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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.
- 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.
- ItemReference shape effects on Fourier transform holography(Washington, DC : Soc., 2022) Malm, Erik; Pfau, Bastian; Schneider, Michael; Günther, Christian M.; Hessing, Piet; Büttner, Felix; Mikkelsen, Anders; Eisebitt, StefanSoft-x-ray holography which utilizes an optics mask fabricated in direct contact with the sample, is a widely applied x-ray microscopy method, in particular, for investigating magnetic samples. The optics mask splits the x-ray beam into a reference wave and a wave to illuminate the sample. The reconstruction quality in such a Fourier-transform holography experiment depends primarily on the characteristics of the reference wave, typically emerging from a small, high-aspect-ratio pinhole in the mask. In this paper, we study two commonly used reference geometries and investigate how their 3D structure affects the reconstruction within an x-ray Fourier holography experiment. Insight into these effects is obtained by imaging the exit waves from reference pinholes via high-resolution coherent diffraction imaging combined with three-dimensional multislice simulations of the x-ray propagation through the reference pinhole. The results were used to simulate Fourier-transform holography experiments to determine the spatial resolution and precise location of the reconstruction plane for different reference geometries. Based on our findings, we discuss the properties of the reference pinholes with view on application in soft-x-ray holography experiments.