Search Results
Experimental evaluation of signal-to-noise in spectro-holography via modified uniformly redundant arrays in the soft x-ray and extreme ultraviolet spectral regime
2017-05-08, Günther, Christian M., Guehrs, Erik, Schneider, Michael, Pfau, Bastian, von Korff Schmising, Clemens, Geilhufe, Jan, Schaffert, Stefan, Eisebitt, Stefan
We present dichroic x-ray lensless magnetic imaging by Fourier transform holography with an extended reference scheme via a modified uniformly redundant array (mURA). Holographic images of magnetic domains simultaneously generated by a single pinhole reference as well as by a mURA reference are compared with respect to the signal-to-noise ratio (SNR) as a function of exposure time. We apply this approach for spectro-holographic imaging of ferromagnetic domain patterns in Co/Pt multilayer films. Soft x-rays with wavelengths of 1.59 nm (Co L3 absorption edge) and 20.8 nm (Co M2,3 absorption edges) are used for image formation and to generate contrast via x-ray magnetic circular dichroism. For a given exposure time, the mURA-based holography allows to decouple the reconstruction SNR from the spatial resolution. For 1.59 nm wavelength, the reconstruction via the extended reference scheme shows no significant loss of spatial resolution compared to the single pinhole reference. In contrast, at 20.8 nm wavelength the single pinhole reveals some very intricate features which are lost in the image generated by the mURA, although overall a high-quality image is generated. The SNR-advantage of the mURA scheme is most notable when the hologram has to be encoded with few photons, while errors associated with the increased complexity of the reconstruction process reduce the advantage for high-photon-number experiments.
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.
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.
Multi-color imaging of magnetic Co/Pt heterostructures
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, Stefan
We 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.