Extreme ultraviolet resonant inelastic X-ray scattering (RIXS) at a seeded free-electron laser

The MagneDyn beamline at the FERMI free electron laser

The scope of this paper is to outline the main marks and performances of the MagneDyn beamline, which was designed and built to perform ultrafast magnetodynamic studies in solids. Open to users since 2019, MagneDyn operates with variable circular and linear polarized femtosecond pulses delivered by the externally laser-seeded FERMI free-electron laser (FEL). The very high degree of polarization, the high pulse-to-pulse stability, and the photon energy tunability in the 50-300 eV range allow performing advanced time-resolved magnetic dichroic experiments at the K-edge of light elements, e.g., carbon and at the M- and N-edge of the 3d-transition-metals and rare earth elements, respectively. To this end, two experimental end-stations are available. The first is equipped with an in situ dedicated electromagnet, a cryostat, and an extreme ultraviolet Wollaston-like polarimeter. The second, designed for carry-in user instruments, hosts also a spectrometer for pump-probe resonant x-ray emission and inelastic spectroscopy experiments with a sub-eV energy resolution. A Kirkpatrick-Baez active optics system provides a minimum focus of ∼20×20μm² FWHM at the sample. A pump laser setup, synchronized with the FEL-laser seeding system, delivers sub-picosecond pulses with photon energies ranging from the mid-IR to near-UV for optical pump-FEL probe experiments with a minimal pump-probe jitter of few femtoseconds. The overall combination of these features renders MagneDyn a unique state-of-the-art tool for studying ultrafast magnetic and resonant emission phenomena in solids.

AC/DC: The FERMI FEL Split and Delay Optical Device for Ultrafast X-ray Science

Free-electron lasers (FELs) are the most advanced class of light-sources, by virtue of their unique capability to lase high-brightness pulses characterized by wavelengths spanning the extreme-ultraviolet, the soft and hard X-ray spectral domains, as well as by temporal lengths lying in the femtosecond (fs) timescale. The next step to push the current standards in ultrafast X-ray science is strongly linked to the possibility of engineering and exploiting time-resolved experiments exclusively for FELs pulses, ideally having different colors tunable at specific electronic resonance of the chemical elements. At the seeded FERMI FEL (Trieste, Italy) this goal is committed to the optical device known as AC/DC, which stands for the auto correlator/delay creator. AC/DC is designed to double the incoming FEL pulse splitting the photon beam by inserting a grazing incidence flat mirror, thus preserving the spectral and temporal properties, and further delaying one of these two pulses in time. It can independently tune the FEL pulses fluence on the two optical paths by means of solid-state filters, too. Here, we present a detailed description about this optical device. Strong emphasis is dedicated to the AC/DC opto-mechanical design and to the laser-based feedback systems implemented to compensate for any mismatch affecting the FEL optical trajectory, ascribable to both mechanical imperfections and paraxial errors rising during a temporal delay scan.

Tomography of a seeded free-electron laser focal spot: qualitative and quantitative comparison of two reconstruction methods for spot size characterization

Performing experiments at free-electron lasers (FELs) requires an exhaustive knowledge of the pulse temporal and spectral profile, as well as the focal spot shape and size. Operating FELs in the extreme ultraviolet (EUV) and soft X-ray (SXR) spectral regions calls for designing ad-hoc optical layouts to transport and characterize the EUV/SXR beam, as well as tailoring its spatial dimensions at the focal plane down to sizes in the few micrometers range. At the FERMI FEL (Trieste, Italy) this task is carried out by the Photon Analysis Delivery and Reduction System (PADReS). In particular, to meet the different experimental requests on the focal spot shape and size, a proper tuning of the optical systems is required, and this should be monitored by means of dedicated techniques. Here, we present and compare two reconstruction methods for spot characterization: single-shot imprints captured via ablation on a poly(methyl methacrylate) sample (PMMA) and pulse profiles retrieved by means of a Hartmann wavefront sensor (WFS). By recording complementary datasets at and nearby the focal plane, we exploit the tomography of the pulse profile along the beam propagation axis, as well as a qualitative and quantitative comparison between these two reconstruction methods.

Probing the Electronic Structure of Warm Dense Nickel via Resonant Inelastic X-Ray Scattering

The development of bright free-electron lasers (FEL) has revolutionized our ability to create and study matter in the high-energy-density (HED) regime. Current diagnostic techniques have been successful in yielding information on fundamental thermodynamic plasma properties, but provide only limited or indirect information on the detailed quantum structure of these systems, and on how it is affected by ionization dynamics. Here we show how the valence electronic structure of solid-density nickel, heated to temperatures of around 10 of eV on femtosecond timescales, can be probed by single-shot resonant inelastic x-ray scattering (RIXS) at the Linac Coherent Light Source FEL. The RIXS spectrum provides a wealth of information on the HED system that goes well beyond what can be extracted from x-ray absorption or emission spectroscopy alone, and is particularly well suited to time-resolved studies of electronic-structure dynamics.

The TRIXS end-station for femtosecond time-resolved resonant inelastic x-ray scattering experiments at the soft x-ray free-electron laser FLASH

We present the experimental end-station TRIXS dedicated to time-resolved soft x-ray resonant inelastic x-ray scattering (RIXS) experiments on solid samples at the free-electron laser FLASH. Using monochromatized ultrashort femtosecond XUV/soft x-ray photon pulses in combination with a synchronized optical laser in a pump-probe scheme, the TRIXS setup allows measuring sub-picosecond time-resolved high-resolution RIXS spectra in the energy range from 35 eV to 210 eV, thus spanning the M-edge (M₁ and M_2,3) absorption resonances of 3d transition metals and N_4,5-edges of rare earth elements. A Kirkpatrick-Baez refocusing mirror system at the first branch of the plane grating monochromator beamline (PG1) provides a focus of (6 × 6) μm² (FWHM) at the sample. The RIXS spectrometer reaches an energy resolution of 35-160 meV over the entire spectral range. The optical laser system based on a chirped pulse optical parametric amplifier provides approximately 100 fs (FWHM) long photon pulses at the fundamental wavelength of 800 nm and a fluence of 120 mJ/cm² at a sample for optical pump-XUV probe measurements. Furthermore, optical frequency conversion enables experiments at 400 nm or 267 nm with a fluence of 80 and 30 mJ/cm², respectively. Some of the first (pump-probe) RIXS spectra measured with this setup are shown. The measured time resolution for time-resolved RIXS measurements has been characterized as 287 fs (FWHM) for the used energy resolution.

Ultrafast dynamics of spin and orbital correlations in quantum materials: an energy- and momentum-resolved perspective

Many remarkable properties of quantum materials emerge from states with intricate coupling between the charge, spin and orbital degrees of freedom. Ultrafast photo-excitation of these materials holds great promise for understanding and controlling the properties of these states. Here, we introduce time-resolved resonant inelastic X-ray scattering (tr-RIXS) as a means of measuring the charge, spin and orbital excitations out of equilibrium. These excitations encode the correlations and interactions that determine the detailed properties of the states generated. After outlining the basic principles and instrumentations of tr-RIXS, we review our first observations of transient antiferromagnetic correlations in quasi two dimensions in a photo-excited Mott insulator and present possible future routes of this fast-developing technique. The increasing number of X-ray free electron laser facilities not only enables tackling long-standing fundamental scientific problems, but also promises to unleash novel inelastic X-ray scattering spectroscopies. This article is part of the theme issue 'Measurement of ultrafast electronic and structural dynamics with X-rays'.

Non-linear soft x-ray methods on solids with MUSIX-the multi-dimensional spectroscopy and inelastic x-ray scattering endstation

With the intense and coherent x-ray pulses available from free-electron lasers, the possibility to transfer non-linear spectroscopic methods from the laser lab to the x-ray world arises. Advantages especially regarding selectivity and thus information content as well as an improvement of signal levels are expected. The use of coherences is especially fruitful and the example of coherent x-ray/optical sum-frequency generation is discussed. However, many non-linear x-ray methods still await discovery, partially due to the necessity for extremely adaptable and versatile instrumentation that can be brought to free-electron lasers for the analysis of the spectral content emitted from the sample into a continuous range of emission angles. Such an instrument (called MUSIX) is being developed and employed at FLASH, the free-electron laser in Hamburg and is described in this contribution together with first results.

Compact coherence enhancement by subharmonic self-seeding in X-ray free-electron laser facilities

X-ray free-electron lasers (FELs) are cutting-edge scientific tools able to generate transversely coherent radiation with very high power and ultra-short pulse durations. The self-seeding mechanism has been proven to increase the longitudinal coherence of the FEL radiation but its efficiency could be significantly improved, especially for soft X-rays. This paper proposes the enhancement of the performance of self-seeding by combining it with the harmonic generation mechanism. In particular, by starting the process with a subharmonic of the wavelength of interest, the coherence of the produced radiation is improved, the undulator beamline becomes more compact, and the monochromator realization is simplified. Numerical simulations for SwissFEL are presented showing that the method can be employed, within a given space, to increase the spectral brightness by one order of magnitude or more with respect to standard self-seeding. This coherence enhancement will be fundamental for many photon science applications and techniques such as resonant inelastic X-ray scattering.

Ultrafast magnetodynamics with free-electron lasers

The study of ultrafast magnetodynamics has entered a new era thanks to the groundbreaking technological advances in free-electron laser (FEL) light sources. The advent of these light sources has made possible unprecedented experimental schemes for time-resolved x-ray magneto-optic spectroscopies, which are now paving the road for exploring the ultimate limits of out-of-equilibrium magnetic phenomena. In particular, these studies will provide insights into elementary mechanisms governing spin and orbital dynamics, therefore contributing to the development of ultrafast devices for relevant magnetic technologies. This topical review focuses on recent advancement in the study of non-equilibrium magnetic phenomena from the perspective of time-resolved extreme ultra violet (EUV) and soft x-ray spectroscopies at FELs with highlights of some important experimental results.