Ultrafast manipulation of the NiO antiferromagnetic order via sub-gap optical excitation.

Wide-band-gap insulators such as NiO offer the exciting prospect of coherently manipulating electronic correlations with strong optical fields. Contrary to metals where rapid dephasing of optical excitation via electronic processes occurs, the sub-gap excitation in charge-transfer insulators has been shown to couple to low-energy bosonic excitations. However, it is currently unknown if the bosonic dressing field is composed of phonons or magnons. Here we use the prototypical charge-transfer insulator NiO to demonstrate that 1.5 eV sub-gap optical excitation leads to a renormalised NiO band-gap in combination with a significant reduction of the antiferromagnetic order. We employ element-specific X-ray reflectivity at the FLASH free-electron laser to demonstrate the reduction of the upper band-edge at the O 1s-2p core-valence resonance (K-edge) whereas the antiferromagnetic order is probed via X-ray magnetic linear dichroism (XMLD) at the Ni 2p-3d resonance (L2-edge). Comparing the transient XMLD spectral line shape to ground-state measurements allows us to extract a spin temperature rise of 65 ± 5 K for time delays longer than 400 fs while at earlier times a non-equilibrium spin state is formed. We identify transient mid-gap states being formed during the first 200 fs accompanied by a band-gap reduction lasting at least up to the maximum measured time delay of 2.4 ps. Electronic structure calculations indicate that magnon excitations significantly contribute to the reduction of the NiO band gap.

Study of temporal, spectral, arrival time and energy fluctuations of SASE FEL pulses.

Self-amplified spontaneous emission (SASE) pulses delivered by free electron lasers (FELs) are inherently fluctuating sources; each pulse varies in energy, duration, arrival time and spectral shape. Therefore, there is strong demand for a full characterization of the properties of SASE radiation, which will facilitate more precise interpretation of the experimental data taken at SASE FELs. In this paper, we present an investigation into the fluctuations of pulse duration, spectral distribution, arrival time and pulse energy of SASE XUV pulses at FLASH, both on a shot-to-shot basis and on average over many pulses. With the aid of simulations, we derived scaling laws for these parameters and disentangled the statistical SASE fluctuations from accelerator-based fluctuations and measurement uncertainties.

XUV-Initiated Dissociation Dynamics of Molecular Oxygen (O2).

We performed a time-resolved spectroscopy experiment on the dissociation of oxygen molecules after the interaction with intense extreme-ultraviolet (XUV) light from the free-electron laser in Hamburg at Deutsches Elektronen-Synchrotron. Using an XUV-pump/XUV-probe transient-absorption geometry with a split-and-delay unit, we observe the onset of electronic transitions in the O2+ cation near 50 eV photon energy, marking the end of the progression from a molecule to two isolated atoms. We observe two different time scales of 290 ± 53 and 180 ± 76 fs for the emergence of different ionic transitions, indicating different dissociation pathways taken by the departing oxygen atoms. With regard to the emerging opportunities of tuning the central frequencies of pump and probe pulses and of increasing the probe-pulse bandwidth, future pump-probe transient-absorption experiments are expected to provide a detailed view of the coupled nuclear and electronic dynamics during molecular dissociation.

Observation of an Excitonic Mott Transition through Ultrafast Core-cum-Conduction Photoemission Spectroscopy.

Time-resolved soft-x-ray photoemission spectroscopy is used to simultaneously measure the ultrafast dynamics of core-level spectral functions and excited states upon excitation of excitons in WSe_{2}. We present a many-body approximation for the Green's function, which excellently describes the transient core-hole spectral function. The relative dynamics of excited-state signal and core levels clearly show a delayed core-hole renormalization due to screening by excited quasifree carriers resulting from an excitonic Mott transition. These findings establish time-resolved core-level photoelectron spectroscopy as a sensitive probe of subtle electronic many-body interactions and ultrafast electronic phase transitions.

Alignment of the aberration-free XUV Raman spectrometer at FLASH.

An extreme-ultraviolet (XUV) double-stage Raman spectrometer is permanently installed as an experimental end-station at the PG1 beamline of the soft X-ray/XUV free-electron laser in Hamburg, FLASH. The monochromator stages are designed according to the Czerny-Turner optical scheme, adapted for the XUV photon energy range, with optical elements installed at grazing-incidence angles. Such an optical scheme along with the usage of off-axis parabolic mirrors for light collimation and focusing allows for aberration-free spectral imaging on the optical axis. Combining the two monochromators in additive dispersion mode allows for reaching high resolution and superior stray light rejection, but puts high demands on the quality of the optical alignment. In order to align the instrument with the highest precision and to quantitatively characterize the instrument performance and thus the quality of the alignment, optical laser interferometry, Hartmann-Shack wavefront-sensing measurements as well as off-line soft X-ray measurements and extensive optical simulations were conducted. In this paper the concept of the alignment scheme and the procedure of the internal optical alignment are presented. Furthermore, results on the imaging quality and resolution of the first monochromator stage are shown.

Wavefront-propagation simulations supporting the design of a time-delay compensating monochromator beamline at FLASH2.

Wavefront-propagation simulations have been performed to complete the design of a monochromator beamline for FLASH2, the variable-gap undulator line at the soft X-ray free-electron laser in Hamburg (FLASH). Prior to propagation through the beamline optical elements, the parameters of the photon source were generated using the GENESIS code which includes the free-electron laser experimental data. Threshold tolerances for the misalignment of mirror angles are calculated and, since diffraction effects were included in the simulations, the minimum quality with respect to the slope errors required for the optics is determined.

Linear array detector for online diagnostics of spectral distributions at MHz repetition rates.

Free-electron lasers (FELs) based on superconducting accelerator technology and storage ring facilities operate with bunch repetition rates in the MHz range, and the need arises for bunch-by-bunch electron and photon diagnostics. For photon-pulse-resolved measurements of spectral distributions, fast one-dimensional profile monitors are required. The linear array detector KALYPSO (KArlsruhe Linear arraY detector for MHz-rePetition rate SpectrOscopy) has been developed for electron bunch or photon pulse synchronous read-out with frame rates of up to 2.7 MHz. At the FLASH facility at DESY, a current version of KALYPSO with 256 pixels has been installed at a grating spectrometer as online diagnostics to monitor the pulse-resolved spectra of the high-repetition-rate FEL pulses. Application-specific front-end electronics based on MicroTCA standard have been developed for data acquisition and processing. Continuous data read-out with low latency in the microsecond range enables the integration into fast feedback applications. In this paper, pulse-resolved FEL spectra recorded at 1.0 MHz repetition rate for various operation conditions at FLASH are presented, and the first application of an adaptive feedback for accelerator control based on photon beam diagnostics is demonstrated.

Normalized single-shot X-ray absorption spectroscopy at a free-electron laser.

A setup for dispersive X-ray absorption spectroscopy (XAS), employing a new reference scheme, has been implemented and tested at the soft X-ray free-electron laser (FEL) FLASH in Hamburg. A transmission grating was used to split the FEL beam into two copies (signal and reference). The spectral content of both beams was simultaneously measured for intensity normalization within the FEL bandwidth on a shot-to-shot basis. Excellent correlation between the two beams was demonstrated within a few percent for single bunch operation at 143 eV photon energy. Applying the normalization scheme, an absorption spectrum of a Gd2O3thin film sample was recorded around the Gd N4,5-edge photon energy of 143 eV, showing excellent agreement with a reference spectrum measured at a synchrotron. This scheme opens the door for time-resolved single-shot XAS with femtosecond time resolution at FELs.

FLASH free-electron laser single-shot temporal diagnostic: terahertz-field-driven streaking.

The commissioning of a terahertz-field-driven streak camera installed at the free-electron laser (FEL) FLASH at DESY in Hamburg, being able to deliver photon pulse duration as well as arrival time information with ∼10 fs resolution for each single XUV FEL pulse, is reported. Pulse durations between 300 fs and <15 fs have been measured for different FLASH FEL settings. A comparison between the XUV pulse arrival time and the FEL electron bunch arrival time measured at the FLASH linac section exhibits a correlation width of 20 fs r.m.s., thus demonstrating the excellent operation stability of FLASH. In addition, the terahertz-streaking setup was operated simultaneously to an alternative method to determine the FEL pulse duration based on spectral analysis. FLASH pulse duration derived from simple spectral analysis is in good agreement with that from terahertz-streaking measurement.

Double-grating monochromatic beamline with ultrafast response for FLASH2 at DESY.

The preliminary design of a monochromatic beamline for FLASH2 at DESY is presented. The monochromator is tunable in the 50-1000 eV energy range with resolving power higher than 1000 and temporal response below 50 fs over the whole energy range. A time-delay-compensated configuration using the variable-line-spacing monochromator design with two gratings is adopted: the first grating disperses the radiation on its output plane, where the intermediate slit performs the spectral selection; the second grating compensates for the pulse-front tilt and for the spectral dispersion due to diffraction from the first grating.

Diffraction gratings metrology and ray-tracing results for an XUV Raman spectrometer at FLASH.

The extreme-ultraviolet double-stage imaging Raman spectrometer is a permanent experimental endstation at the plane-grating monochromator beamline branch PG1 at FLASH at DESY in Hamburg, Germany. This unique instrument covers the photon energy range from 20 to 200 eV with high energy resolution of about 2 to 20 meV (design values) featuring an efficient elastic line suppression as well as effective stray light rejection. Such a design enables studies of low-energy excitations like, for example, phonons in solids close to the vicinity of the elastic line. The Raman spectrometer effectively operates with four reflective off-axial parabolic mirrors and two plane-grating units. The optics quality and their precise alignment are crucial to guarantee best performance of the instrument. Here, results on a comprehensive investigation of the quality of the spectrometer diffraction gratings are presented. The gratings have been characterized by ex situ metrology at the BESSY-II Optics Laboratory, employing slope measuring deflectometry and interferometry as well as atomic force microscopy studies. The efficiency of these key optical elements has been measured at the at-wavelength metrology laboratory using the reflectometer at the BESSY-II Optics beamline. Also, the metrology results are discussed with respect to the expected resolving power of the instrument by including them in ray-tracing studies of the instrument.

CAMP@FLASH: an end-station for imaging, electron- and ion-spectroscopy, and pump-probe experiments at the FLASH free-electron laser.

The non-monochromatic beamline BL1 at the FLASH free-electron laser facility at DESY was upgraded with new transport and focusing optics, and a new permanent end-station, CAMP, was installed. This multi-purpose instrument is optimized for electron- and ion-spectroscopy, imaging and pump-probe experiments at free-electron lasers. It can be equipped with various electron- and ion-spectrometers, along with large-area single-photon-counting pnCCD X-ray detectors, thus enabling a wide range of experiments from atomic, molecular, and cluster physics to material and energy science, chemistry and biology. Here, an overview of the layout, the beam transport and focusing capabilities, and the experimental possibilities of this new end-station are presented, as well as results from its commissioning.

Quasi-real-time photon pulse duration measurement by analysis of FEL radiation spectra.

For photon diagnostics at free-electron lasers (FELs), the determination of the photon pulse duration is an important challenge and a complex task. This is especially true for SASE FELs with strongly fluctuating pulse parameters. However, most techniques require an extensive experimental setup, data acquisition and evaluation time, limiting the usability in all-day operation. In contrast, the presented work uses an existing approach based on the analysis of statistical properties of measured SASE FEL spectra and implements it as a software tool, integrated in FLASH's data acquisition system. This allows the calculation of the average pulse durations from a set of measured spectral distributions with only seconds of delay, whenever high-resolution spectra are recorded.

A non-invasive online photoionization spectrometer for FLASH2.

The stochastic nature of the self-amplified spontaneous emission (SASE) process of free-electron lasers (FELs) effects pulse-to-pulse fluctuations of the radiation properties, such as the photon energy, which are determinative for processes of photon-matter interactions. Hence, SASE FEL sources pose a great challenge for scientific investigations, since experimenters need to obtain precise real-time feedback of these properties for each individual photon bunch for interpretation of the experimental data. Furthermore, any device developed to deliver the according information should not significantly interfere with or degrade the FEL beam. Regarding the spectral properties, a device for online monitoring of FEL wavelengths has been developed for FLASH2, which is based on photoionization of gaseous targets and the measurements of the corresponding electron and ion time-of-flight spectra. This paper presents experimental studies and cross-calibration measurements demonstrating the viability of this online photoionization spectrometer.

Microfocusing at the PG1 beamline at FLASH.

The Kirkpatrick-Baez (KB) refocusing mirror system installed at the PG1 branch of the plane-grating monochromator beamline at the soft X-ray/XUV free-electron laser in Hamburg (FLASH) is designed to provide tight aberration-free focusing down to 4 µm × 6 µm full width at half-maximum (FWHM) on the sample. Such a focal spot size is mandatory to achieve ultimate resolution and to guarantee best performance of the vacuum-ultraviolet (VUV) off-axis parabolic double-monochromator Raman spectrometer permanently installed at the PG1 beamline as an experimental end-station. The vertical beam size on the sample of the Raman spectrometer, which operates without entrance slit, defines and limits the energy resolution of the instrument which has an unprecedented design value of 2 meV for photon energies below 70 eV and about 15 meV for higher energies up to 200 eV. In order to reach the designed focal spot size of 4 µm FWHM (vertically) and to hold the highest spectrometer resolution, special fully motorized in-vacuum manipulators for the KB mirror holders have been developed and the optics have been aligned employing wavefront-sensing techniques as well as ablative imprints analysis. Aberrations like astigmatism were minimized. In this article the design and layout of the KB mirror manipulators, the alignment procedure as well as microfocus optimization results are presented.

Free-electron laser multiplex driven by a superconducting linear accelerator.

Free-electron lasers (FELs) generate femtosecond XUV and X-ray pulses at peak powers in the gigawatt range. The FEL user facility FLASH at DESY (Hamburg, Germany) is driven by a superconducting linear accelerator with up to 8000 pulses per second. Since 2014, two parallel undulator beamlines, FLASH1 and FLASH2, have been in operation. In addition to the main undulator, the FLASH1 beamline is equipped with an undulator section, sFLASH, dedicated to research and development of fully coherent extreme ultraviolet photon pulses using external seed lasers. In this contribution, the first simultaneous lasing of the three FELs at 13.4 nm, 20 nm and 38.8 nm is presented.

Out-of-equilibrium charge redistribution in a copper-oxide based superconductor by time-resolved X-ray photoelectron spectroscopy.

Charge-transfer excitations are of paramount importance for understanding the electronic structure of copper-oxide based high-temperature superconductors. In this study, we investigate the response of a Bi 2 Sr 2 CaCu 2 O 8 + δ crystal to the charge redistribution induced by an infrared ultrashort pulse. Element-selective time-resolved core-level photoelectron spectroscopy with a high energy resolution allows disentangling the dynamics of oxygen ions with different coordination and bonds thanks to their different chemical shifts. Our experiment shows that the O 1s component arising from the Cu-O planes is significantly perturbed by the infrared light pulse. Conversely, the apical oxygen, also coordinated with Sr ions in the Sr-O planes, remains unaffected. This result highlights the peculiar behavior of the electronic structure of the Cu-O planes. It also unlocks the way to study the out-of-equilibrium electronic structure of copper-oxide-based high-temperature superconductors by identifying the O 1s core-level emission originating from the oxygen ions in the Cu-O planes. This ability could be critical to gain information about the strongly-correlated electron ultrafast dynamical mechanisms in the Cu-O plane in the normal and superconducting phases.

Optical control of 4f orbital state in rare-earth metals.

A change of orbital state alters the coupling between ions and their surroundings drastically. Orbital excitations are hence key to understand and control interaction of ions. Rare-earth elements with strong magneto-crystalline anisotropy (MCA) are important ingredients for magnetic devices. Thus, control of their localized 4f magnetic moments and anisotropy is one major challenge in ultrafast spin physics. With time-resolved x-ray absorption and resonant inelastic scattering experiments, we show for Tb metal that 4f-electronic excitations out of the ground-state multiplet occur after optical pumping. These excitations are driven by inelastic 5d-4f-electron scattering, altering the 4f-orbital state and consequently the MCA with important implications for magnetization dynamics in 4f-metals and more general for the excitation of localized electronic states in correlated materials.

Time-Resolved X-ray Photoelectron Spectroscopy: Ultrafast Dynamics in CS2 Probed at the S 2p Edge.

Recent developments in X-ray free-electron lasers have enabled a novel site-selective probe of coupled nuclear and electronic dynamics in photoexcited molecules, time-resolved X-ray photoelectron spectroscopy (TRXPS). We present results from a joint experimental and theoretical TRXPS study of the well-characterized ultraviolet photodissociation of CS2, a prototypical system for understanding non-adiabatic dynamics. These results demonstrate that the sulfur 2p binding energy is sensitive to changes in the nuclear structure following photoexcitation, which ultimately leads to dissociation into CS and S photoproducts. We are able to assign the main X-ray spectroscopic features to the CS and S products via comparison to a first-principles determination of the TRXPS based on ab initio multiple-spawning simulations. Our results demonstrate the use of TRXPS as a local probe of complex ultrafast photodissociation dynamics involving multimodal vibrational coupling, nonradiative transitions between electronic states, and multiple final product channels.

Probing electron and hole colocalization by resonant four-wave mixing spectroscopy in the extreme ultraviolet.

Extending nonlinear spectroscopic techniques into the x-ray domain promises unique insight into photoexcited charge dynamics, which are of fundamental and applied interest. We report on the observation of a third-order nonlinear process in lithium fluoride (LiF) at a free-electron laser. Exploring the yield of four-wave mixing (FWM) in resonance with transitions to strongly localized core exciton states versus delocalized Bloch states, we find resonant FWM to be a sensitive probe for the degree of charge localization: Substantial sum- and difference-frequency generation is observed exclusively when in a one- or three-photon resonance with a LiF core exciton, with a dipole forbidden transition affecting details of the nonlinear response. Our reflective geometry-based approach to detect FWM signals enables the study of a wide variety of condensed matter sample systems, provides atomic selectivity via resonant transitions, and can be easily scaled to shorter wavelengths at free-electron x-ray lasers.