The investigation and characterization of materials with a spatial resolution in the range of a few picometers (pm) and a time resolution in the range of femtoseconds (fs) and below is becoming increasingly important for understanding the nature of physical processes. Due to the small wavelength, X-rays are indispensable as a “tool” for the production and investigation of nanostructures. This is taken into account, among other things, by providing the next generation of high-brilliance X-ray light sources, such as free electron lasers (FEL) [1] or linear accelerators with energy recovery (ERL) [2]. At the same time, laboratory X-ray sources such as high harmonics (HHG) [3] or laser-generated plasma sources (LPP) [4] are increasingly being developed as flexible, widely applicable supplements to these large-scale research facilities. This applies in particular to the extremely short pulse duration in the sub-fs range, which so far can only be realized with HHG sources [5].

The spectroscopy of molecular systems with soft X-rays has become increasingly important in recent years. Highly brilliant synchrotron radiation sources were used in X-ray absorption (X-Ray Absorption = XAS) and emission spectroscopy (X-Ray Emission = XES) as well as in X-ray scattering experiments (Resonant Inelastic X-Ray Scattering = RIXS) to study a variety of molecular interactions. Examples include the investigation of the structural dynamics of 3d metal-ligand complexes or hydrogen bonds in liquids, especially in water [6, 7] using oxygen K-edges XAS, XES and RIXS. Recently, time-resolved XAS, XES and RIXS experiments in connection with special short-pulse setups at synchrotrons and FELs have played an important role. Experimental “table-top” HHG setups have been available for a few years, which also provide soft X-ray radiation in the so-called water window (photon energy range 285 to 540 eV) in the laboratory, for example for XAS investigations on the carbon K-edge of organic molecules [8, 9]. There are first attempts to make the region of the nitrogen and oxygen K-edges available for these sources [10, 11].

The properties of many materials are largely determined by their structure on the nanometer scale. Exact knowledge of the structure and its connection with the material properties are therefore of fundamental importance in order to understand the functionality of the materials and to influence them in a targeted manner. The investigation of these materials with the help of ultra-short pulses in the soft X-ray range offers e.g. the possibility to track the magnetization dynamics with fs time resolution and even to map them on the nanometer scale [12]. To do this, the wavelength of the radiation must be adjusted to specific absorption edges of the materials used, which allows targeted access to the magnetization of the individual elementary components. For optically induced magnetic switching processes, some rare earths (Gd, Tb, Dy) are of particular interest, which show a strong magnetic contrast at their N5 edges around 150 eV. Holographic methods in particular can be used for spatial imaging, which benefit directly from the good coherence properties of HHG radiation or FEL radiation and – in contrast to lens-based methods – allow simple implementation of the optical excitation [13-15].

The advantage of laser-rubbed short-pulse X-ray sources for spatially and time-resolved structural investigations results from the fact that the pulsed driver laser can be used both to excite the system to be examined and to “generate” the X-rays. In this way, “pump-probe” experiments can be carried out relatively easily and the so-called “jitter” problem of synchronizing X-ray and optical pulses on the fs time scale can be avoided. On the other hand, laboratory sources emit an X-ray photon flux several orders of magnitude lower than synchrotron or FEL sources. Therefore, for practical applications of short-pulse laboratory X-ray sources, it is necessary to develop a new type of equipment with a significantly higher efficiency of the optical elements compared to previously known solutions.