The soft X-ray monochromator at the SASE3 beamline of the European XFEL: from design to operation

The SASE3 soft X-ray beamline at the European XFEL has been designed and built to provide experiments with a pink or monochromatic beam in the photon energy range 250-3000 eV. Here, the focus is monochromatic operation of the SASE3 beamline, and the design and performance of the SASE3 grating monochromator are reported. The unique capability of a free-electron laser source to produce short femtosecond pulses of a high degree of coherence challenges the monochromator design by demanding control of both photon energy and temporal resolution. The aim to transport close to transform-limited pulses poses very high demands on the optics quality, in particular on the grating. The current realization of the SASE3 monochromator is discussed in comparison with optimal design performance. At present, the monochromator operates with two gratings: the low-resolution grating is optimized for time-resolved experiments and allows for moderate resolving power of about 2000-5000 along with pulse stretching of a few to a few tens of femtoseconds RMS, and the high-resolution grating reaches a resolving power of 10 000 at the cost of larger pulse stretching.

The SASE1 X-ray beam transport system

SASE1 is the first beamline of the European XFEL that became operational in 2017. It consists of the SASE1 undulator system, the beam transport system, and the two scientific experiment stations: Single Particles, Clusters, and Biomolecules and Serial Femtosecond Crystallography (SPB/SFX), and Femtosecond X-ray Experiments (FXE). The beam transport system comprises mirrors to offset and guide the beam to the instruments and a set of X-ray optical components to align, manipulate and diagnose the beam. The SASE1 beam transport system is described here in its initial configuration, and results and experiences from the first year of user operation are reported.

Installation and commissioning of the European XFEL beam transport in the first two beamlines from a metrology point of view

The European XFEL is a large x-ray free-electron laser facility under construction in the Hamburg area of Germany. It is designed to provide a transversally fully coherent x-ray radiation with outstanding characteristics: high repetition rate (up to 2700 pulses with a 0.6 ms long pulse train at 10 Hz, for a total of 27 000 pulses/s), short wavelength (down to 0.05 nm), short pulse (in the femtosecond scale), and high average brilliance [1.6 × 10²⁵ photons/s/(mm²/mrad²)/0.1% bandwidth]. Five main beamlines are foreseen, with three fully financed and installed, called SASEs (from "self-amplified spontaneous-emission"): SASE1 (hard x-rays, 3-25 KeV), SASE2 (hard x-rays, 3 to possibly 60 KeV with the use of a third harmonic), and SASE3 (soft x-rays, 0.3-3 KeV). For each beamline, two separate scientific instruments will be served using the beam alternately in 24-h, 7-day shifts. The installation and commissioning of the European XFEL beamlines are proceeding rapidly. So far, the hard x-ray SASE1 beamline and the soft x-ray SASE3 beamline, both injected with the same electron beam, have been installed and fully commissioned. SASE1 already delivers beam to the corresponding stations and has been open for external users since September 2017. The SASE3 beamline was successfully commissioned in February 2018, and the simultaneous operation of SASE3 and SASE1 was also demonstrated. In the meantime, the SASE2 beamline is being equipped and will be commissioned starting October 2018. We present the last results in the SASE1 and SASE3 beam transport, taking consideration in particular of the metrology carried out before the installation, the installation itself, and the final commissioning. The different stages were crucial to have good quality optical beam and fast commissioning to proceed with the delivery to experiments and users.

Refractive index variation in a free-standing diamond thin film induced by irradiation with fully transmitted high-energy protons

Ion irradiation is a widely employed tool to fabricate diamond micro- and nano-structures for applications in integrated photonics and quantum optics. In this context, it is essential to accurately assess the effect of ion-induced damage on the variation of the refractive index of the material, both to control the side effects in the fabrication process and possibly finely tune such variations. Several partially contradictory accounts have been provided on the effect of the ion irradiation on the refractive index of single crystal diamond. These discrepancies may be attributable to the fact that in all cases the ions are implanted in the bulk of the material, thus inducing a series of concurrent effects (volume expansion, stress, doping, etc.). Here we report the systematic characterization of the refractive index variations occurring in a 38 µm thin artificial diamond sample upon irradiation with high-energy (3 MeV and 5 MeV) protons. In this configuration the ions are fully transmitted through the sample, while inducing an almost uniform damage profile with depth. Therefore, our findings conclusively identify and accurately quantify the change in the material polarizability as a function of ion beam damage as the primary cause for the modification of its refractive index.

Large aperture Fizeau interferometer commissioning and preliminary measurements of a long x-ray mirror at European X-ray Free Electron Laser

The European XFEL (X-ray Free Electron Laser) is a large facility under construction in Hamburg, Germany. It will provide a transversally fully coherent x-ray radiation with outstanding characteristics: high repetition rate (up to 2700 pulses with a 0.6 ms long pulse train at 10 Hz), short wavelength (down to 0.05 nm), short pulse (in the femtoseconds scale), and high average brilliance (1.6 ⋅ 10(25) (photons s(-1) mm(-2) mrad(-2))/0.1% bandwidth). The beam has very high pulse energy; therefore, it has to be spread out on a relatively long mirror (about 1 m). Due to the very short wavelength, the mirrors need to have a high quality surface on their entire length, and this is considered very challenging even with the most advanced polishing methods. In order to measure the mirrors and to characterize their interaction with the mechanical mount, we equipped a metrology laboratory with a large aperture Fizeau interferometer. The system is a classical 100 mm diameter commercial Fizeau, with an additional expander providing a 300 mm diameter beam. Despite the commercial nature of the system, special care has been taken in the polishing of the reference flats and in the expander quality. We report the first commissioning of the instrument, its calibration, and performance characterization, together with some preliminary results with the measurement of a 950 mm silicon substrate. The intended application is to characterize the final XFEL mirrors with nanometer accuracy.

Complex refractive index variation in proton-damaged diamond

An accurate control of the optical properties of single crystal diamond during microfabrication processes such as ion implantation plays a crucial role in the engineering of integrated photonic devices. In this work we present a systematic study of the variation of both real and imaginary parts of the refractive index of single crystal diamond, when damaged with 2 and 3 MeV protons at low-medium fluences (range: 10(15) - 10(17) cm(-2)). After implanting in 125 × 125 μm(2) areas with a scanning ion microbeam, the variation of optical pathlength of the implanted regions was measured with laser interferometric microscopy, while their optical transmission was studied using a spectrometric set-up with micrometric spatial resolution. On the basis of a model taking into account the strongly non-uniform damage profile in the bulk sample, the variation of the complex refractive index as a function of damage density was evaluated.

Relaxation time and viscosity of fused silica glass at room temperature

Cases of long-term deformation of fused silica glass at room temperature attributed to the action of gravity have been reported. Further experimental investigations now provide evidence of time-dependent viscous behavior, with a time constant of the order of 10 years. Data relating to a pair of fused silica reference plates are presented, showing the overall deformation occurred over the years; considerations on the pertaining viscosity with aging are also given. An account of the observed relaxation process in terms of the Kelvin-Voigt model for linear viscoelasticity is provided.

Evidence of light guiding in ion-implanted diamond

We demonstrate the feasibility of fabricating light-waveguiding microstructures in bulk single-crystal diamond by means of direct ion implantation with a scanning microbeam, resulting in the modulation of the refractive index of the ion-beam damaged crystal. Direct evidence of waveguiding through such buried microchannels is obtained with a phase-shift micro-interferometric method allowing the study of the multimodal structure of the propagating electromagnetic field. The possibility of defining optical and photonic structures by direct ion writing opens a range of new possibilities in the design of quantum-optical devices in bulk single-crystal diamond.

One-Step 360 degrees Rainbow Holography with Two Spherical Mirrors

A new optical configuration producing one-step 360 degrees rainbow holograms is presented. The method makes use of two concave mirrors in confocal position, with an annular aperture between the two. An experimental demonstration is given.