PhotonDiag2017 workshop: introductory overview

This issue of the Journal of Synchrotron Radiation is a special issue including papers from the PhotonDiag2017 workshop. Here, a brief introduction is given

Time-delay-compensated grating monochromator for FEL beamlines

We present the design of a time-delay-compensated monochromator explicitly designed for extreme-ultraviolet FEL sources, in particular the upcoming FLASH II at DESY (Hamburg). The design originates from the variable-line-spaced (VLS) grating monochromator by adding a second grating to compensate for the pulse-front tilt given by the first grating after the diffraction. The covered spectral range is 6-60 nm, the spectral resolution is in the range 1000–2000, while the residual temporal broadening is lower than 15 fs. Accounting for typical FLASH II divergences, the grazing angles on the different optics have been chosen so that the mirrors and gratings are respectively shorter than 500 mm and 300 mm. The proposed design: 1) minimizes the number of optical elements, since just one grating is added with respect to a standard VLS monochromator B-L; 2) guarantees high focusing properties in the whole spectral range of operation; 3) requires simple mechanical movements, since only rotations are needed to perform the spectral scan

Grating monochromator with ultrafast response for FLASH2 at DESY

We discuss the design of grating-based monochromators for coherent ultrafast pulses in the extreme-ultraviolet. The main application of such instruments is the monochromatization of ultrafast high-order laser harmonics and freeelectron-laser pulses. We present the conditions to be fulfilled by a grating monochromator that doesn’t increase the pulse duration significantly longer than the Fourier limit. A full correction of the pulse-front tilt requires the use of two gratings in a time-delay compensating configuration. The grating-monochromator configuration is applied to the design of the monochromatic beamline for FLASH2 at DESY. The monochromator has to be tunable in the 50-1000 eV energy range with a resolving power higher than 1000 and an instrumental response shorter than 100 fs in the whole energy range. Given the actual parameters of the FLASH2 radiation and the restrictions in the positioning of the optical elements, the tilt of the pulse-front given by a single grating would give an unacceptable temporal stretching of the pulse. This has to be corrected by a second grating in the compensated configuration. The residual distortion of the pulse-front after the second grating is well below 10 fs

Beam characterization of FLASH from beam profile measurement by intensity transport equation and reconstruction of the Wigner distribution function

Beam parameters of the free-electron laser FLASH @13.5 nm in two different operation modes were determined from beam profile measurements and subsequent reconstruction of the Wigner distribution function behind the ellipsoidal focusing mirror at beamline BL2. 40 two-dimensional single pulse intensity distributions were recorded at each of 65 axial positions around the waist of the FEL beam with a magnifying EUV sensitized CCD camera. From these beam profile data the Wigner distribution function based on different levels of averaging could be reconstructed by an inverse Radon transform. For separable beams this yields the complete Wigner distribution, and for beams with zero twist the information is still sufficient for wavefront determination and beam propagation through stigmatic systems. The obtained results are compared to wavefront reconstructions based on the transport of intensity equation. A future setup for Wigner distribution measurements of general beams is discussed

High-harmonic generation wave front dependence on a driving infrared wave front

With high-harmonic generation (HHG), spatially and temporally coherent XUV to soft x-ray (100 nm to 10 nm)table-top sources can be realized by focusing a driving infrared (IR) laser on a gas target. For applications such ascoherent diffraction imaging, holography, plasma diagnostics, or pump–probe experiments, it is desirable to havecontrol over the wave front (WF) of the HHs to maximize the number of XUV photons on target or to tailor the WF.Here, we demonstrate control of the XUV WF by tailoring the driving IR WF with a deformable mirror. The WFsof both IR and XUV beams are monitored with WF sensors. We present a systematic study of the dependence of theaberrations of the HHs on the aberrations of the driving IR laser and explain the observations with propagationsimulations. We show that we can control the astigmatism of the HHs by changing the astigmatism of the drivingIR laser without compromising the HH generation efficiency with a WF quality fromλ/8 toλ/13.3. This allows usto shape the XUV beam without changing any XUV optical element

Single-shot ptychography at a soft X-ray free-electron laser

Ptychograhy is a scanning coherent diffraction imaging technique capable of providing images of extended samples withdiffraction-limited resolution. However, ptychography experiments are time-consuming due to their scanning nature which alsoprevents their use for imaging of dynamical processes. Recently, setups based on two con-focal lenses were proposed toperform single-shot ptychography in the visible regime by measuring the diffraction pattern produced by multiple overlappingbeams in one shot. However, this approach cannot be extended straightforwardly to X-ray wavelengths due to the application ofrefractive optics. In this work, we demonstrate a novel and nascent single-shot ptychography setup utilizing the combination ofX-ray focusing optics with a two-dimensional beam-splitting diffraction grating. It allows single-shot imaging of extended samplesat X-ray wavelengths. As a proof of concept, we performed single-shot ptychography in the XUV range at the free-electronlaser FLASH and obtained a high-resolution reconstruction of the sample