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- ItemSingleshot polychromatic coherent diffractive imaging with a high-order harmonic source(Washington, DC : Soc., 2020) Malm, Erik; Wikmark, Hampus; Pfau, Bastian; Villanueva-Perez, Pablo; Rudawski, Piotr; Peschel, Jasper; Maclot, Sylvain; Schneider, Michael; Eisebitt, Stefan; Mikkelsen, Anders; L’Huillier, Anne; Johnsson, PerSingleshot polychromatic coherent diffractive imaging is performed with a high-intensity high-order harmonic generation source. The coherence properties are analyzed and several reconstructions show the shot-to-shot fluctuations of the incident beam wavefront. The method is based on a multi-step approach. First, the spectrum is extracted from double-slit diffraction data. The spectrum is used as input to extract the monochromatic sample diffraction pattern, then phase retrieval is performed on the quasi-monochromatic data to obtain the sample’s exit surface wave. Reconstructions based on guided error reduction (ER) and alternating direction method of multipliers (ADMM) are compared. ADMM allows additional penalty terms to be included in the cost functional to promote sparsity within the reconstruction. © 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement.
- ItemUltrafast Demagnetization Dominates Fluence Dependence of Magnetic Scattering at Co M Edges(College Park, Md. : APS, 2020) Schneider, Michael; Pfau, Bastian; Günther, Christian M.; von Korff Schmising, Clemens; Weder, David; Geilhufe, Jan; Perron, Jonathan; Capotondi, Flavio; Pedersoli, Emanuele; Manfredda, Michele; Hennecke, Martin; Vodungbo, Boris; Lüning, Jan; Eisebitt, StefanWe systematically study the fluence dependence of the resonant scattering cross-section from magnetic domains in Co/Pd-based multilayers. Samples are probed with single extreme ultraviolet (XUV) pulses of femtosecond duration tuned to the Co M3,2 absorption resonances using the FERMI@Elettra free-electron laser. We report quantitative data over 3 orders of magnitude in fluence, covering 16 mJ/cm2/pulse to 10 000 mJ/cm2/pulse with pulse lengths of 70 fs and 120 fs. A progressive quenching of the diffraction cross-section with fluence is observed. Compression of the same pulse energy into a shorter pulse—implying an increased XUV peak electric field—results in a reduced quenching of the resonant diffraction at the Co M3,2 edge. We conclude that the quenching effect observed for resonant scattering involving the short-lived Co 3p core vacancies is noncoherent in nature. This finding is in contrast to previous reports investigating resonant scattering involving the longer-lived Co 2p states, where stimulated emission has been found to be important. A phenomenological model based on XUV-induced ultrafast demagnetization is able to reproduce our entire set of experimental data and is found to be consistent with independent magneto-optical measurements of the demagnetization dynamics on the same samples.
- ItemIn situ single-shot diffractive fluence mapping for X-ray free-electron laser pulses([London] : Nature Publishing Group UK, 2018) Schneider, Michael; Günther, Christian M.; Pfau, Bastian; Capotondi, Flavio; Manfredda, Michele; Zangrando, Marco; Mahne, Nicola; Raimondi, Lorenzo; Pedersoli, Emanuele; Naumenko, Denys; Eisebitt, StefanFree-electron lasers (FELs) in the extreme ultraviolet (XUV) and X-ray regime opened up the possibility for experiments at high power densities, in particular allowing for fluence-dependent absorption and scattering experiments to reveal non-linear light-matter interactions at ever shorter wavelengths. Findings of such non-linear effects are met with tremendous interest, but prove difficult to understand and model due to the inherent shot-to-shot fluctuations in photon intensity and the often structured, non-Gaussian spatial intensity profile of a focused FEL beam. Presently, the focused beam is characterized and optimized separately from the actual experiment. Here, we present the simultaneous measurement of XUV diffraction signals from solid samples in tandem with the corresponding single-shot spatial fluence distribution on the actual sample. Our in situ characterization scheme enables direct monitoring of the sample illumination, providing a basis to optimize and quantitatively understand FEL experiments.
- ItemAchieving diffraction-limited resolution in soft-X-ray Fourier-transform holography(Amsterdam : Elsevier Science, 2020) Geilhufe, Jan; Pfau, Bastian; Günther, Christian M.; Schneider, Michael; Eisebitt, StefanThe spatial resolution of microscopic images acquired via X-ray Fourier-transform holography is limited by the source size of the reference wave and by the numerical aperture of the detector. We analyze the interplay between both influences and show how they are matched in practice. We further identify, how high spatial frequencies translate to imaging artifacts in holographic reconstructions where mainly the reference beam limits the spatial resolution. As a solution, three methods are introduced based on numerical post-processing of the reconstruction. The methods comprise apodization of the hologram, refocusing via wave propagation, and deconvolution using the transfer function of the imaging system. In particular for the latter two, we demonstrate that image details smaller than the source size of the reference beam can be recovered up to the diffraction limit of the hologram. Our findings motivate the intentional application of a large reference-wave source enhancing the image contrast in applications with low photon numbers such as single-shot experiments at free-electron lasers or imaging at laboratory sources.
- ItemMulti-color imaging of magnetic Co/Pt heterostructures(Melville, NY : AIP Publishing LLC, 2017) Willems, Felix; von Korff Schmising, Clemens; Weder, David; Günther, Christian M.; Schneider, Michael; Pfau, Bastian; Meise, Sven; Guehrs, Erik; Geilhufe, Jan; Merhe, Alaa El Din; Jal, Emmanuelle; Vodungbo, Boris; Lüning, Jan; Mahieu, Benoit; Capotondi, Flavio; Pedersoli, Emanuele; Gauthier, David; Manfredda, Michele; Eisebitt, StefanWe present an element specific and spatially resolved view of magnetic domainsin Co/Pt heterostructures in the extreme ultraviolet spectral range. Resonantsmall-angle scattering and coherent imaging with Fourier-transform holographyreveal nanoscale magnetic domain networks via magnetic dichroism of Co at theM2,3 edges as well as via strong dichroic signals at the O2,3 and N6,7 edges of Pt.We demonstrate for the first time simultaneous, two-color coherent imaging at afree-electron laser facility paving the way for a direct real space access toultrafast magnetization dynamics in complex multicomponent material systems.
- ItemReal-time spatial characterization of micrometer-sized X-ray free-electron laser beams focused by bendable mirrors(Washington, DC : Soc., 2022) Mercurio, Giuseppe; Chalupský, Jaromír; Nistea, Ioana-Theodora; Schneider, Michael; Hájková, Věra; Gerasimova, Natalia; Carley, Robert; Cascella, Michele; Le Guyader, Loïc; Mercadier, Laurent; Schlappa, Justine; Setoodehnia, Kiana; Teichmann, Martin; Yaroslavtsev, Alexander; Burian, Tomáš; Vozda, Vojtĕch; Vyšín, Luděk; Wild, Jan; Hickin, David; Silenzi, Alessandro; Stupar, Marijan; Torben Delitz, Jan; Broers, Carsten; Reich, Alexander; Pfau, Bastian; Eisebitt, Stefan; La Civita, Daniele; Sinn, Harald; Vannoni, Maurizio; Alcock, Simon G.; Juha, Libor; Scherz, AndreasA real-time and accurate characterization of the X-ray beam size is essential to enable a large variety of different experiments at free-electron laser facilities. Typically, ablative imprints are employed to determine shape and size of μm-focused X-ray beams. The high accuracy of this state-of-the-art method comes at the expense of the time required to perform an ex-situ image analysis. In contrast, diffraction at a curved grating with suitably varying period and orientation forms a magnified image of the X-ray beam, which can be recorded by a 2D pixelated detector providing beam size and pointing jitter in real time. In this manuscript, we compare results obtained with both techniques, address their advantages and limitations, and demonstrate their excellent agreement. We present an extensive characterization of the FEL beam focused to ≈1 μm by two Kirkpatrick-Baez (KB) mirrors, along with optical metrology slope profiles demonstrating their exceptionally high quality. This work provides a systematic and comprehensive study of the accuracy provided by curved gratings in real-time imaging of X-ray beams at a free-electron laser facility. It is applied here to soft X-rays and can be extended to the hard X-ray range. Furthermore, curved gratings, in combination with a suitable detector, can provide spatial properties of μm-focused X-ray beams at MHz repetition rate.
- ItemSpatial distribution of electric-field enhancement across the gap of terahertz bow-tie antennas(Washington, DC : Soc., 2020) Runge, Matthias; Engel, Dieter; Schneider, Michael; Reimann, Klaus; Woerner, Michael; Elsaesser, ThomasThe electric-field enhancement in terahertz (THz) antennas designed for nonlinear THz spectroscopy of soft matter is characterized by spatially resolved electrooptic sampling. To mimic the relevant interaction geometry, metallic, resonant bow-tie antennas are deposited on a thin zinc telluride crystal of 10 µm thickness. The THz electric field transmitted through the antenna gap is recorded by electrooptic sampling. By focusing the 800 nm, sub-20 fs sampling pulses, we achieve a spatial resolution of some 3 µm, which is 1/3 to 1/8 of the antenna-gap width. The THz field in the gap displays an enhancement by a factor of up to 4.5 with a pronounced spectral variation, depending sensitively on the antenna-arm length and the gap width. By scanning the 800 nm probe spot laterally through the antenna gap, the spatial variation of the enhancement is determined, reaching the highest values at the edges of the gap. The results are in agreement with simulations of the electric-field distributions by finite-element calculations. © 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
- ItemEnabling time-resolved 2D spatial-coherence measurements using the Fourier-analysis method with an integrated curved-grating beam monitor(Washington, DC : Soc., 2020) Bagschik, Kai; Schneider, Michael; Wagner, Jochen; Buss, Ralph; Riepp, Matthias; Philippi-Kobs, Andre; Müller, Leonard; Roseker, Wojciech; Trinter, Florian; Hoesch, Moritz; Viefhaus, Jens; Eisebitt, Stefan; Grübel, Gerhard; Oepen, Hans Peter; Frömter, RobertDirect 2D spatial-coherence measurements are increasingly gaining importance at synchrotron beamlines, especially due to present and future upgrades of synchrotron facilities to diffraction-limited storage rings. We present a method to determine the 2D spatial coherence of synchrotron radiation in a direct and particularly simple way by using the Fourier-analysis method in conjunction with curved gratings. Direct photon-beam monitoring provided by a curved grating circumvents the otherwise necessary separate determination of the illuminating intensity distribution required for the Fourier-analysis method. Hence, combining these two methods allows for time-resolved spatial-coherence measurements. As a consequence, spatial-coherence degradation effects caused by beamline optics vibrations, which is one of the key issues of state-of-the-art X-ray imaging and scattering beamlines, can be identified and analyzed. © 2020 Optical Society of America.
- ItemQuantification of silver nanoparticle uptake and distribution within individual human macrophages by FIB/SEM slice and view(London : Biomed Central, 2017-3-21) Guehrs, Erik; Schneider, Michael; Günther, Christian M.; Hessing, Piet; Heitz, Karen; Wittke, Doreen; López-Serrano Oliver, Ana; Jakubowski, Norbert; Plendl, Johanna; Eisebitt, Stefan; Haase, AndreaBackground: Quantification of nanoparticle (NP) uptake in cells or tissues is very important for safety assessment. Often, electron microscopy based approaches are used for this purpose, which allow imaging at very high resolution. However, precise quantification of NP numbers in cells and tissues remains challenging. The aim of this study was to present a novel approach, that combines precise quantification of NPs in individual cells together with high resolution imaging of their intracellular distribution based on focused ion beam/ scanning electron microscopy (FIB/SEM) slice and view approaches. Results: We quantified cellular uptake of 75 nm diameter citrate stabilized silver NPs (Ag 75 Cit) into an individual human macrophage derived from monocytic THP-1 cells using a FIB/SEM slice and view approach. Cells were treated with 10 μg/ml for 24 h. We investigated a single cell and found in total 3138 ± 722 silver NPs inside this cell. Most of the silver NPs were located in large agglomerates, only a few were found in clusters of fewer than five NPs. Furthermore, we cross-checked our results by using inductively coupled plasma mass spectrometry and could confirm the FIB/SEM results. Conclusions: Our approach based on FIB/SEM slice and view is currently the only one that allows the quantification of the absolute dose of silver NPs in individual cells and at the same time to assess their intracellular distribution at high resolution. We therefore propose to use FIB/SEM slice and view to systematically analyse the cellular uptake of various NPs as a function of size, concentration and incubation time.
- ItemCoherent correlation imaging for resolving fluctuating states of matter(London : Macmillan Publishers Limited, 2023) Klose, Christopher; Büttner, Felix; Hu, Wen; Mazzoli, Claudio; Litzius, Kai; Battistelli, Riccardo; Lemesh, Ivan; Bartell, Jason M.; Huang, Mantao; Günther, Christian M.; Schneider, Michael; Barbour, Andi; Wilkins, Stuart B.; Beach, Geoffrey S. D.; Eisebitt, Stefan; Pfau, BastianFluctuations and stochastic transitions are ubiquitous in nanometre-scale systems, especially in the presence of disorder. However, their direct observation has so far been impeded by a seemingly fundamental, signal-limited compromise between spatial and temporal resolution. Here we develop coherent correlation imaging (CCI) to overcome this dilemma. Our method begins by classifying recorded camera frames in Fourier space. Contrast and spatial resolution emerge by averaging selectively over same-state frames. Temporal resolution down to the acquisition time of a single frame arises independently from an exceptionally low misclassification rate, which we achieve by combining a correlation-based similarity metric1,2 with a modified, iterative hierarchical clustering algorithm3,4. We apply CCI to study previously inaccessible magnetic fluctuations in a highly degenerate magnetic stripe domain state with nanometre-scale resolution. We uncover an intricate network of transitions between more than 30 discrete states. Our spatiotemporal data enable us to reconstruct the pinning energy landscape and to thereby explain the dynamics observed on a microscopic level. CCI massively expands the potential of emerging high-coherence X-ray sources and paves the way for addressing large fundamental questions such as the contribution of pinning5–8 and topology9–12 in phase transitions and the role of spin and charge order fluctuations in high-temperature superconductivity13,14.