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The intensity correlations in incoherently scattered X-rays from a free-electron laser can be exploited to image 2D objects with a resolution close to or below the diffraction limit. The advent of accelerator-driven free-electron lasers (FEL) has opened new avenues for high-resolution structure determination via diffraction methods that go far beyond conventional X-ray crystallography methods1,2,3,4,5,6,7,8,9,10. These techniques rely on coherent scattering processes that require the maintenance of first-order coherence of the radiation field throughout the imaging procedure. Here we show that higher-order degrees of coherence, displayed in the intensity correlations of incoherently scattered X-rays from an FEL, can be used to image two-dimensional objects with a spatial resolution close to or even below the Abbe limit. This constitutes a new approach towards structure determination based on incoherent processes11,12, including fluorescence emission or wavefront distortions, generally considered detrimental for imaging applications. Our method is an extension of the landmark intensity correlation measurements of Hanbury Brown and Twiss13 to higher than second order, paving the way towards determination of structure and dynamics of matter in regimes where coherent imaging methods have intrinsic limitations14.

The realization of the strong coupling regime between a single cavity mode and an electromagnetic resonance is a centrepiece of quantum optics. In this regime, the reversible exchange of a photon between the two components of the system leads to so-called Rabi oscillations. Strong coupling is used in the optical and infrared regimes, for instance, to produce non-classical states of light, enhance optical nonlinearities and control quantum states. Here, we report the first observation of Rabi oscillations of an X-ray photon between two resonant 57Fe layers embedded in two coupled cavities. The system is described by an effective Hamiltonian, in which the two layers couple strongly. We observe sinusoidal beating as the signature of the Rabi oscillations in the system’s temporal evolution, as well as the splitting of nuclear resonances in the reflected light spectrum. Our results significantly advance the development of the new field of X-ray quantum optics. Rabi oscillations in the regime of hard X-rays are observed between two ensembles of the nuclear resonant isotope 57Fe. The sample system consists of two thin-film cavities stacked on top of each other, coupled through a thin interlayer.

Transient magnetization dynamics are calculated from the Landau-Lifshitz-Gilbert equation. The internal fields are described by partial derivatives of the sample's free energy density. This description allows the calculation of the magnetization trajectory of homogeneously magnetized samples for all kinds of energy contributions. It provides a general solution for transient magnetization dynamics. The model is used to determine the magnetization response to excitation profiles like rectangular or Gaussian pulses, frequency bursts, wavelet excitations, and chirped THz pulses.

Strong coupling between X-rays and matter excitations in arrays of alternating 57Fe and 56Fe layers is demonstrated. The results may open the door to X-ray quantum optics and strong coupling phenomena.

Scientific reports 8(1), 11261 (2018). doi:10.1038/s41598-018-29596-z, We introduce a method to study the spatial profiles of standing spin waves in ferromagnetic microstructures. The method relies on Nuclear Resonant Scattering of 57Fe using a microfocused beam of synchrotron radiation, the transverse coherence length of which is smaller than the length scale of lateral variations in the magnetization dynamics. Using this experimental method, the nuclear resonant scattering signal due to a confined spin wave is determined on the basis of an incoherent superposition model. From the fits of the Nuclear Resonant Scattering time spectra, the precessional amplitude profile across the stripe predicted by an analytical model is reconstructed. Our results pave the way for studying non-homogeneous dynamic spin configurations in microstructured magnetic systems using nuclear resonant scattering of synchrotron light., Published by Nature Publishing Group, London

Physical review materials 1(2), 023001(2017). doi:10.1103/PhysRevMaterials.1.023001, We present a synchrotron-based x-ray scattering technique which allows disentangling magnetic propertiesof heterogeneous systems with nanopatterned surfaces. This technique combines the nanometer-range spatialresolution of surface morphology features provided by grazing incidence small angle x-ray scattering and thehigh sensitivity of nuclear resonant scattering to magnetic order. A single experiment thus allows attributingmagnetic properties to structural features of the sample; chemical and structural properties may be correlatedanalogously. We demonstrate how this technique shows the correlation between structural growth and evolutionof magnetic properties for the case of a remarkable magnetization reversal in a structurally and magneticallynanopatterned sample system., Published by APS, College Park, MD

Physical review letters 118(25), 257202(2017). doi:10.1103/PhysRevLett.118.257202, The generation of short spin current pulses is the basis for fast spintronic devices. In thin bilayer systems consisting of a nonmagnetic metal and a ferromagnet, a pure spin current is induced by a precessing magnetization into the nonmagnetic layer by spin pumping. This effect has been experimentally demonstrated at ferromagnetic resonance at GHz frequencies. Here, it is theoretically shown that transient magnetization dynamics efficiently generates short spin current pulses that exhibit two transient contributions. An effective coherent spin current generation is found far above the ferromagnetic resonance up to THz frequencies although dynamic magnetization amplitudes are very small in this regime. The results provide a concept for coherent THz spin current generation., Published by APS, College Park, Md.

Physical review letters 118(23), 237204(2017). doi:10.1103/PhysRevLett.118.237204, Nuclear resonant x-ray diffraction in grazing incidence geometry is used to determine the lateral magnetic configuration in a one-dimensional lattice of ferromagnetic nanostripes. During magnetic reversal, strong nuclear superstructure diffraction peaks appear in addition to the electronic ones due to an antiferromagnetic order in the nanostripe lattice. We show that the analysis of the angular distribution together with the time dependence of the resonantly diffracted x rays reveals surface spin structures with very high sensitivity. This scattering technique provides unique access to laterally correlated spin configurations in magnetically ordered nanostructures and, in perspective, also to their dynamics., Published by APS, College Park, Md.

We demonstrate the feasibility of investigating periodically driven magnetization dynamics in a scanning electron microscope with polarization analysis based on spin-polarized low-energy electron diffraction. With the present setup, analyzing the time structure of the scattering events, we obtain a temporal resolution of 700 ps, which is demonstrated by means of imaging the field-driven 100 MHz gyration of the vortex in a soft-magnetic FeCoSiB square. Owing to the efficient intrinsic timing scheme, high-quality movies, giving two components of the magnetization simultaneously, can be recorded on the time scale of hours.