Free Electron Laser Facility Research Articles

Element-Specific Magnetization Dynamics of Complex Magnetic Systems Probed by Ultrafast Magneto-Optical Spectroscopy

The vision to manipulate and control magnetism with light is driven on the one hand by fundamental questions of direct and indirect photon-spin interactions, and on the other hand by the necessity to cope with ever growing data volumes, requiring radically new approaches on how to write, read and process information. Here, we present two complementary experimental geometries to access the element-specific magnetization dynamics of complex magnetic systems via ultrafast magneto-optical spectroscopy in the extreme ultraviolet spectral range. First, we employ linearly polarized radiation of a free electron laser facility to demonstrate decoupled dynamics of the two sublattices of an FeGd alloy, a prerequisite for all-optical magnetization switching. Second, we use circularly polarized radiation generated in a laboratory-based high harmonic generation setup to show optical inter-site spin transfer in a CoPt alloy, a mechanism which only very recently has been predicted to mediate ultrafast metamagnetic phase transitions.

Time-resolved site-selective imaging of predissociation and charge transfer dynamics: the CH3I B-band

The predissociation dynamics of the 6s (B2E) Rydberg state of gas-phase CH3I were investigated by time-resolved Coulomb-explosion imaging using extreme ultraviolet (XUV) free-electron laser pulses. Inner-shell ionization at the iodine 4d edge was utilized to provide a site-specific probe of the ensuing dynamics. The combination of a velocity-map imaging (VMI) spectrometer coupled with the pixel imaging mass spectrometry (PImMS) camera permitted three-dimensional ionic fragment momenta to be recorded simultaneously for a wide range of iodine charge states. In accord with previous studies, initial excitation at 201.2 nm results in internal conversion and subsequent dissociation on the lower-lying A-state surface on a picosecond time scale. Examination of the time-dependent yield of low kinetic energy iodine fragments yields mechanistic insights into the predissociation and subsequent charge transfer following multiple ionization of the iodine products. The effect of charge transfer was observed through differing delay-dependencies of the various iodine charge states, from which critical internuclear distances for charge transfer could be inferred and compared to a classical over-the-barrier model. Time-dependent photofragment angular anisotropy parameters were extracted from the central slice of the Newton sphere, without Abel inversion, and highlight the effect of rotation of the parent molecule before dissociation, as observed in previous works. Our results demonstrate the ability to perform three-dimensional ion imaging at high event rates and showcase the potential benefits of this approach, particularly in relation to further time-resolved studies at free-electron laser facilities.

Enhanced polarization transfer to the characteristic Lα x-ray lines near the nonlinear Cooper minimum of two-photon ionization

It has been shown that for nonlinear atomic ionization, transfer of the degree of circular polarization from incident to fluorescence light is maximum at the Cooper minimum [J. Hofbrucker, A. V. Volotka, and S. Fritzsche, Phys. Rev. A 100, 011401(R) (2019)]. However, it is still a challenge to produce and detect circularly polarized light at xuv and x-ray photon energies. We here show that transfer of linear polarization is strongly enhanced at the Cooper minimum in two-photon ionization and can be readily detected using current free-electron laser facilities by measurements of the degree of linear polarization of the characteristic $L{\ensuremath{\alpha}}_{1}$ and $L{\ensuremath{\alpha}}_{2}$ lines. Two-photon ionization of a $2{p}_{3/2}$ electron of a tungsten atom is proposed to demonstrate the presented phenomena and realization of such an experiment is discussed in detail.

Record high extraction efficiency of free electron laser oscillator

The highest extraction efficiency (9.4%) of a free electron laser (FEL) oscillator has been achieved at the midinfrared FEL facility of Kyoto University. Because of the interaction between the electron beam and FEL electromagnetic field, a maximum electron energy decrease of 16% was observed. The measured energy decrease was consistent with the measured FEL spectrum. An FEL micropulse energy of ∼100 μJ with the expected few-cycle pulse duration at a wavelength of 11 μm was observed. This result is an important milestone for the high-extraction-efficiency FEL oscillator and will contribute to the strong-field physics of atoms and molecules.

Hard x-ray single-shot spectrometer at the European X-ray Free-Electron Laser.

The European X-ray Free-Electron Laser Facility in Germany delivers x-ray pulses with femtosecond pulse duration at a repetition rate of up to 4.5 MHz. The free-electron laser radiation is created by the self-amplified spontaneous emission (SASE) process, whose stochastic nature gives rise to shot-to-shot fluctuations in most beam properties, including spectrum, pulse energy, spatial profile, wavefront, and temporal profile. Each spectrum consisting of many spikes varies in width and amplitude that appear differently within the envelope of the SASE spectrum. In order to measure and study the SASE spectrum, the HIgh REsolution hard X-ray single-shot (HIREX) spectrometer was installed in the photon tunnel of the SASE1 undulator beamline. It is based on diamond gratings, bent crystals as a dispersive element, and a MHz-repetition-rate strip detector. It covers a photon energy range of 3 keV-25 keV and a bandwidth of 0.5% of the SASE beam. The SASE spikes are resolved with 0.15 eV separation using the Si 440 reflection, providing a resolving power of 60 000 at a photon energy of 9.3 keV. The measured SASE bandwidth is 25 eV. In this paper, we discuss the design specifications, installation, and commissioning of the HIREX spectrometer. The spectral results using Si (110), Si (111), and C (110) crystals are presented.

Approach of Serial Crystallography

Radiation damage and cryogenic sample environment are an experimental limitation observed in the traditional X-ray crystallography technique. However, the serial crystallography (SX) technique not only helps to determine structures at room temperature with minimal radiation damage, but it is also a useful tool for profound understanding of macromolecules. Moreover, it is a new tool for time-resolved studies. Over the past 10 years, various sample delivery techniques and data collection strategies have been developed in the SX field. It also has a wide range of applications in instruments ranging from the X-ray free electron laser (XFEL) facility to synchrotrons. The importance of the various approaches in terms of the experimental techniques and a brief review of the research carried out in the field of SX has been highlighted in this editorial.

Ronchi shearing interferometry for wavefronts with circular symmetry.

Ronchi testing of a focused electromagnetic wave has in the last few years been used extensively at X-ray free-electron laser (FEL) facilities to qualitatively evaluate the wavefront of the beam. It is a quick and straightforward test, is easy to interpret on the fly, and can be used to align phase plates that correct the focus of aberrated beams. In general, a single Ronchigram is not sufficient to gain complete quantitative knowledge of the wavefront. However the compound refractive lenses that are commonly used at X-ray FELs exhibit a strong circular symmetry in their aberration, and this can be exploited. Here, a simple algorithm that uses a single recorded Ronchigram to recover the full wavefront of a nano-focused beam, assuming circular symmetry, is presented, and applied to experimental measurements at the Matter in Extreme Conditions instrument at the Linac Coherent Light Source.

Spin-dependent two-photon Bragg scattering in the Kapitza-Dirac effect

We present the possibility of spin-dependent Kapitza-Dirac scattering based on a two-photon interaction only. The interaction scheme is inspired from a Compton scattering process, for which we explicitly show the mathematical correspondence to the spin-dynamics of an electron diffraction process in a standing light wave. The spin effect has the advantage that it already appears in a Bragg scattering setup with arbitrary low field amplitudes, for which we have estimated the diffraction count rate in a realistic experimental setup at available X-ray free-electron laser facilities.

Transient magnetic gratings on the nanometer scale.

Laser-driven non-local electron dynamics in ultrathin magnetic samples on a sub-10 nm length scale is a key process in ultrafast magnetism. However, the experimental access has been challenging due to the nanoscopic and femtosecond nature of such transport processes. Here, we present a scattering-based experiment relying on a laser-induced electro- and magneto-optical grating in a Co/Pd ferromagnetic multilayer as a new technique to investigate non-local magnetization dynamics on nanometer length and femtosecond timescales. We induce a spatially modulated excitation pattern using tailored Al near-field masks with varying periodicities on a nanometer length scale and measure the first four diffraction orders in an x-ray scattering experiment with magnetic circular dichroism contrast at the free-electron laser facility FERMI, Trieste. The design of the periodic excitation mask leads to a strongly enhanced and characteristic transient scattering response allowing for sub-wavelength in-plane sensitivity for magnetic structures. In conjunction with scattering simulations, the experiment allows us to infer that a potential ultrafast lateral expansion of the initially excited regions of the magnetic film mediated by hot-electron transport and spin transport remains confined to below three nanometers.

Characterization of megahertz X-ray laser beams by multishot desorption imprints in PMMA.

Proper diagnostics of intense free-electron laser (FEL) X-ray pulses is indisputably important for experimental data analysis as well as for the protection of beamline optical elements. New challenges for beam diagnostic methods are introduced by modern FEL facilities capable of delivering powerful pulses at megahertz (MHz) repetition rates. In this paper, we report the first characterization of a defocused MHz 13.5-nm beam generated by the free-electron laser in Hamburg (FLASH) using the method of multi-pulse desorption imprints in poly(methyl methacrylate)(PMMA). The beam fluence profile is reconstructed in a novel and highly accurate way that takes into account the nonlinear response of material removal to total dose delivered by multiple pulses. The algorithm is applied to experimental data of single-shot ablation imprints and multi-shot desorption imprints at both low (10 Hz) and high (1 MHz) repetition rates. Reconstructed response functions show a great agreement with the theoretical desorption response function model.

Microfluidic polyimide gas dynamic virtual nozzles for serial crystallography

Free liquid jets are a common sample delivery method in serial femtosecond x-ray (SFX) crystallography. Gas dynamic virtual nozzles (GDVNs) use an outer gas stream to focus a liquid jet down to a few micrometers in diameter. Such nozzles can be fabricated through various methods (capillary grinding, soft lithography, digital light processing, and two-photon polymerization) and materials, such as glass, polydimethylsiloxane, and photosensitive polyacrylates. Here, we present a broadly accessible, rapid prototyping laser ablation approach to micromachine solvent-resistant and inert Kapton polyimide foils with highly reproducible geometric features that result in 3D flow-focused GDVNs suitable for crystallography experiments at synchrotrons and free-electron laser facilities.

The Radiation Beamline of Novosibirsk Free-Electron Laser Facility Operating in Terahertz, Far-Infrared, and Mid-Infrared Ranges

Unlike synchrotrons with multiple beamlines, free-electron lasers (FELs) are single-beam facilities, which nevertheless have a number of endstations. The latter requires development of complex radiation transport line, which should be efficient enough to avoid scaling down the FEL capabilities. Keeping the beam shape and radiation power level along the beamline is a challenge because the total length of the FEL radiation transport line can exceed a hundred meters. The FELs around the world differ from each other in both radiation parameters and endstations’ layout requiring individual design of their radiation transport lines. In this work, we describe the 120-m beamline for transporting the radiation of the Novosibirsk FEL facility, consisting of three FELs of the terahertz (THz), far-infrared, and mid-infrared ranges. The radiation of these three FELs is directed to the same beam transport channel, which is able to deliver the radiation to any of 14 endstations already commissioned. To compare the expected beam parameters with the actual ones, the radiation intensity distribution was measured in a number of places of the beamline that are THz radiation outputs for some endstations. Possible causes of the parameters’ mismatching observed for distant endstations are discussed. The problem of radiation absorption by water vapor is considered in detail.

3D printing of gas-dynamic virtual nozzles and optical characterization of high-speed microjets.

Gas dynamic virtual nozzles (GDVNs) produce microscopic flow-focused liquid jets and droplets and play an important role at X-ray free-electron laser (XFEL) facilities where they are used to steer a stream of hydrated biomolecules into an X-ray focus during diffraction measurements. Highly stable and reproducible microjet and microdroplets are desired, as are flexible fabrication methods that enable integrated mixing microfluidics, droplet triggering mechanisms, laser illumination, and other customized features. In this study, we develop the use of high-resolution 3D nano-printing for the production of monolithic, asymmetric GDVN designs that are difficult to fabricate by other means. We also develop a dual-pulsed nanosecond image acquisition and analysis platform for the characterization of GDVN performance, including jet speed, length, diameter, and directionality, among others. We show that printed GDVNs can form microjets with very high degree of reproducibility, down to sub-micron diameters, and with water jet speeds beyond 170 m/s.

Ptychographic X-ray speckle tracking with multi-layer Laue lens systems.

The ever-increasing brightness of synchrotron radiation sources demands improved X-ray optics to utilize their capability for imaging and probing biological cells, nano-devices and functional matter on the nanometre scale with chemical sensitivity. Hard X-rays are ideal for high-resolution imaging and spectroscopic applications owing to their short wavelength, high penetrating power and chemical sensitivity. The penetrating power that makes X-rays useful for imaging also makes focusing them technologically challenging. Recent developments in layer deposition techniques have enabled the fabrication of a series of highly focusing X-ray lenses, known as wedged multi-layer Laue lenses. Improvements to the lens design and fabrication technique demand an accurate, robust, in situ and at-wavelength characterization method. To this end, a modified form of the speckle tracking wavefront metrology method has been developed. The ptychographic X-ray speckle tracking method is capable of operating with highly divergent wavefields. A useful by-product of this method is that it also provides high-resolution and aberration-free projection images of extended specimens. Three separate experiments using this method are reported, where the ray path angles have been resolved to within 4 nrad with an imaging resolution of 45 nm (full period). This method does not require a high degree of coherence, making it suitable for laboratory-based X-ray sources. Likewise, it is robust to errors in the registered sample positions, making it suitable for X-ray free-electron laser facilities, where beam-pointing fluctuations can be problematic for wavefront metrology.

Photon beam line of the water window FEL for the EuPRAXIA@SPARC_LAB project

A proposal for building a new Free Electron Laser facility at the Laboratori Nazionali di Frascati, EuPRAXIA@SPARC_LAB, is at present under consideration. This FEL facility will exploit plasma acceleration to produce ultra-bright photon pulses with durations of few femtoseconds down to the wavelengths between 2 and 4 nm, in the so called “water window”. The main class of experiments to be performed will include coherent diffraction imaging, soft X-ray absorption spectroscopy, Raman and photofragmentation measurements. In this article we present the updates on the photon beamlines design for the facility.

RF design optimization for the SHINE 3.9 GHz cavity

The Shanghai high-repetition-rate XFEL and extreme light facility (SHINE) under construction is designed to be one of the most advanced free electron laser (FEL) facilities in the world. The main part of the SHINE facility is an 8 GeV superconducting Linac operating in continuous wave (CW) mode. The Linac consists of seventy-five 1.3 GHz and two 3.9 GHz cryomodules. Based on the design and experience of 3.9 GHz cavities in the European X-ray free electron laser (E-XFEL) and the Linac coherent light source-II (LCLS-II) projects, we optimize the SHINE 3.9 GHz cavity design to adapt it for CW mode operation. In this paper, we present a particular redesign of the end group for the SHINE 3.9 GHz superconducting cavity that includes a redesign of the end cell and beam pipe to shift away the potentially troublesome lowest high-order modes (HOMs), a modification of the main coupler antenna, and a tuning of the HOM notch filter to meet the cavity requirements. RF losses calculations on the HOM coupler antennas show that the overheating on the inner conductor at the operating mode is diminished significantly. Furthermore, we have also studied the HOMs to ensure there are no dangerously trapped modes in the optimized cavity design.

The first decade of XFELs

Over the past decade, several X-ray free-electron laser (XFEL) facilities have been constructed and started operation worldwide. New, high-repetition XFELs are expected to open to users in the next 5 years.

A high-power, high-repetition-rate THz source for pump-probe experiments at Linac Coherent Light Source II.

Experiments using a THz pump and an X-ray probe at an X-ray free-electron laser (XFEL) facility like the Linac Coherent Light Source II (LCLS II) require frequency-tunable (3 to 20 THz), narrow bandwidth (∼10%), carrier-envelope-phase-stable THz pulses that produce high fields (>1 MV cm-1) at the repetition rate of the X-rays and are well synchronized with them. In this paper, a two-bunch scheme to generate THz radiation at LCLS II is studied: the first bunch produces THz radiation in an electromagnet wiggler immediately following the LCLS II undulator that produces X-rays from the second bunch. The initial time delay between the two bunches is optimized to compensate for the path difference in THz transport. The two-bunch beam dynamics, the THz wiggler and radiation are described, as well as the transport system bringing the THz pulses from the wiggler to the experimental hall.

Creating a diverse international user facility

The European XFEL is the first hard X-ray high-repetition-rate free-electron laser facility. Sakura Pascarelli, Serguei Molodtsov and Thomas Tschentscher, scientific directors of the European XFEL, discuss the challenges that lie ahead before the European XFEL can reach its full potential and cater for an international and diverse community of users.

Development and application of electron beam optical diagnostics for the multi-turn ERL of the Novosibirsk FEL facility

The optical diagnostics system is under development for the multi-turn energy recovery linac (ERL), which is used as a source of electrons for the Novosibirsk free electron laser (FEL) facility. Part of the diagnostic system has already been installed on the last track of the ERL. Its full configuration includes stations for measurement of the transversal and longitudinal electron beam profiles upstream and downstream of the FEL undulators. The stations for measurement of the electron beam energy spread and undulator radiation spectrum are at the stage of installation and commissioning. A radiation power sensor will also be set in the special output radiation line, mounted at an angle to the optical axis of the FEL. The new optical diagnostics is mostly required for fine tuning of the magnet system for electron outcoupling experiments. This paper presents a detailed overview of the new diagnostics and discusses the first results of the transverse beam profile measurements and experiments planned for the near future.