TY - JOUR A1 - Opolka, Alexander A1 - Müller, Dominik A1 - Fella, Christian A1 - Balles, Andreas A1 - Mohr, Jürgen A1 - Last, Arndt T1 - Multi-lens array full-field X-ray microscopy JF - Applied Sciences N2 - X-ray full-field microscopy at laboratory sources for photon energies above 10 keV suffers from either long exposure times or low resolution. The photon flux is mainly limited by the objectives used, having a limited numerical aperture NA. We show that this can be overcome by making use of the cone-beam illumination of laboratory sources by imaging the same field of view (FoV) several times under slightly different angles using an array of X-ray lenses. Using this technique, the exposure time can be reduced drastically without any loss in terms of resolution. A proof-of-principle is given using an existing laboratory metal-jet source at the 9.25 keV Ga K\(_α\)-line and compared to a ray-tracing simulation of the setup. KW - X-ray microscopy KW - full-field microscopy KW - compound refractive X-ray lenses KW - CRLs Y1 - 2021 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-244974 SN - 2076-3417 VL - 11 IS - 16 ER - TY - JOUR A1 - Tufarelli, Tommaso A1 - Friedrich, Daniel A1 - Groß, Heiko A1 - Hamm, Joachim A1 - Hess, Ortwin A1 - Hecht, Bert T1 - Single quantum emitter Dicke enhancement JF - Physical Review Research N2 - Coupling N identical emitters to the same field mode is a well-established method to enhance light-matter interaction. However, the resulting √N boost of the coupling strength comes at the cost of a “linearized” (effectively semiclassical) dynamics. Here, we instead demonstrate a new approach for enhancing the coupling constant of a single quantum emitter, while retaining the nonlinear character of the light-matter interaction. We consider a single quantum emitter with N nearly degenerate transitions that are collectively coupled to the same field mode. We show that in such conditions an effective Jaynes-Cummings model emerges with a boosted coupling constant of order √N. The validity and consequences of our general conclusions are analytically demonstrated for the instructive case N=2. We further observe that our system can closely match the spectral line shapes and photon autocorrelation functions typical of Jaynes-Cummings physics, proving that quantum optical nonlinearities are retained. Our findings match up very well with recent broadband plasmonic nanoresonator strong-coupling experiments and will, therefore, facilitate the control and detection of single-photon nonlinearities at ambient conditions. KW - Cavity quantum electrodynamics KW - Collective effects in quantum optics KW - Quantum optics with artificial atoms KW - Superradiance & subradiance Y1 - 2021 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-261459 VL - 3 ER - TY - JOUR A1 - Weissenseel, Sebastian A1 - Gottscholl, Andreas A1 - Bönnighausen, Rebecca A1 - Dyakonov, Vladimir A1 - Sperlich, Andreas T1 - Long-lived spin-polarized intermolecular exciplex states in thermally activated delayed fluorescence-based organic light-emitting diodes JF - Science Advances N2 - Spin-spin interactions in organic light-emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF) are pivotal because radiative recombination is largely determined by triplet-to-singlet conversion, also called reverse intersystem crossing (RISC). To explore the underlying process, we apply a spin-resonance spectral hole-burning technique to probe electroluminescence. We find that the triplet exciplex states in OLEDs are highly spin-polarized and show that these states can be decoupled from the heterogeneous nuclear environment as a source of spin dephasing and can even be coherently manipulated on a spin-spin relaxation time scale T-2* of 30 ns. Crucially, we obtain the characteristic triplet exciplex spin-lattice relaxation time T-1 in the range of 50 mu s, which far exceeds the RISC time. We conclude that slow spin relaxation rather than RISC is an efficiency-limiting step for intermolecular donor:acceptor systems. Finding TADF emitters with faster spin relaxation will benefit this type of TADF OLEDs. KW - detected magnetic-resonance KW - population oscillations KW - polaron delocalization KW - charge separation KW - hole KW - phosphorescence KW - singlet KW - absorption KW - tryptophan KW - emission Y1 - 2021 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-265508 VL - 7 IS - 47 ER - TY - JOUR A1 - Graetz, Jonas T1 - Simulation study towards quantitative X-ray and neutron tensor tomography regarding the validity of linear approximations of dark-field anisotropy JF - Scientific Reports N2 - Tensor tomography is fundamentally based on the assumption of a both anisotropic and linear contrast mechanism. While the X-ray or neutron dark-field contrast obtained with Talbot(-Lau) interferometers features the required anisotropy, a preceding detailed study of dark-field signal origination however found its specific orientation dependence to be a non-linear function of the underlying anisotropic mass distribution and its orientation, especially challenging the common assumption that dark-field signals are describable by a function over the unit sphere. Here, two approximative linear tensor models with reduced orientation dependence are investigated in a simulation study with regard to their applicability to grating based X-ray or neutron dark-field tensor tomography. By systematically simulating and reconstructing a large sample of isolated volume elements covering the full range of feasible anisotropies and orientations, direct correspondences are drawn between the respective tensors characterizing the physically based dark-field model used for signal synthesization and the mathematically motivated simplified models used for reconstruction. The anisotropy of freely rotating volume elements is thereby confirmed to be, for practical reconstruction purposes, approximable both as a function of the optical axis' orientation or as a function of the interferometer's grating orientation. The eigenvalues of the surrogate models' tensors are found to exhibit fuzzy, yet almost linear relations to those of the synthesization model. Dominant orientations are found to be recoverable with a margin of error on the order of magnitude of 1 degrees. Although the input data must adequately address the full orientation dependence of dark-field anisotropy, the present results clearly support the general feasibility of quantitative X-ray dark-field tensor tomography within an inherent yet acceptable statistical margin of uncertainty. KW - applied mathematics KW - applied physics KW - imaging techniques Y1 - 2021 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-261844 VL - 11 ER - TY - JOUR A1 - Gottscholl, Andreas A1 - Diez, Matthias A1 - Soltamov, Victor A1 - Kasper, Christian A1 - Krauße, Dominik A1 - Sperlich, Andreas A1 - Kianinia, Mehran A1 - Bradac, Carlo A1 - Aharonovich, Igor A1 - Dyakonov, Vladimir T1 - Spin defects in hBN as promising temperature, pressure and magnetic field quantum sensors JF - Nature Communications N2 - Spin defects in solid-state materials are strong candidate systems for quantum information technology and sensing applications. Here we explore in details the recently discovered negatively charged boron vacancies (V\(_B\)\(^−\)) in hexagonal boron nitride (hBN) and demonstrate their use as atomic scale sensors for temperature, magnetic fields and externally applied pressure. These applications are possible due to the high-spin triplet ground state and bright spin-dependent photoluminescence of the V\(_B\)\(^−\). Specifically, we find that the frequency shift in optically detected magnetic resonance measurements is not only sensitive to static magnetic fields, but also to temperature and pressure changes which we relate to crystal lattice parameters. We show that spin-rich hBN films are potentially applicable as intrinsic sensors in heterostructures made of functionalized 2D materials. KW - electronic properties and materials KW - qubits Y1 - 2021 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-261581 VL - 12 IS - 1 ER - TY - JOUR A1 - Ünzelmann, M. A1 - Bentmann, H. A1 - Figgemeier, T. A1 - Eck, P. A1 - Neu, J. N. A1 - Geldiyev, B. A1 - Diekmann, F. A1 - Rohlf, S. A1 - Buck, J. A1 - Hoesch, M. A1 - Kalläne, M. A1 - Rossnagel, K. A1 - Thomale, R. A1 - Siegrist, T. A1 - Sangiovanni, G. A1 - Di Sante, D. A1 - Reinert, F. T1 - Momentum-space signatures of Berry flux monopoles in the Weyl semimetal TaAs JF - Nature Communications N2 - Since the early days of Dirac flux quantization, magnetic monopoles have been sought after as a potential corollary of quantized electric charge. As opposed to magnetic monopoles embedded into the theory of electromagnetism, Weyl semimetals (WSM) exhibit Berry flux monopoles in reciprocal parameter space. As a function of crystal momentum, such monopoles locate at the crossing point of spin-polarized bands forming the Weyl cone. Here, we report momentum-resolved spectroscopic signatures of Berry flux monopoles in TaAs as a paradigmatic WSM. We carried out angle-resolved photoelectron spectroscopy at bulk-sensitive soft X-ray energies (SX-ARPES) combined with photoelectron spin detection and circular dichroism. The experiments reveal large spin- and orbital-angular-momentum (SAM and OAM) polarizations of the Weyl-fermion states, resulting from the broken crystalline inversion symmetry in TaAs. Supported by first-principles calculations, our measurements image signatures of a topologically non-trivial winding of the OAM at the Weyl nodes and unveil a chirality-dependent SAM of the Weyl bands. Our results provide directly bulk-sensitive spectroscopic support for the non-trivial band topology in the WSM TaAs, promising to have profound implications for the study of quantum-geometric effects in solids. Weyl semimetals exhibit Berry flux monopoles in momentum-space, but direct experimental evidence has remained elusive. Here, the authors reveal topologically non-trivial winding of the orbital-angular-momentum at the Weyl nodes and a chirality-dependent spin-angular-momentum of the Weyl bands, as a direct signature of the Berry flux monopoles in TaAs. KW - electronic properties and materials KW - topological insulators Y1 - 2021 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-260719 VL - 12 IS - 1 ER - TY - JOUR A1 - Winter, Patrick M. A1 - Andelovic, Kristina A1 - Kampf, Thomas A1 - Hansmann, Jan A1 - Jakob, Peter Michael A1 - Bauer, Wolfgang Rudolf A1 - Zernecke, Alma A1 - Herold, Volker T1 - Simultaneous measurements of 3D wall shear stress and pulse wave velocity in the murine aortic arch JF - Journal of Cardiovascular Magnetic Resonance N2 - Purpose Wall shear stress (WSS) and pulse wave velocity (PWV) are important parameters to characterize blood flow in the vessel wall. Their quantification with flow-sensitive phase-contrast (PC) cardiovascular magnetic resonance (CMR), however, is time-consuming. Furthermore, the measurement of WSS requires high spatial resolution, whereas high temporal resolution is necessary for PWV measurements. For these reasons, PWV and WSS are challenging to measure in one CMR session, making it difficult to directly compare these parameters. By using a retrospective approach with a flexible reconstruction framework, we here aimed to simultaneously assess both PWV and WSS in the murine aortic arch from the same 4D flow measurement. Methods Flow was measured in the aortic arch of 18-week-old wildtype (n = 5) and ApoE\(^{−/−}\) mice (n = 5) with a self-navigated radial 4D-PC-CMR sequence. Retrospective data analysis was used to reconstruct the same dataset either at low spatial and high temporal resolution (PWV analysis) or high spatial and low temporal resolution (WSS analysis). To assess WSS, the aortic lumen was labeled by semi-automatically segmenting the reconstruction with high spatial resolution. WSS was determined from the spatial velocity gradients at the lumen surface. For calculation of the PWV, segmentation data was interpolated along the temporal dimension. Subsequently, PWV was quantified from the through-plane flow data using the multiple-points transit-time method. Reconstructions with varying frame rates and spatial resolutions were performed to investigate the influence of spatiotemporal resolution on the PWV and WSS quantification. Results 4D flow measurements were conducted in an acquisition time of only 35 min. Increased peak flow and peak WSS values and lower errors in PWV estimation were observed in the reconstructions with high temporal resolution. Aortic PWV was significantly increased in ApoE\(^{−/−}\) mice compared to the control group (1.7 ± 0.2 versus 2.6 ± 0.2 m/s, p < 0.001). Mean WSS magnitude values averaged over the aortic arch were (1.17 ± 0.07) N/m\(^2\) in wildtype mice and (1.27 ± 0.10) N/m\(^2\) in ApoE\(^{−/−}\) mice. Conclusion The post processing algorithm using the flexible reconstruction framework developed in this study permitted quantification of global PWV and 3D-WSS in a single acquisition. The possibility to assess both parameters in only 35 min will markedly improve the analyses and information content of in vivo measurements. KW - 4D flow KW - pulse wave velocity KW - wall shear stress KW - radial KW - self-navigation KW - mouse KW - aortic arch KW - atherosclerosis KW - mice KW - flow KW - plaque KW - CMR KW - quantification KW - microscopy Y1 - 2021 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-259152 VL - 23 IS - 1 ER - TY - JOUR A1 - Kiermasch, David A1 - Fischer, Mathias A1 - Gil-Escrig, Lidón A1 - Baumann, Andreas A1 - Bolink, Henk J. A1 - Dyakonov, Vladimir A1 - Tvingstedt, Kristofer T1 - Reduced Recombination Losses in Evaporated Perovskite Solar Cells by Postfabrication Treatment JF - Solar RRL N2 - The photovoltaic perovskite research community has now developed a large set of tools and techniques to improve the power conversion efficiency (PCE). One such arcane trick is to allow the finished devices to dwell in time, and the PCE often improves. Herein, a mild postannealing procedure is implemented on coevaporated perovskite solar cells confirming a substantial PCE improvement, mainly attributed to an increased open-circuit voltage (V\(_{OC}\)). From a V\(_{OC}\) of around 1.11 V directly after preparation, the voltage improves to more than 1.18 V by temporal and thermal annealing. To clarify the origin of this annealing effect, an in-depth device experimental and simulation characterization is conducted. A simultaneous reduction of the dark saturation current, the ideality factor (n\(_{id}\)), and the leakage current is revealed, signifying a substantial impact of the postannealing procedure on recombination losses. To investigate the carrier dynamics in more detail, a set of transient optoelectrical methods is first evaluated, ascertaining that the bulk carrier lifetime is increased with device annealing. Second, a drift-diffusion simulation is used, confirming that the beneficial effect of the annealing has its origin in effective bulk trap passivation that accordingly leads to a reduction of Shockley–Read–Hall recombination rates. KW - defects KW - heating KW - lifetimes KW - passivation KW - perovskite solar cells KW - recombination KW - Shockley–Read–Hall Y1 - 2021 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-258003 VL - 5 IS - 11 ER - TY - JOUR A1 - Andelovic, Kristina A1 - Winter, Patrick A1 - Kampf, Thomas A1 - Xu, Anton A1 - Jakob, Peter Michael A1 - Herold, Volker A1 - Bauer, Wolfgang Rudolf A1 - Zernecke, Alma T1 - 2D Projection Maps of WSS and OSI Reveal Distinct Spatiotemporal Changes in Hemodynamics in the Murine Aorta during Ageing and Atherosclerosis JF - Biomedicines N2 - Growth, ageing and atherosclerotic plaque development alter the biomechanical forces acting on the vessel wall. However, monitoring the detailed local changes in wall shear stress (WSS) at distinct sites of the murine aortic arch over time has been challenging. Here, we studied the temporal and spatial changes in flow, WSS, oscillatory shear index (OSI) and elastic properties of healthy wildtype (WT, n = 5) and atherosclerotic apolipoprotein E-deficient (Apoe\(^{−/−}\), n = 6) mice during ageing and atherosclerosis using high-resolution 4D flow magnetic resonance imaging (MRI). Spatially resolved 2D projection maps of WSS and OSI of the complete aortic arch were generated, allowing the pixel-wise statistical analysis of inter- and intragroup hemodynamic changes over time and local correlations between WSS, pulse wave velocity (PWV), plaque and vessel wall characteristics. The study revealed converse differences of local hemodynamic profiles in healthy WT and atherosclerotic Apoe\(^{−/−}\) mice, and we identified the circumferential WSS as potential marker of plaque size and composition in advanced atherosclerosis and the radial strain as a potential marker for vascular elasticity. Two-dimensional (2D) projection maps of WSS and OSI, including statistical analysis provide a powerful tool to monitor local aortic hemodynamics during ageing and atherosclerosis. The correlation of spatially resolved hemodynamics and plaque characteristics could significantly improve our understanding of the impact of hemodynamics on atherosclerosis, which may be key to understand plaque progression towards vulnerability. KW - atherosclerosis KW - mouse KW - 4D flow MRI KW - aortic arch KW - flow dynamics KW - WSS KW - mapping KW - PWV KW - plaque characteristics Y1 - 2021 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-252164 SN - 2227-9059 VL - 9 IS - 12 ER - TY - JOUR A1 - Beierlein, J. A1 - Egorov, O. A. A1 - Harder, T. H. A1 - Gagel, P. A1 - Emmerling, M. A1 - Schneider, C. A1 - Höfling, S. A1 - Peschel, U. A1 - Klembt, S. T1 - Bloch Oscillations of Hybrid Light‐Matter Particles in a Waveguide Array JF - Advanced Optical Materials N2 - Bloch oscillations are a phenomenon well known from quantum mechanics where electrons in a lattice experience an oscillatory motion in the presence of an electric field gradient. Here, the authors report on Bloch oscillations of hybrid light−matter particles, called exciton‐polaritons (polaritons), being confined in an array of coupled microcavity waveguides. To this end, the waveguide widths and their mutual couplings are carefully designed such that a constant energy gradient is induced perpendicular to the direction of motion of the propagating polaritons. This technique allows us to directly observe and study Bloch oscillations in real‐ and momentum‐space. Furthermore, the experimental findings are supported by numerical simulations based on a modified Gross–Pitaevskii approach. This work provides an important transfer of basic concepts of quantum mechanics to integrated solid state devices, using quantum fluids of light. KW - Bloch oscillations KW - exciton‐polaritons KW - polariton condensation KW - waveguides Y1 - 2021 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-239814 VL - 9 IS - 13 ER -