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Enabling time-resolved 2D spatial-coherence measurements using the Fourier-analysis method with an integrated curved-grating beam monitor

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, Robert

Direct 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.

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Optical inter-site spin transfer probed by energy and spin-resolved transient absorption spectroscopy

2020, Willems, Felix, von Korff Schmising, Clemens, Strüber, Christian, Schick, Daniel, Engel, Dieter W., Dewhurst, J. K., Elliott, Peter, Sharma, Sangeeta, Eisebitt, Stefan

Optically driven spin transport is the fastest and most efficient process to manipulate macroscopic magnetization as it does not rely on secondary mechanisms to dissipate angular momentum. In the present work, we show that such an optical inter-site spin transfer (OISTR) from Pt to Co emerges as a dominant mechanism governing the ultrafast magnetization dynamics of a CoPt alloy. To demonstrate this, we perform a joint theoretical and experimental investigation to determine the transient changes of the helicity dependent absorption in the extreme ultraviolet spectral range. We show that the helicity dependent absorption is directly related to changes of the transient spin-split density of states, allowing us to link the origin of OISTR to the available minority states above the Fermi level. This makes OISTR a general phenomenon in optical manipulation of multi-component magnetic systems.

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Real-time spatial characterization of micrometer-sized X-ray free-electron laser beams focused by bendable mirrors

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, Andreas

A 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.

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27 W 2.1 µm OPCPA system for coherent soft X-ray generation operating at 10 kHz

2020, Feng, Tianli, Heilmann, Anke, Bock, Martin, Ehrentraut, Lutz, Witting, Tobias, Yu, Haohai, Stiel, Holger, Eisebitt, Stefan, Schnürer, Matthias

We developed a high power optical parametric chirped-pulse amplification (OPCPA) system at 2.1 µm harnessing a 500 W Yb:YAG thin disk laser as the only pump and signal generation source. The OPCPA system operates at 10 kHz with a single pulse energy of up to 2.7 mJ and pulse duration of 30 fs. The maximum average output power of 27 W sets a new record for an OPCPA system in the 2 µm wavelength region. The soft X-ray continuum generated through high harmonic generation with this driver laser can extend to around 0.55 keV, thus covering the entire water window (284 eV - 543 eV). With a repetition rate still enabling pump-probe experiments on solid samples, the system can be used for many applications. © 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

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Element-specific magnetization dynamics of complex magnetic systems probed by ultrafast magneto-optical spectroscopy

2020, Korff Schmising, Clemens von, Willems, Felix, Sharma, Sangeeta, Yao, Kelvin, Borchert, Martin, Hennecke, Martin, Schick, Daniel, Radu, Ilie, Strüber, Christian, Engel, Dieter W., Shokeen, Vishal, Buck, Jens, Bagschik, Kai, Viefhaus, Jens, Hartmann, Gregor, Manschwetus, Bastian, Grunewald, Soeren, Düsterer, Stefan, Jal, Emmanuelle, Vodungbo, Boris, Lüning, Jan, Eisebitt, Stefan

The vision to manipulate and control magnetism with light is driven on the one hand by fundamental questions of direct and indirect photon-spin interactions, and on the other hand by the necessity to cope with ever growing data volumes, requiring radically new approaches on how to write, read and process information. Here, we present two complementary experimental geometries to access the element-specific magnetization dynamics of complex magnetic systems via ultrafast magneto-optical spectroscopy in the extreme ultraviolet spectral range. First, we employ linearly polarized radiation of a free electron laser facility to demonstrate decoupled dynamics of the two sublattices of an FeGd alloy, a prerequisite for all-optical magnetization switching. Second, we use circularly polarized radiation generated in a laboratory-based high harmonic generation setup to show optical inter-site spin transfer in a CoPt alloy, a mechanism which only very recently has been predicted to mediate ultrafast metamagnetic phase transitions. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.

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Ultrafast Demagnetization Dominates Fluence Dependence of Magnetic Scattering at Co M Edges

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, Stefan

We 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.

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Reference shape effects on Fourier transform holography

2022, Malm, Erik, Pfau, Bastian, Schneider, Michael, Günther, Christian M., Hessing, Piet, Büttner, Felix, Mikkelsen, Anders, Eisebitt, Stefan

Soft-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.

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Singleshot polychromatic coherent diffractive imaging with a high-order harmonic source

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, Per

Singleshot 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.

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Achieving diffraction-limited resolution in soft-X-ray Fourier-transform holography

2020, Geilhufe, Jan, Pfau, Bastian, Günther, Christian M., Schneider, Michael, Eisebitt, Stefan

The 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.