Soft X-ray Monochromator Research Articles

Highly efficient and aberration-free off-plane grating spectrometer and monochromator for EUV—soft X-ray applications

We demonstrate a novel flat-field, dual-optic imaging EUV—soft X-ray spectrometer and monochromator that attains an unprecedented throughput efficiency exceeding 60% by design, along with a superb spectral resolution of λ/Δλ > 200 accomplished without employing variable line spacing gratings. Exploiting the benefits of the conical diffraction geometry, the optical system is globally optimized in multidimensional parameter space to guarantee optimal imaging performance over a broad spectral range while maintaining circular and elliptical polarization states at the first, second, and third diffraction orders. Moreover, our analysis indicates minimal temporal dispersion, with pulse broadening confined within 80 fs tail-to-tail and an FWHM value of 29 fs, which enables ultrafast spectroscopic and pump-probe studies with femtosecond accuracy. Furthermore, the spectrometer can be effortlessly transformed into a monochromator spanning the EUV—soft X-ray spectral region using a single grating with an aberration-free spatial profile. Such capability allows coherent diffractive imaging applications to be conducted with highly monochromatic light in a broad spectral range and extended to the soft X-ray region with minimal photon loss, thus facilitating state-of-the-art imaging of intricate nano- and bio-systems, with a significantly enhanced spatiotemporal resolution, down to the nanometer–femtosecond level.

Status and future of the soft X-ray free-electron laser beamline at the SHINE

The Shanghai High repetition rate XFEL and Extreme light facility (SHINE) is under construction and aims at generating X-rays between 0.4 and 25 keV with three FEL beamlines at repetition rates of up to 1 MHz. The soft X-ray FEL beamline, FEL-II, will be ready for commissioning in 2025. It is designed to cover the photon energy from 0.4 to 3 keV, in which the baselines of the FEL operation modes are self-amplified spontaneous emission (SASE), self-seeding, echo-enabled harmonic generation (EEHG), and polarization control. Therefore, a high repetition-rate external seed laser, large period length modulator, soft X-ray monochromator, planar undulator, and elliptically polarized undulator have been adopted in the FEL-II beamline. Several potentials such as an ultra-short pulse mode and a multi-color mode are also foreseeable without significant equipment changes in the follow-up operation. A dual-period undulator design is suggested for the echo-enabled harmonic generation (EEHG) commissioning, and it has great potential to break through the unreachable energy of the fully coherent X-ray in the future. The FEL-II beamline will deliver SASE radiation and fully coherent radiation in all the wavelengths of interest.

Короткопериодные многослойные зеркала для высокоразрешающего монохроматора многослойное зеркало/кристалл

The paper presents the results of studies of W/B4C multilayer structures with small periods for their use as the first mirror in a two-mirror soft X-ray monochromator at the KOSMOS station of the VEPP-4M synchrotron. It is shown that even if the periods of the mirror and the RbAP crystal coincide, when operating in a wide wavelength range, the Bragg angle must be adjusted due to the stronger refraction in the W/B4C multilayer mirror.

Design of nano-ARPES beamline at 3-GeV next-generation synchrotron radiation facility, NanoTerasu

The beamline for angle-resolved photoemission spectroscopy (ARPES) with a nano-focused beam (nano-ARPES) is planned as one of the public beamlines in NanoTerasu, the 3-GeV next-generation synchrotron radiation facility at Tohoku, Japan, which is scheduled to start operation in 2024. A 4 m long APPLE-II type undulator provides brilliant soft X-rays with various polarizations (linear horizontal and vertical, and left and right circular) in the energy range of 50-1,000 eV. A collimated plane grating monochromator is adopted as a soft X-ray monochromator because we can select the demagnification factor of grating cf f = cosα/cosβ between the high energy-resolution (HR) mode for HR-ARPES experiments and the low divergence mode for high-flux nano-ARPES measurements. This beamline aims to produce the high flux nano-focused beam by using ultrahigh precision reflective mirror optics, not a Fresnel zone plate used as a focusing optics in the previous nano-ARPES station at other synchrotron facilities.

A WISER way for simulating the performance of gratings.

Soft X-ray monochromators for synchrotron radiation sources have been continuously developed over the years, improving energy resolution and stability. Great effort has been made in improving the surface quality of the optics involved, reaching values permitting diffraction-limited images. Still, one problem has not been solved, nor fully understood, yet: groove placing errors. Nowadays, these are one of the major factors responsible for resolving the power reduction of diffraction-gratings-based X-ray monochromators. Despite decades of use of gratings, there is not yet a universally established method for predicting and simulating the effect of groove placing errors on monochromator performance. This is especially important in the new scenario of high-coherent X-ray sources, i.e. diffraction-limited storage rings and free-electron lasers. To address this problem, in this article an approach based on WISER (Wavefront propagatIon Simulation codE libRary) is presented. WISER is a physical optics simulation package, also available in the user-friendly Orange Synchrotron Radiation Suite - OASYS. Even though it was originally conceived to assess the focusing performance of X-ray mirrors in the presence of height defects, it perfectly simulates the performance of a periodic (or quasi-periodic) structure like a diffraction grating. In this article, the way to use WISER and its application to a specific case, e.g. the design of a monochromator for the upgrade of a beamline at the Advanced Light Source, are shown. A simple rule for estimating how well the grooves are placed on a grating, based on calculation of the Strehl ratio, is also presented.

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.

Understanding the mechanical limitations of the performance of soft X-ray monochromators at MAX IV laboratory.

MAX IV is a fourth-generation, or diffraction-limited, synchrotron light source with a number of state-of-the-art beamlines. The performance of a beamline is, to a high degree, set by the energy resolution it can achieve, which in turn is governed to a large extent by the monochromator. During the design phase of a monochromator, the mechanical requirements must be fully understood and met with margin. During commissioning, the performance must be verified and optimized. In this paper, six soft X-ray monochromators at MAX IV beamlines (Bloch, Veritas, HIPPIE, SPECIES, FinEstBeAMS and SoftiMAX) are examined with a focus on their resolving power, energy range and the time required to change measurement range, as those parameters are dependent on each other. The monochromators have a modern commercial design, planned and developed in close collaboration with the vendors. This paper aims to present the current status of the commissioning at MAX IV with emphasis on elucidating the mechanical limitations on the performance of the monochromators. It contains analysis of the outcome and our approach to achieve fast and high-resolution monochromators.

On the advantages of operation in second-order diffraction of blazed gratings in soft X-ray monochromators.

The fact that a diffraction grating can provide twofold-smaller bandwidth when operated in second-order diffraction is long known and applied routinely in the laboratory for spectroscopy in the visible and ultraviolet spectral range. A similar routine operation in monochromators for the soft X-ray range is not reported yet. This study will thus address the feasibility of efficient diffraction of soft X-rays in the second order at reflection gratings when operated at grazing angles of incidence. The related systematic study could make profitable use of a recently introduced simple analytical equation for the prediction of the diffraction efficiency of blazed gratings with an ideal sawtooth profile. The predictions are then verified by use of rigorous calculations. The principle finding is that, by operation of gratings with lower groove densities, and thus with higher efficiencies, in higher order diffraction, one can extend the tuning in existing instruments with mechanical/optical limitations to larger photon energies. The performance in terms of transmission and spectral resolving power can be very similar to the performance of a grating with a larger groove density, which would otherwise have to be used for accessing the same energy range. This would allow operation of a single highly efficient grating over a larger photon energy interval at a modern synchrotron radiation source, e.g. from 0.3 to 2.2 keV. Without any requirement for a sophisticated grating exchange scheme, a related instrument promises to be sufficiently stable for the needs imposed by the improvements in source point stability at diffraction-limited storage rings.

Operation of photon diagnostic imagers for beam commissioning at the European XFEL.

X-ray photon beam diagnostic imagers are located at 24 positions in the European XFEL beam transport system to characterize the X-ray beam properties, and to give feedback for tuning and optimization of the electron acceleration and orbit, the undulators, and the X-ray optics. One year of commissioning allowed experience to be gained with these imagers, which will be reported here. The sensitive Spontaneous Radiation imager is useful for various investigations in spontaneous radiation mode: for undulator adjustments and for low-signal imaging applications. Thehigh-resolution Free-Electron Laser imager, 10 µm spatial resolution, is extensively used for the monitoring of beam position, spot size and shape, gain curve measurements, and also for beam-intensity monitoring. The wide field-of-view pop-in monitors (up to 200 mm) are regularly used for alignment and tuning of the various X-ray optical components like mirrors, slits and monochromators, and also for on-line beam control of a stable beam position at the instruments. The Exit Slit imager after the soft X-ray monochromator provides spectral information of the beam together with multi-channel plate based single-pulse gating. For particular use cases, these special features of the imagers are described. Some radiation-induced degradation of scintillators took place in this initial commissioning phase, providing useful information for better understanding of damage thresholds. Visible-light radiation in the beam pipe generated by upstream bending magnets caused spurious reflections in the optical system of some of the imagers which can be suppressed by aluminium-coated scintillating screens.

On smart optimization of blazed soft X-ray gratings.

The first attempts to calculate the diffraction efficiencies of gratings in the soft X-ray range were made on a scalar model. The results were simple analytical equations, that always severely overestimated the performance of real objects. In this respect, computer programs were found to be more successful, which rigorously consider all diffracted and refracted waves. Consequently soft X-ray gratings are presently optimized using these tools, which requires rather extensive calculations for any instrument optimization as general trends are not immediately obvious. Here it will be shown that the results of the rigorous calculations for gratings with blaze or sawtooth profile can be approximated rather well with a simple analytical equation. This equation contains three multiplicative factors, which deal independently with the effect of the reflectivity, the blaze angle and the groove density. This opens the possibility to initially ignore the effects of the blaze angle and thus to start an optimization in a very general way. Such optimization can be based on isoreflectivity curves and it can then provide `blaze maximum efficiency maps', i.e. simple images. In these latter images, one can identify directly the optimum parameters for a grating, i.e. the groove density providing best efficiency for a requested spectral resolving power. Only successively will the blaze angle have to be fixed. Its choice is then not the result of an extensive optimization process but of a simple calculation applied for the photon energy at which maximum efficiency performance is requested. The maps presented here are used for the optimization of a medium-resolving-power soft X-ray monochromator, which can scan the photon energy range 300-2000 eV.

Quantum efficiency measurement of an x-ray charge-coupled device by means of an x-ray generator monochromatized over the 0.2-1.2 keV spectral region.

The Atomic Energy Commission has been developing an x-ray calibration laboratory for several years close to the Laser MégaJoule Facility mainly to calibrate x-ray cameras. Based on x-ray generators, great effort is being carried out to obtain x-ray monochromatic sources over the 0.1-10 keV spectral range. Moreover, their routine availability is a prerequisite while getting reduced uncertainties. To achieve this purpose, two x-ray monochromators have been recently developed, the former running below 1.5 keV and the latter in the harder x-ray region. The work presented in this paper constitutes, to the best of our knowledge, the first radiometric calibration of an x-ray camera, namely, a charge-coupled device (CCD). The CCD is thin and back illuminated with a 1300×1340 pixel, 20 μm×20 μm area. It has been calibrated by means of our soft x-ray monochromator based on Rowland circle geometry. This apparatus is able to take on board simultaneously two detectors under vacuum: the CCD to be calibrated and a silicon drift spectrometer (SDD), which acts as a reference detector to obtain the x-ray monochromatic intensity. An internal specific vacuum manipulator allows putting successively each detector in front of the scanning exit slit. Quantum efficiency (QE) of the CCD has thus been measured over the 0.2-1.2 keV region, due to the reference SDD. The latter was previously calibrated on the same monochromator by cross-calibration with another SDD calibrated at the Physikalisch-Technische Bundesanstalt (PTB) Radiometry Laboratory, on the synchrotron radiation facility BESSY. QE measurements are then compared to the CCD manufacturer's model and with previous ones performed on the same camera by means of our hard x-ray monochromator (2-10 keV). Comparison of these results with further PTB efficiency calibration of the reference SDD used to calibrate the CCD is discussed and allows concluding definitively about the validity of measurements performed by our x-ray monochromators.

Damage thresholds for blaze diffraction gratings and grazing incidence optics at an X-ray free-electron laser.

The Linac Coherent Light Source is upgrading its machine to high repetition rate and to extended ranges. Novel coatings, with limited surface oxidation, which are able to work at the carbon edge, are required. In addition, high-resolution soft X-ray monochromators become necessary. One of the big challenges is to design the mirror geometry and the grating profile to have high reflectivity (or efficiency) and at the same time survive the high peak energy of the free-electron laser pulses. For these reasons the experimental damage threshold, at 900 eV, of two platinum-coated gratings with different blazed angles has been investigated. The gratings were tested at 1° grazing incidence. To validate a model for which the damage threshold on the blaze grating can be estimated by calculating the damage threshold of a mirror with an angle of incidence identical to the angle of incidence on the grating plus the blaze angle, tests on Pt-coated substrates have also been performed. The results confirmed the prediction. Uncoated silicon, platinum and SiB3 (both deposited on a silicon substrate) were also investigated. In general, the measured damage threshold at grazing incidence is higher than that calculated under the assumption that there is no energy transport from the volume where the photons are absorbed. However, it was found that, for the case of the SiB3 coating, the grazing incidence condition did not increase the damage threshold, indicating that the energy transport away from the extinction volume is negligible.

Absolute, high-accuracy characterization of a variable line spacing grating for the European XFEL soft X-ray monochromator.

The European XFEL is a free-electron laser (FEL) with a superconducting linear accelerator and three beamlines (SASE1, SASE2, and SASE3) covering the energy range from 250 eV to 24 keV. The SASE3 beamline is dedicated to the soft x-ray range (0.25-3 keV) and is designed to operate in both monochromatic and non-monochromatic mode. A variable line spacing - plane grating (VLS-PG) monochromator is placed along the beam transport system for the monochromatic mode. The VLS parameters of the grating profile are challenging from a manufacturing and measuring perspective, especially at the desired length of 530 mm. A shorter grating of 150 mm has been procured to allow early operation of the facility. We describe the characterization method that was used to assess the VLS parameters of the grating using Fizeau interferometry. The method is intrinsically absolute and limited only by the quality of the test grating and the noise level. Further measurements using a white-light-interferometry profilometer are also reported. We discuss the possibility of extending the method to the future 530 mm long grating.

On the contribution of gratings with laterally graded groove depths to the design and performances of SOLEIL soft X-ray monochromators

The so called varied groove depth (VGD) etching technology, developed by Horiba-Jobin-Yvon, which consists in laterally grading the groove depth of a lamellar grating, is exploited in the monochromators of the seven soft x-ray beamlines of SOLEIL. With a VGD profile a single grating can perform in more than a decade wide energy band and some control is given over the harmonic content. It also requires a specific, but easily achieved design of the beamline optics to match the lateral footprint of the beam to the depth variation. Performances predicted by computation with electromagnetic propagation code are supported by flux measurements of the two first harmonics in the experimental chamber of a photoemission beamline, and by measurements of grating efficiencies on SOLEIL etrology beamline.

Growth of Multilayer Optics for Synchrotron Radiation Sources

W/B4C multilayers have been deposited using magnetron sputtering with period ranging from 3.5 nm to 1.6 nm and layer pairs up to 300 to be used as soft x-ray /hard x-ray monochromators and soft x-ray polarizers. Normal incidence mirrors have been developed to be used in 12.5−13.5 nm using Mo/Si with reflectivity of ~ 67 % at 13 nm. In addition, high thermal stability NbC/Si multilayers in this wavelength range have been developed. The reflectivity of these mirrors is comparable with Mo/Si but have much higher thermal stability compared to Mo/Si. We have also deposited thermally stable C/B4C, low Z/low Z multilayer. These mirrors can be used as polarizer and high resolution soft x-ray monochromators.

A time-preserving soft X-ray monochromator for a FEL source: design optimisation using Genesis simulations and wavefront propagation

A design concept for a soft X-ray monochromator that can preserve the length of a FEL pulse at the femtosecond level is presented. The design is based around a plane diffraction grating in a classical mount operating in collimated light. Designing the beamline requires a thorough knowledge of the source behaviour, which is achieved by wavefront propagation of the simulations of the FEL output. The effect on the spectral filtering on the pulse profile has been explored and is found to improve the contrast ratio without significantly reducing the peak power.

Optical polarizer integrated with suppression of higher harmonics in the vacuum ultraviolet and soft x-ray spectral regions

Based on a concept of Au–Si–Au triple reflection, we present a low pass polarizer that suppresses higher-order harmonics from vacuum ultraviolet and soft x-ray monochromators. By appropriate optimization of the material and the angle of incidence, the polarizer enables the possibility to tune its cutoff edge of photon energy, and therefore to suppress higher-order diffraction light. Vacuum ultraviolet ellipsometry experiments with BESSY II synchrotron radiation in 9–23eV have demonstrated the suppression of higher-order harmonics.

Design and performance of a tunable polarizer for VUV and soft x-ray spectral regions

A low pass polarizer is presented for suppressing higher order diffraction light from vacuum ultraviolet and soft x-ray monochromators. By optimization of materials combination and angle of incidence, the polarizer demonstrates the capability to match to the desired photon energy cut-off. With the proposed hybrid polarizer, a higher order suppression unit, such as a thin film filter, gas filter or double reflecting mirror, is no longer required. The filtering effect of higher harmonics is integrated within the proposed polarizer.

Tunable thin film polarizer for the vacuum ultraviolet and soft x-ray spectral regions

A low pass polarizer that suppresses higher-order diffraction light from vacuum ultraviolet and soft x-ray monochromators is presented in this paper. This vacuum ultraviolet and soft x-ray polarizer is based on a concept of sandwiched metal-dielectric-metal triple reflection configuration. By appropriate optimization of material and angle of incidence, the proposed Au–SiC–Au polarizer demonstrates the capability of matching to desired cutoff edge of photon energy. Furthermore, the optimized soft x-ray polarizer shows the possibility to tune cutoff photon energy in a broadband spectral region ranging from 80 down to down to 20eV.

Core Excitation and De-excitation Spectroscopies of Free Atoms and Molecules

This article provides a review of the current status of core excitation and de-excitation spectroscopy studies of free atoms and molecules using a high-resolution soft X-ray monochromator and a high-resolution electron energy analyzer, installed in the soft X-ray photochemistry beam line at SPring-8. Experimental results are discussed for 1s excitation of Ne, O 1s excitation of CO and H 2 O, and F 1s excitation of CF 4 .