Jens Buck

Circular dichroism measurements at an x-ray free-electron laser with polarization control

A non-destructive diagnostic method for the characterization of circularly polarized, ultraintense, short wavelength free-electron laser (FEL) light is presented. The recently installed Delta undulator at the LCLS (Linac Coherent Light Source) at SLAC National Accelerator Laboratory (USA) was used as showcase for this diagnostic scheme. By applying a combined two-color, multi-photon experiment with polarization control, the degree of circular polarization of the Delta undulator has been determined. Towards this goal, an oriented electronic state in the continuum was created by non-resonant ionization of the O2 1s core shell with circularly polarized FEL pulses at hν ≃ 700 eV. An also circularly polarized, highly intense UV laser pulse with hν ≃ 3.1 eV was temporally and spatially overlapped, causing the photoelectrons to redistribute into so-called sidebands that are energetically separated by the photon energy of the UV laser. By determining the circular dichroism of these redistributed electrons using angle resolving electron spectroscopy and modeling the results with the strong-field approximation, this scheme allows to unambiguously determine the absolute degree of circular polarization of any pulsed, ultraintense XUV or X-ray laser source.

Single Shot Polarization Characterization of XUV FEL Pulses from Crossed Polarized Undulators

Polarization control is a key feature of light generated by short-wavelength free-electron lasers. In this work, we report the first experimental characterization of the polarization properties of an extreme ultraviolet high gain free-electron laser operated with crossed polarized undulators. We investigate the average degree of polarization and the shot-to-shot stability and we analyze aspects such as existing possibilities for controlling and switching the polarization state of the emitted light. The results are in agreement with predictions based on Gaussian beams propagation.

Polarization measurement of free electron laser pulses in the VUV generated by the variable polarization source FERMI

FERMI, based at Elettra (Trieste, Italy) is the first free electron laser (FEL) facility operated for user experiments in seeded mode. Another unique property of FERMI, among other FEL sources, is to allow control of the polarization state of the radiation. Polarization dependence in the study of the interaction of coherent, high field, short-pulse ionizing radiation with matter, is a new frontier with potential in a wide range of research areas. The first measurement of the polarization-state of VUV light from a single-pass FEL was performed at FERMI FEL-1 operated in the 52 nm-26 nm range. Three different experimental techniques were used. The experiments were carried out at the end-station of two different beamlines to assess the impact of transport optics and provide polarization data for the end user. In this paper we summarize the results obtained from different setups. The results are consistent with each other and allow a general discussion about the viability of permanent diagnostics aimed at monitoring the polarization of FEL pulses.

Transmission zone plates as analyzers for efficient parallel 2D RIXS-mapping

We have implemented and successfully tested an off-axis transmission Fresnel zone plate as spectral analyzer for resonant inelastic X-ray scattering (RIXS). The imaging capabilities of zone plates allow for advanced two-dimensional (2D) mapping applications. By varying the photon energy along a line focus on the sample, we were able to simultaneously record the emission spectra over a range of excitation energies. Moreover, by scanning a line focus across the sample in one dimension, we efficiently recorded RIXS spectra spatially resolved in 2D, increasing the throughput by two orders of magnitude. The presented scheme opens up a variety of novel measurements and efficient, ultra-fast time resolved investigations at X-ray Free-Electron Laser sources.

Accurate prediction of X-ray pulse properties from a free-electron laser using machine learning

A. Sanchez-Gonzalez,1 P. Micaelli,1 C. Olivier,1 T. R. Barillot,1 M. Ilchen,2, 3 A. A. Lutman,4 A. Marinelli,4 T. Maxwell,4 A. Achner,3 M. Agåker,5 N. Berrah,6 C. Bostedt,4, 7 J. Buck,8 P. H. Bucksbaum,2, 9 S. Carron Montero,4, 10 B. Cooper,1 J. P. Cryan,2 M. Dong,5 R. Feifel,11 L. J. Frasinski,1 H. Fukuzawa,12 A. Galler,3 G. Hartmann,8, 13 N. Hartmann,4 W. Helml,4, 14 A. S. Johnson,1 A. Knie,13 A. O. Lindahl,2, 11 J. Liu,3 K. Motomura,12 M. Mucke,5 C. O’Grady,4 J-E. Rubensson,5 E. R. Simpson,1 R. J. Squibb,11 C. Såthe,15 K. Ueda,12 M. Vacher,16, 17 D. J. Walke,1 V. Zhaunerchyk,11 R. N. Coffee,4 and J. P. Marangos1 1Department of Physics, Imperial College, London, SW7 2AZ, United Kingdom 2Stanford PULSE Institute, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA 3European XFEL GmbH, Holzkoppel 4, 22869 Schenefeld, Germany 4Linac Coherent Light Source, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA 5Department of Physics and Astronomy, Uppsala University, Uppsala, 75120, Sweden 6Department of Physics, University of Connecticut, 2152 Hillside Road, U-3046, Storrs, CT 06269, USA 7Argonne National Laboratory, Lemont, IL 60439, USA 8Deutsches Elektronen-Synchrotron DESY, Notkestrasse 85, Hamburg, 22607, Germany 9Department of Physics, Stanford University, 382 Via Pueblo Mall, Stanford, CA 94305, USA 10California Lutheran University, 60 W Olsen Rd, Thousand Oaks, CA 91360, USA 11Department of Physics, University of Gothenburg, Origovägen 6B, 41296 Gothenburg, Sweden 12Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan 13Institut für Physik und CINSaT, Universität Kassel, Heinrich-Plett-Str. 40, 34132 Kassel, Germany 14Physics Department, TU Munich, James-Franck-Str. 1, 85748 Garching, Germany 15MAX IV Laboratory, Lund University, Box 118, SE-221 00 Lund, Sweden 16Department of Chemistry, Imperial College London, London SW7 2AZ, United Kingdom 17Department of Chemistry Ångtröm, Uppsala University, Uppsala, 75120, SwedenFree-electron lasers providing ultra-short high-brightness pulses of X-ray radiation have great potential for a wide impact on science, and are a critical element for unravelling the structural dynamics of matter. To fully harness this potential, we must accurately know the X-ray properties: intensity, spectrum and temporal profile. Owing to the inherent fluctuations in free-electron lasers, this mandates a full characterization of the properties for each and every pulse. While diagnostics of these properties exist, they are often invasive and many cannot operate at a high-repetition rate. Here, we present a technique for circumventing this limitation. Employing a machine learning strategy, we can accurately predict X-ray properties for every shot using only parameters that are easily recorded at high-repetition rate, by training a model on a small set of fully diagnosed pulses. This opens the door to fully realizing the promise of next-generation high-repetition rate X-ray lasers.

Generating circularly polarized radiation in the extreme ultraviolet spectral range at the free-electron laser FLASH

A new device for polarization control at the free electron laser facility FLASH1 at DESY has been commissioned for user operation. The polarizer is based on phase retardation upon reflection off metallic mirrors. Its performance is characterized in three independent measurements and confirms the theoretical predictions of efficient and broadband generation of circularly polarized radiation in the extreme ultraviolet spectral range from 35 eV to 90 eV. The degree of circular polarization reaches up to 90% while maintaining high total transmission values exceeding 30%. The simple design of the device allows straightforward alignment for user operation and rapid switching between left and right circularly polarized radiation.

Sample refreshment schemes for high repetition rate FEL experiments

Free-electron lasers offer a variety of unique properties for spectroscopy and imaging. The combination of high peak brilliance and a high repetition rate opens a window to experiments that have not been feasible so far but also introduces challenges in sample preparation and refreshment. First experiments at the Linac Coherent Light Source (LCLS) in Stanford showed the potential of free electron lasers for serial X-ray crystallography as well as for imaging non-reproducible objects. Owing to the superconducting accelerator technology, the European X-ray Free-Electron Laser Facility (European XFEL) will allow an average repetition rate of up to 27 kHz with bunch separation in the order of 200 ns. This extremely high repetition rate gives great chances for the scientific impact of the European XFEL, but it also comes with challenges for providing fresh samples for each bunch. This contribution will give an overview of the sample environment techniques that are in consideration for the European XFEL Facility. These techniques include gas phase, liquid, and aerosol sources for life science and physics experiments.

Development status of the X-ray beam diagnostics devices for the commissioning and user operation of the European XFEL

X-ray Free-Electron-Lasers (XFEL) as the Linac Coherent Light Source (LCLS) in the USA, SACLA in Japan, and the European XFEL under construction in Germany are 4th generation light sources which allow research of at the same time extremely small structures (Angstrom resolution) and extremely fast phenomena (femtosecond resolution). Unlike the pulses from a conventional optical laser, the radiation in these sources is created by the Self-Amplified Spontaneous Emission (SASE) process when electron bunches pass through very long segmented undulators. The shot noise at the origin of this process leads to significant pulse-to-pulse variations of pulse intensity, spectrum, wavefront, temporal properties etc. so that for user experiments an online monitoring of these properties is mandatory. Also, the adjustment of the long segmented undulators requires dedicated diagnostics such as an undulator commissioning spectrometer and spontaneous radiation analysis. The extremely high brilliance and resulting single-shot damage issue are difficult to handle for any XFEL diagnostics. Apart from the large energy range of operation of the facility from 280 eV to 25 keV in FEL fundamental, the particular challenge for the European XFEL diagnostics is the high intra bunch train photon pulse repetition rate of 4.5 MHz, potentially causing additional damage by high heat loads and making shot-to-shot diagnostics very demanding. This contribution reports on the facility concepts, recent progress in instrumentation development, and the optimization of diagnostics performance with respect to resolution/accuracy, shot-to-shot capabilities and energy range.

Time-of-flight photoemission spectroscopy from rare gases fornon-invasive, pulse-to-pulse x-ray photon diagnostics at theEuropean XFEL

The European X-ray Free Electron Laser (XFEL.EU) under construction will provide highly brilliant soft to hard X-ray (<280 eV - <20 keV) radiation with an intra-bunch train repetition rate of 4.5 MHz by employing the self-amplified spontaneous emission process. The resulting statistical fluctuations of important beam characteristics makes pulse-to-pulse diagnostics data of the photon beam a mandatory reference during user experiments. We present our concepts of analysing the photoemission from rare gases with a time-of-flight spectrometer for non-invasive, pulse-to-pulse measurements of the photon spectrum and polarization with a special emphasis on real-time processing with a low latency of ≤ 10−5 s.

Angle resolved photoelectron spectroscopy of two-color XUV-NIR ionization with polarization control.

Electron emission caused by extreme ultraviolet (XUV) radiation in the presence of a strong near infrared (NIR) field leads to multiphoton interactions that depend on several parameters. Here, a comprehensive study of the influence of the angle between the polarization directions of the NIR and XUV fields on the two-color angle-resolved photoelectron spectra of He and Ne is presented. The resulting photoelectron angular distribution strongly depends on the orientation of the NIR polarization plane with respect to that of the XUV field. The prevailing influence of the intense NIR field over the angular emission characteristics for He(1s) and Ne(2p) ionization lines is shown. The underlying processes are modeled in the frame of the strong field approximation (SFA) which shows very consistent agreement with the experiment reaffirming the power of the SFA for multicolor-multiphoton ionization in this regime.