| 1. |
- Engel, Robin Y., et al.
(författare)
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Electron population dynamics in resonant non-linear x-ray absorption in nickel at a free-electron laser
- 2023
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Ingår i: Structural Dynamics. - : American Institute of Physics (AIP). - 2329-7778. ; 10:5
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Tidskriftsartikel (refereegranskat)abstract
- Free-electron lasers provide bright, ultrashort, and monochromatic x-ray pulses, enabling novel spectroscopic measurements not only with femtosecond temporal resolution: The high fluence of their x-ray pulses can also easily enter the regime of the non-linear x-ray-matter interaction. Entering this regime necessitates a rigorous analysis and reliable prediction of the relevant non-linear processes for future experiment designs. Here, we show non-linear changes in the L-3-edge absorption of metallic nickel thin films, measured with fluences up to 60 J/cm(2). We present a simple but predictive rate model that quantitatively describes spectral changes based on the evolution of electronic populations within the pulse duration. Despite its simplicity, the model reaches good agreement with experimental results over more than three orders of magnitude in fluence, while providing a straightforward understanding of the interplay of physical processes driving the non-linear changes. Our findings provide important insights for the design and evaluation of future high-fluence free-electron laser experiments and contribute to the understanding of non-linear electron dynamics in x-ray absorption processes in solids at the femtosecond timescale.
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| 2. |
- Geilhufe, J., et al.
(författare)
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Extracting depth information of 3-dimensional structures from a single-view X-ray Fourier-transform hologram
- 2014
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Ingår i: Optics Express. - 1094-4087. ; 22:21, s. 24959-24969
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Tidskriftsartikel (refereegranskat)abstract
- We demonstrate how information about the three-dimensional structure of an object can be extracted from a single Fourier-transform Xray hologram. In contrast to lens-based 3D imaging approaches that provide depth information of a specimen utilizing several images from different angles or via adjusting the focus to different depths, our method capitalizes on the use of the holographically encoded phase and amplitude information of the object's wavefield. It enables single-shot measurements of 3D objects at coherent X-ray sources. As the ratio of longitudinal resolution over transverse resolution scales proportional to the diameter of the reference beam aperture over the X-ray wavelength, we expect the approach to be particularly useful in the extreme ultraviolet and soft-X-ray regime. (C) 2014 Optical Society of America
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| 3. |
- Hennecke, Martin, et al.
(författare)
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Ultrafast opto-magnetic effects in the extreme ultraviolet spectral range
- 2024
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Ingår i: Communications Physics. - : Springer Nature. - 2399-3650. ; 7:1
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Tidskriftsartikel (refereegranskat)abstract
- Coherent light-matter interactions mediated by opto-magnetic phenomena like the inverse Faraday effect (IFE) are expected to provide a non-thermal pathway for ultrafast manipulation of magnetism on timescales as short as the excitation pulse itself. As the IFE scales with the spin-orbit coupling strength of the involved electronic states, photo-exciting the strongly spin-orbit coupled core-level electrons in magnetic materials appears as an appealing method to transiently generate large opto-magnetic moments. Here, we investigate this scenario in a ferrimagnetic GdFeCo alloy by using intense and circularly polarized pulses of extreme ultraviolet radiation. Our results reveal ultrafast and strong helicity-dependent magnetic effects which are in line with the characteristic fingerprints of an IFE, corroborated by ab initio opto-magnetic IFE theory and atomistic spin dynamics simulations.
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| 4. |
- Hessing, Piet, et al.
(författare)
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Holography-guided ptychography with soft X-rays
- 2016
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Ingår i: Optics Express. - 1094-4087. ; 24:2, s. 1840-1851
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Tidskriftsartikel (refereegranskat)abstract
- Ptychography is a lensless imaging technique that aims to reconstruct an object from a set of coherent diffraction patterns originating from different and partially overlapping sample illumination areas. For a successful convergence of the iterative algorithms used, the sample scan positions have to be known with very high accuracy. Here, we present a method that allows to directly encode this information in the diffraction patterns without the need of accurate position encoders. Our approach relies on combining ptychography with another coherent imaging method, namely Fourier-transform holography. We have imaged two different objects using coherent soft-X-ray illumination and investigate the influence of experimental and numerical position refinement on the reconstruction result. We demonstrate that holographically encoded positions significantly reduce the experimental and numerical requirements. Our ptychographic reconstructions cover a large field of view with diffraction-limited resolution and high sensitivity in the reconstructed phase shift and absorption of the objects.
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| 5. |
- Malm, Erik, et al.
(författare)
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Reference shape effects on Fourier transform holography
- 2022
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Ingår i: Optics Express. - 1094-4087. ; 30:21, s. 38424-38438
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Tidskriftsartikel (refereegranskat)abstract
- 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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| 6. |
- Malm, Erik, et al.
(författare)
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Singleshot polychromatic coherent diffractive imaging with a high-order harmonic source
- 2020
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Ingår i: Optics Express. - 1094-4087. ; 28:1, s. 394-404
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 7. |
- Pfau, Bastian, et al.
(författare)
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Influence of stray fields on the switching-field distribution for bit-patterned media based on pre-patterned substrates
- 2014
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 105:13
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Tidskriftsartikel (refereegranskat)abstract
- Using a direct imaging method, we experimentally investigate the reversal of magnetic islands in a bit-patterned media sample based on a pre-patterned substrate. Due to systematic variation of the island distances in the media, we are able to study the influence of the dipolar interaction on the switching-field distribution of the island ensemble. The experimental findings are explained by an analytical magnetostatic model that allows us to quantify the different components of the demagnetizing field in the system and to distinguish intrinsic and dipolar broadening of the switching-field distribution. Besides the well-known dipolar broadening due to stray fields from neighboring islands, we find strong influence from the magnetized trench material on the island switching. (C) 2014 Author(s).
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| 8. |
- Pfau, Bastian, et al.
(författare)
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X-ray holography
- 2016
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Ingår i: Synchrotron Light Sources and Free-Electron Lasers: Accelerator Physics, Instrumentation and Science Applications. - Cham : Springer International Publishing. - 9783319143934 - 9783319143941 ; , s. 1093-1133
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Bokkapitel (refereegranskat)abstract
- With the dramatic increase in x-ray source brilliance and associated coherent photon flux, holography with x-rays has become feasible and is increasingly used for real-space investigations of static and dynamic properties of nanoscale systems. We describe the basic principles and capabilities of the method and present the current state of the art. A particular emphasis is put on the investigation of magnetic nanostructures, where x-ray holography has been applied at synchrotron radiation sources and free-electron x-ray lasers to investigate dynamic effects on time scales down to the femtosecond regime.
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| 9. |
- Pfau, Bastian, et al.
(författare)
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X-ray holography
- 2020
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Ingår i: Synchrotron Light Sources and Free-Electron Lasers : Accelerator Physics, Instrumentation and Science Applications - Accelerator Physics, Instrumentation and Science Applications. - Cham : Springer International Publishing. - 9783030232009 - 9783030232016 ; , s. 1295-1335
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Bokkapitel (refereegranskat)abstract
- With the dramatic increase in x-ray source brilliance and associated coherent photon flux, holography with x-rays has become feasible and is increasingly used for real-space investigations of static and dynamic properties of nanoscale systems. We describe the basic principles and capabilities of the method and present the current state of the art. A particular emphasis is put on the investigation of magnetic nanostructures, where x-ray holography has been applied at synchrotron radiation sources and free-electron x-ray lasers to investigate dynamic effects on time scales down to the femtosecond regime.
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| 10. |
- Vodungbo, Boris, et al.
(författare)
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Indirect excitation of ultrafast demagnetization.
- 2016
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Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322. ; 6
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Tidskriftsartikel (refereegranskat)abstract
- Does the excitation of ultrafast magnetization require direct interaction between the photons of the optical pump pulse and the magnetic layer? Here, we demonstrate unambiguously that this is not the case. For this we have studied the magnetization dynamics of a ferromagnetic cobalt/palladium multilayer capped by an IR-opaque aluminum layer. Upon excitation with an intense femtosecond-short IR laser pulse, the film exhibits the classical ultrafast demagnetization phenomenon although only a negligible number of IR photons penetrate the aluminum layer. In comparison with an uncapped cobalt/palladium reference film, the initial demagnetization of the capped film occurs with a delayed onset and at a slower rate. Both observations are qualitatively in line with energy transport from the aluminum layer into the underlying magnetic film by the excited, hot electrons of the aluminum film. Our data thus confirm recent theoretical predictions.
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