillatory near discontinuities. For this reason, the WENO formulation is particularly effective when solving conservation laws containing discontinuities. In this work, the FD and FV methods are extended to very high-order accuracy on regular Cartesian meshes by making use of the arbitrary high-order reconstruction WENO operator. The time discretization is carried out with a strong stability-preserving Runge-Kutta (SSPRK) method. The MHD, SRHD and SRMHD equations are then solved with these two methods for problems having strong shock configurations. The discontinuous Galerkin (DG) methods combine the ideas of the finite element (FE) and the finite volume methods. From the FE methods, the solution and test functions in the variational formulation of the conservation law are locally represented by polynomials, allowing to be discontinuous at element faces. In order to stabilize the scheme, from the FV methods are borrowed the ideas of using Riemann solvers, which permit to connect a given element with its direct neighboring ones. One special case in the family of DG methods is the DGSEM. In these methods, the domain is decomposed into quadrilateral/hexahedral elements, and the solution and the fluxes are represented by tensor-product basis functions (high-order Lagrangian interpolants). The integrals are approximated by quadrature, and the nodal points, where the solution is computed, are the Gauss-Legendre quadrature points. With these choices, the DG operator has a dimension-by-dimension splitting form, which yields more efficiency due to less operations and less memory consumption. In this work, the DGSEM has been also extended to the equations of computational astrophysics on flat spacetimes, but restricted only to the MHD and SRHD equations. Because discontinuous solutions form part of the nature of the hyperbolic conservation laws, shock capturing strategies have to be devised, especially for the discontinuous Galerkin method. For the DGSEM, a hybrid DG/FV shock capturing approach is used as the main building block for stabilization of the solution when shocks take place. The hybrid DGSEM/FV is constructed in such a way that, in regions of smooth flows, the DGSEM method is employed, and those parts of the flow having shocks, the DGSEM elements are interpreted as quadrilateral/hexahedral subdomains. In each of these subdomains, the nodal DG solution values are used to build a new local domain composed now of finite volume subcells, which are evolved with a robust finite volume method with third order WENO reconstruction. This new numerical framework for computational astrophysics based on the hybridization of high-order methods brings very promising results.Item Open Access Fluoreszenzdetektiertes Schalten im optischen Nahfeld(2018) Heilig, Mark; Wrachtrup, Jörg (Prof. Dr.)In dieser Abhandlung wird ein neuer Ansatz für hochdichte optische Speichersysteme auf Basis isolierter, dicht gepackter, photochromer Nanostrukturen (Dots) überprüft und weiterentwickelt. Zum Nachweis der dicht gepackten Fulgid-Nanostrukturen mit Strukturabständen weit unterhalb des Beugungslimits über deren Fluoreszenemission wurde ein multifunktionales Raster-Mikroskopiesystem (Konfokal, AFM, SNOM) entwickelt und aufgebaut. Als Sonde werden geätzte Glasfaserspitzen verwendet. Die Fluoreszenzeigenschaften der verwendeten Fulgide bei Raumtemperatur werden bestimmt. Die Korrelation zwischen Schaltzustand und Fluoreszenzemission des C-Isomer der Fulgide wird nachgewiesen. Schaltexperimente an unregelmäßig strukturierten Fulgid-Schichten zeigen die prinzipielle Eignung des Konzepts. An regelmäßig angeordneten Strukturen werde in programmgesteuerten Speicherversuchen Informationen reversibel in die einzelnen Speicherzellen geschrieben, gelesen und gelöscht. Für kleinste Fulgid-Strukturen in hexagonaler Anordnung mit einem Strukturabstand von 0.100µm ergibt sich eine Speicherdichte von 173GBit=in². Aufgrund der Anordnung treten in hochauflösenden SNOM-Messungen Überlagerungseffekte auf. Diese werden mit Hilfe der FEM in der Simulation untersucht, analysiert und erklärt. Eine Alternative zu Apertur-Sitzen im SNOM stellen lokal emittierende Lichtquellen dar. Ein Nanodiamant mit Stickstoff-Defekt-Zelle eignet sich als Emitter. Die Wechselwirkung mit einer Gold-Struktur wird in der FEM simuliert und experimentell bestätigt.Item Open Access Ultrafast near- and mid-infrared laser sources for linear and nonlinear spectroscopy(2016) Steinle, Tobias; Giessen, Harald (Prof. Dr.)