Direct 2D spatial coherence determination using the Fourier analysis method Multi parameter characterization of the P04 beamline at PETRA III

We present a systematic 2D spatial-coherence analysis of the soft-X-ray beamline P04 at PETRA III for various beamline configurations. The influence of two different beam-defining apertures on the spatial coherence properties of the beam is discussed and optimal conditions for coherence-based experiments are found. A significant degradation of the spatial coherence in the vertical direction has been measured and sources of this degradation are identified and discussed. The Fourier-analysis method, which gives fast and simple access to the 2D spatial coherence function of the X-ray beam, is used for the experiment. Here, we exploit the charge scattering of a disordered nanodot sample allowing the use of arbitrary X-ray photon energies with this method

Application concepts for ultrafast laser induced skyrmion creation and annihilation

Magnetic skyrmions can be created and annihilated in ferromagnetic multilayers using single femtosecond infrared laser pulses above a material dependent fluence threshold. From the perspective of applications, optical control of skyrmions offers a route to a faster and, potentially, more energy efficient new class of information technology devices. Here, we investigate laser induced skyrmion generation in two different materials, mapping out the dependence of the process on the applied field and the laser fluence. We observe that sample properties like strength of the Dzyaloshinskii Moriya interaction and pinning do not considerably influence the initial step of optical creation. In contrast, the number of skyrmions created can be directly and robustly controlled via the applied field and the laser fluence. Based on our findings, we propose concepts for applications, such as all optical writing and deletion, an ultrafast skyrmion reshuffling device for probabilistic computing, and a combined optical and spin orbit torque controlled racetrac

Observation of fluctuation-mediated picosecond nucleation of a topological phase

peer reviewedTopological states of matter exhibit fascinating physics combined with an intrinsic stability. A key challenge is the fast creation of topological phases, which requires massive reorientation of charge or spin degrees of freedom. Here we report the picosecond emergence of an extended topological phase that comprises many magnetic skyrmions. The nucleation of this phase, followed in real time via single-shot soft X-ray scattering after infrared laser excitation, is mediated by a transient topological fluctuation state. This state is enabled by the presence of a time-reversal symmetry-breaking perpendicular magnetic field and exists for less than 300 ps. Atomistic simulations indicate that the fluctuation state largely reduces the topological energy barrier and thereby enables the observed rapid and homogeneous nucleation of the skyrmion phase. These observations provide fundamental insights into the nature of topological phase transitions, and suggest a path towards ultrafast topological switching in a wide variety of materials through intermediate fluctuating states. © 2020, The Author(s), under exclusive licence to Springer Nature Limited.Leibniz Association Grant no. K162/2018 (OptiSPIN

BUSCA: A Telescope Instrumentation for Simultaneous Imaging in 4 Optical Bands

The "Bonn University Simultaneous CAmera" (BUSCA) is a CCD camera system which allows simultaneous direct imaging of the same sky area in four colors. The optics are designed for an f/8 beam and four 4Kx4K CCDs with 15m pixels covering a field of view of 12 arcmin x 12 arcmin at a 2m class telescope. In September 1998 BUSCA has seen "First Light". The instrument is based on three dichroic beam splitters which separate optical wavelength bands such (at 430nm, 540nm, 730nm) that standard astronomical intermediate-band filter systems can be used. The dichroics are made of plane-parallel glass plates mounted at an angle of 45 degrees. Astigmatism in the transmitted beams (f/8) is completely cancelled by identical plane-parallel glass plates of suitable orientation. BUSCA offers new perspectives in astronomical multicolor photometry: i) The broadband spectral properties (e.g. color indices) of astronomical objects in the optical can be determined with high reliability even in non-photometric atmospheric conditions. ii) Precious observing time is used very efficiently. iii) With the large field of view, extended objects like globular and open star clusters or galaxies are covered in a single exposure. iv) Each exposure results in a complete data set

BUSCA: Simultaneous imaging in 4 optical bands.

The "Bonn University Simultaneous CAmera" (BUSCA) is a CCD camera system for the CA 2.2.m telescope which allows simultaneous direct imaging of the same area in four colors. It is based on three dichroic beam splitters and four large area detectors (6x6 cm^2). The field of view at the 2.2 m telescope is 12'x12'. BUSCA offers new perspectives for multicolor photometry: i) Color indices in the optical can be determined with high reliability even in non-photometric atmospheric conditions. ii) Precious observing time is used very efficiently. iii) With the large field of view extended objects are covered in a single exposure. iv) Each exposure results in a complete dataset. The dichroics --- plane-parallel glass plates operating at an angle of 45 degrees --- split the optical wavelength range such that standard intermediate-band filter systems (e.g. Stroemgren) can be used. During the commissioning phase the instrument is equipped with four 2k x 2k LORAL CCDs. Three devices will be replaced by (thick) monolithic 4k x 4k, 15\mu CCDs from Lockheed Martin Fairchild Systems. The UV channel will be upgraded to a single thinned 4k x 4k or two thinned 2k x 4k devices depending on availability and funding

Ultrafast Demagnetization by Extreme Ultraviolet Light

Free-electron lasers (FELs) enable the study of dynamics in matter on combined femtosecond time and nanometer length scales [1]. One of the most intriguing topics within contemporary research on magnetism, ultrafast near-infrared (IR) laser-induced demagnetization [2,3], has greatly benefited from the advent of FELs [4] as it was shown that optically generated superdiffusive spin currents [5] contribute to that phenomenon [6]. Following a previous campaign [7], here we report on the observation of a breakdown of the magnetic scattering cross section of Co/Pt multilayers for extreme ultraviolet (XUV) fluences >1 mJ/cm² defining the threshold fluence for FEL experiments where the FEL is meant to be a non-invasive probe. By employing a FEL double-pulse scheme, XUV-induced demagnetization is identified to be the major mechanism behind the breakdown. Besides revealing the existence of ultrafast demagnetization in the XUV regime for the first time, our results demonstrate that it proceeds much faster than the demagnetization when using IR radiation.[1] E. A. Seddon et al., Rep. Prog. Phys. 80, 115901 (2017), [2] E. Beaurepaire et al., Phys. Rev. Lett. 76, 4250 (1996), [3] D. Sander et al., J. Phys. D: Appl. Phys. 50, 363001 (2017), [4] M. Malvestuto, J. Phys. Cond. Mat. 30, 053002 (2018), [5] M. Battiato et al., Phys. Rev. Lett. 105, 027203 (2010), [6] B. Pfau et al., Nat. Commun. 3, 1100 (2012), [7] L. Müller et al., Phys. Rev. Lett. 110, 234801 (2013)

Note: Soft X-ray transmission polarizer based on ferromagnetic thin films

A transmission polarizer for producing elliptically polarized soft X-ray radiation from linearly polarized light is presented. The setup is intended for use at synchrotron and free-electron laser beamlines that do not directly offer circularly polarized light for, e.g., X-ray magnetic circular dichroism (XMCD) measurements or holographic imaging. Here, we investigate the degree of ellipticity upon transmission of linearly polarized radiation through a cobalt thin film. The experiment was performed at a photon energy resonant to the Co L3-edge, i.e., 778 eV, and the polarization of the transmitted radiation was determined using a polarization analyzer that measures the directional dependence of photo electrons emitted from a gas target. Elliptically polarized radiation can be created at any absorption edge showing the XMCD effect by using the respective magnetic element