Search

We present the first lasing results of SwissFEL, a hard X-ray free-electron laser (FEL) that recently came into operation at the Paul Scherrer Institute in Switzerland. SwissFEL is a very stable, compact and cost-effective X-ray FEL facility driven by a low-energy and ultra-low-emittance electron beam travelling through short-period undulators. It delivers stable hard X-ray FEL radiation at 1-A wavelength with pulse energies of more than 500 μJ, pulse durations of ~30 fs (root mean square) and spectral bandwidth below the per-mil level. Using special configurations, we have produced pulses shorter than 1 fs and, in a different set-up, broadband radiation with an unprecedented bandwidth of ~2%. The extremely small emittance demonstrated at SwissFEL paves the way for even more compact and affordable hard X-ray FELs, potentially boosting the number of facilities worldwide and thereby expanding the population of the scientific community that has access to X-ray FEL radiation. The first lasing results at SwissFEL, an X-ray free-electron laser, are presented, highlighting the facility’s unique capabilities. A general comparison to other major facilities is also provided.

The extension of transient grating spectroscopy to the x-ray regime will create numerous opportunities, ranging from the study of thermal transport in the ballistic regime to charge, spin, and energy transfer processes with atomic spatial and femtosecond temporal resolution. Studies involving complicated split-and-delay lines have not yet been successful in achieving this goal. Here we propose a novel, simple method based on the Talbot effect for converging beams, which can easily be implemented at current x-ray free electron lasers. We validate our proposal by analyzing printed interference patterns on polymethyl methacrylate and gold samples using similar to 3 keV X-ray pulses. (C) 2019 Optical Society of America

The Swiss Light Source (SLS) at the Paul Scherrer Institut (PSI) started user operation in 2001. It has been the first medium energy light source to rely on undulators for the production of X-rays up to 20 keV. For soft X-rays, APPLE II-type undulators, providing full polarization control and quasiperiodic electromagnetic undulators, have been installed. The hard X-ray range has been accessed by the newly developed small-gap, short-period, in-vacuum undulators operated on high harmonics. Based on this, FEMTO, the first hard X-ray undulator source for femtosecond (fs) experiments, started operation in 2006.

Next generation X ray sources based on the X ray Free Electron Laser (XFEL) concept will provide highly coherent ultrashort pulses of soft and hard X rays with peak intensity many orders of magnitude higher than that of a synchrotron. These pulses will allow studies of femtosecond dynamics at nanometer resolution and with chemical selectivity. They will produce diffraction images of organic and inorganic nanostructures without deleterious effects of radiation damage. © Schweizerische Chemische Gesellschaft.

The SwissFEL X-ray Free Electron Laser (XFEL) facility started construction at the Paul Scherrer Institute (Villigen, Switzerland) in 2013 and will be ready to accept its first users in 2018 on the Aramis hard X-ray branch. In the following sections we will summarize the various aspects of the project, including the design of the soft and hard X-ray branches of the accelerator, the results of SwissFEL performance simulations, details of the photon beamlines and experimental stations, and our first commissioning results.

Transition metal oxides are among the most promising solar materials, whose properties rely on the generation, transport and trapping of charge carriers (electrons and holes). Identifying the latter's dynamics at room temperature requires tools that combine elemental and structural sensitivity, with the atomic scale resolution of time (femtoseconds, fs). Here, we use fs Ti K-edge X-ray absorption spectroscopy (XAS) upon 3.49 eV (355 nm) excitation of aqueous colloidal anatase titanium dioxide nanoparticles to probe the trapping dynamics of photogenerated electrons. We find that their localization at Titanium atoms occurs in300 fs, forming Ti(3+) centres, in or near the unit cell where the electron is created. We conclude that electron localization is due to its trapping at pentacoordinated sites, mostly present in the surface shell region. The present demonstration of fs hard X-ray absorption capabilities opens the way to a detailed description of the charge carrier dynamics in transition metal oxides.

SwissFEL is a free electron laser facility for hard and soft X-rays at the Paul Scherrer Institut in Switzerland. The first hard X-ray FEL named ARAMIS will deliver photons in the wavelength range from 1 A to 7 A in up to three beamlines alternatively. The beamlines are equipped with crystal monochromators, cover the full wavelength range and offer a variety of operational modes.

The timescales for structural changes in a single crystal of bismuth after excitation with an intense near-infrared laser pulse are studied with femtosecond pump-probe X-ray diffraction. Changes in the intensity and reciprocal-lattice vector of several reflections give quantitative information on the structure factor and lattice strain as a function of time, with a resolution of 200 fs. The results indicate that the majority of excess carrier energy that remains near the surface is transferred to vibrational modes on a timescale of about 10 ps, and that the resultant increase in the variance of the atomic positions at these times is consistent with the overall magnitude of lattice strain that develops.

The first tunable undulator source for femtosecond hard X-rays in the range 4–12 keV is now in operation at the SLS storage ring. The source combines accelerator and laser technology relevant for future seeded free electron lasers. It provides inherently synchronized femtosecond laser ‘pump’ and X-ray ‘probe’ pulses to enable time-resolved diffraction and absorption experiments. By using X-ray diffraction to probe laser-induced coherent optical phonons in bulk bismuth, we estimate an X-ray pulse duration of 140 ± 30 fs FWHM with timing drifts below 30 fs rms measured over 5 days. The excellent spatial and temporal stability of the source allows quantitative measurement of ultrafast lattice dynamics and associated phase transitions in real space with atomic resolution and instrumental time resolution of 85 fs rms. Studying semimetals such as bismuth, we have demonstrated (i) grazing incidence femtosecond X-ray diffraction on single crystals that allows us to more completely characterize the ultrafast structural dynamics of solids, and (ii) optical control of real space coherent atomic motion. Both methods will be used to study the dynamics of photo-induced phase transitions in strongly correlated systems such as manganites. The time resolution of such measurements could be improved by one order of magnitude at future XFEL facilities at much higher flux. Femtosecond linear and non-linear resonant X-ray scattering employing full polarization control both in the soft (0.3–3 keV) and hard (4–15 keV) X-ray regime will become feasible which allows direct measurement of orbital degrees of freedom. However, a laser and hard X-ray cross-correlation technique with sufficient temporal resolution and signal-to-noise ratio appropriate for single shot operation has yet to be realized. Again, laser seeding using a phase stabilized few-cycle (5–7 fs) laser may be a viable option to generate

FEMTO, a femtosecond (fs) X-ray source based on laser interaction with a relativistic electron beam, began operation in the fall of 2006. It is installed at the μXAS beamline of the Swiss Light Source (SLS) at the Paul Scherrer Institut, Villigen. “Laser slicing” of an electron beam has first been proposed and demonstrated at the ALS [1] and has recently been implemented at BESSY [2] to generate fs soft X-rays (1–2 keV) with variable polarization. FEMTO is the first undulator source providing tunable, fs hard X-rays in the range 4.5–12 keV for laser/X-ray pump-probe absorption and diffraction experiments.

The Materials Science Beamline at the Swiss Light Source (SLS) has been designed to produce hard X-rays in the photon-energy range 5–40 keV, at an intermediate energy (2.4 GeV) synchrotron. To this end, it employs a novel “minigap wiggler”. Important issues in the design and realization of the beamline are the high heat load, robust system design, flexibility of operation and user-friendliness. A conventional collimating-mirror/sagittally focusing double-crystal monochromator/focusing mirror optics has been chosen with approximately 1:1 symmetry. Established component designs have been used wherever possible. Three serial end-stations are served with X-rays. Besides the minigap wiggler, other novel or unusual features are: continuous “top-up” injection in the SLS storage-ring, a rotating carbon “cup” filter in the beamline front-end, angles and bending radii of the optics mirrors which are adjusted at each change in photon-energy and special experimental-station equipment including high-speed one- and two-dimensional semiconductor detectors for powder and surface diffraction and a two-dimensional “Bragg magnifier” for tomography. In this work, a comparison is made between predicted and measured beamline properties, and Appendices with useful formulae and algorithms are provided.

The insertion devices under construction for phase I of the Swiss Light Source (SLS) are described. Five undulators and one wiggler will be installed in four straight sections of the third generation 2.4 GeV SLS storage ring, under construction at the Paul Scherrer Institute. To provide undulator radiation in the energy range from 10 eV to 18 keV , both long period and short period, small gap undulators will be installed.

Two APPLE II type twin undulators are installed at BESSY II. They can either be operated in a chicane mode providing variable polarization at high switching rates or in a combined mode using a modulator for phase adaption. Studies on magnet block inhomogeneities have started to further improve the field quality and to reduce the effort on shimming.

At BESSY II the first quasi-periodic hybrid type undulator has been installed. The magnetic structure has been optimized for higher order suppression and simultaneously for high flux in the (shifted) 5th harmonic to provide an useful energy range between 20 and 500 eV . At smallest gap of 15 mm the device operates as a wiggler with a K-value of 16.

We describe the “Surface/Interface: Microscopy” beamline currently under construction at the Paul Scherrer Institut. This beamline will use a photoemission electron microscope to study the electronic and magnetic structure of materials. Its photon energy range will be 94–2000 eV. To allow time resolved measurements with millisecond resolution we use a new scheme allowing to switch the photon helicity optically without perturbing the electron orbit. Two identical elliptical insertion devices are placed behind each other in the same straight section. They can produce light with opposing helicities. These two sources are separated horizontally by introducing a small static parallel offset in the electron orbit. Switching between the two rays of opposite helicities is done mechanically at the horizontal focus behind the monochromator. The two sources are brought to overlap on the sample by slightly defocussing the refocussing mirror. A commercial microscope ( Δx ∼20 nm resolution) is adapted to the synchrotron by various upgrades. These include a software combining microscope and beamline control, a computer controlled mechanical alignment system, a separate sample preparation chamber, and sample cooling using liquid nitrogen. The beamline will be available for user operation in 2002.

The Swiss Light Source (SLS) is a synchrotron radiation user facility (2000 users/year, 18 beamlines, 5000 hrs/year) in operation at Paul Scherrer Institut (PSI) since 2001. Figure 1 presents a schematic view.

We report on the use of grazing-incidence time-resolved x-ray diffraction to investigate the evolution of strain in natural graphite excited by femtosecond-laser pulses in the fluence range of 6-35 mJ/cm(2). Strains corresponding to up to similar to 2.8% c-axis expansion were observed. We show that the experimental data is in good agreement with calculations based on the Thomsen strain model in conjunction with dynamical diffraction theory. Furthermore we find no evidence of nonthermal lattice expansion as reported in recent ultrafast electron-diffraction studies of laser-excited graphite conducted under comparable excitation conditions.

Femtosecond laser-induced strain waves in InSb are studied by means of time-resolved x-ray diffraction. The temporal evolution of the measured x-ray diffracted intensity reveals that the lattice dynamics depends on the time scale of energy transfer from excited carriers to the lattice. A framework that accounts for this energy-transfer time (lattice heating time) is presented and applied to model the fluence dependence of the transient x-ray diffraction data. In this model the initial strain wave dynamics depends crucially on the lattice heating time, which decreases with increasing fluence.

We report on the temporal and spatial stability of the first tunable femtosecond undulator hard-x-ray source for ultrafast diffraction and absorption experiments. The 2.5-1 Angstrom output radiation is driven by an initial 50 fs laser pulse employing the laser-electron slicing technique. By using x-ray diffraction to probe laser-induced coherent optical phonons in bulk bismuth, we estimate an x-ray pulse duration of 140+/-30 fs FWHM with timing drifts below 30 fs rms measured over 5 days. Optical control of coherent lattice motion is demonstrated.

We employ grazing-incidence femtosecond x-ray diffraction to characterize the coherent, femtosecond laser-induced lattice motion of a bismuth crystal as a function of depth from the surface with a temporal resolution of 193 +/- 8 fs. The data show direct consequences on the lattice motion from carrier diffusion and electron-hole interaction, allowing us to estimate an effective diffusion rate of D=2.3 +/- 0.3 cm(2)/s for the highly excited carriers and an electron-hole interaction time of 260 +/- 20 fs.

The Swiss Light Source (SLS) at the Paul Scherrer Institut (PSI) became the first medium energy synchrotron user facility to rely on the high harmonic operation of small gap, short period undulators to extend high brightness radiation into a regime (3–18 keV) otherwise only accessible using lower brightness wigglers or operation at higher electron beam energy. Today several facilities with beam energy 2–3 GeV follow a similar route. A PSI/SPring‐8 collaboration was formed to install and operate the first in‐vacuum undulator shortly after commissioning of the SLS storage ring (2.4 GeV) in 2001. The goal of the joint project was to prove that high harmonic operation of small period undulators at small gaps is a valid concept to operate the PX‐I beamline at 1 A under user operation conditions. The performance of the PX‐I beamline proved to be excellent and launched the installation of meanwhile 4 new in‐vacuum undulators. Having routinely operated such devices for 5 years, our experience confirms that (1) the concept of operating short period undulators (19–24 mm) on higher harmonics (11./13.) is valid, (2) reliable small gap (5–6 mm) undulator operation is feasible in the presence of top‐up injection, and (3) during gap scans the photon beam can be stabilized to sub‐μrad precision using non‐intercepting photon beam monitors.

Corrections of insertion device (ID) induced orbit distortions at the SLS are performed by means of feed forward schemes down to the micron level at the corresponding photon beam position monitors (XBPMs). The remaining orbit fluctuations are suppressed by XBPM feedbacks which are an integral part of the fast orbit feedback system. As a result, sub-μm RMS stability at the XBPMs is achieved while the ID settings are varied.

We report on the feasibility and expected performance of a sub-picosecond X-ray source at the Swiss Light Source (SLS), based on the electron-beam slicing method. This method has recently been demonstrated at the Advanced Light Source (ALS), using a bending magnet to produce ultrafast optical pulses. To improve the flux of such a source for user experiments, we have studied its realization both in the hard and soft X-ray regime, employing small gap insertion devices (IDs) installed either in one or in two successive straight sections. The most favourable geometry is one in which the modulator and the radiator are positioned in the same straight section of the storage ring.

The high-brilliance synchrotron radiation source BESSY II will provide space for the installation of 14 insertion devices. At the time of writing (July 1997), eight insertion devices are funded. The first light for use by scientists will be available in January 1999. The undulators are designed for a scanning operation in parallel with the monochromator. This modus requires tight tolerances concerning the drive system. The first insertion device to be installed, a U-49 undulator, has been assembled and magnetically measured. The r.m.s. phase error is 2°. An endpole configuration using coils with less than 64 amp-turns makes the angular kick smaller than ±3 µrad over a gap range 15–50 mm.

We directly measure by femtosecond time-resolved x-ray diffraction the E-g symmetry coherent phonon excited in bismuth by a strong optical pulse. The magnitude of the E-g mode observed is 0.2 pm peak-to-peak, compared against the 2.7-pm initial displacement of the fully symmetric A(1g) mode. The much smaller motion of the E-g mode is a consequence of the short lifetime of the electronic states that drive the atomic motion. The experimentally measured magnitude of the E-g motion allows us to rule out a previously suggested scenario for explaining the dynamics in bismuth that relies on strong coupling between these modes. DOI: 10.1103/PhysRevB.87.054301

Time resolved x-ray diffraction with sub-picosecond time resolution is used to investigate the fluence dependence of the lattice heating in InSb.

We apply time-resolved resonant x-ray diffraction near the Mn K-edge to directly measure the structural and electronic long-range order changes during ultrafast melting of the charge and orbitally ordered phase in a perovskite manganite.

We use femtosecond x-ray diffraction to make a quantitative study of the structural symmetry-breaking coherent Eg mode of bismuth created by intense laser excitation. Coherent amplitudes on the order of 0.1 pm are observed.