Coherent X-ray Research Articles

4th generation synchrotron source boosts crystalline imaging at the nanoscale

New 4th-generation synchrotron sources, with their increased brilliance, promise to greatly improve the performances of coherent X-ray microscopy. This perspective is of major interest for crystal microscopy, which aims at revealing the 3D crystalline structure of matter at the nanoscale, an approach strongly limited by the available coherent flux. Our results, based on Bragg ptychography experiments performed at the first 4th-generation synchrotron source, demonstrate the possibility of retrieving a high-quality image of the crystalline sample, with unprecedented quality. Importantly, the larger available coherent flux produces datasets with enough information to overcome experimental limitations, such as strongly deteriorated scanning conditions. We show this achievement would not be possible with 3rd-generation sources, a limit that has inhibited the development of this otherwise powerful microscopy method, so far. Hence, the advent of next-generation synchrotron sources not only makes Bragg ptychography suitable for high throughput studies but also strongly relaxes the associated experimental constraints, making it compatible with a wider range of experimental set-ups at the new synchrotrons.

Biomimetic Strain-Stiffening in Chitosan Self-Healing Hydrogels.

The strain-stiffening and self-healing capabilities of biological tissues enable them to preserve the structures and functions from deformation and damage. However, biodegradable hydrogel materials with both of these biomimetic characteristics have not been explored. Here, a series of strain-stiffened, self-healing hydrogels are developed through dynamic imine crosslinking of semiflexible O-carboxymethyl chitosan (main chain) and flexible dibenzaldehyde-terminated telechelic poly(ethylene glycol) (crosslinker). The biomimetic hydrogels can be reversibly stiffened to resist the deformation and can even recover to their original state after repeated damages. The mechanical properties and stiffening responses of the hydrogels are tailored by varying the component contents (1-3%) and the crosslinker length (4 or 8 kDa). A combinatorial system of in situ coherent small-angle X-ray scattering with rheological testing is developed to investigate the network structures (in sizes 1.5-160 nm) of hydrogels under shear strains and reveals that the strain-stiffening originates from the fibrous chitosan network with poly(ethylene glycol) crosslinking fixation. The biomimetic hydrogels with biocompatibility and biodegradability promote wound healing. The study provides an insight into the nanoscale design of biomimetic strain-stiffening self-healing hydrogels for biomedical applications.

Design and performance of a dedicated coherent X-ray scanning diffraction instrument at beamline NanoMAX of MAX IV.

The diffraction endstation of the NanoMAX beamline is designed to provide high-flux coherent X-ray nano-beams for experiments requiring many degrees of freedom for sample and detector. The endstation is equipped with high-efficiency Kirkpatrick-Baez mirror focusing optics and a two-circle goniometer supporting a positioning and scanning device, designed to carry a compact sample environment. A robot is used as a detector arm. The endstation, in continued development, has been in user operation since summer 2017.

Shot-to-shot flat-field correction at X-ray free-electron lasers.

X-ray free-electron lasers (XFELs) provide high-brilliance pulses, which offer unique opportunities for coherent X-ray imaging techniques, such as in-line holography. One of the fundamental steps to process in-line holographic data is flat-field correction, which mitigates imaging artifacts and, in turn, enables phase reconstructions. However, conventional flat-field correction approaches cannot correct single XFEL pulses due to the stochastic nature of the self-amplified spontaneous emission (SASE), the mechanism responsible for the high brilliance of XFELs. Here, we demonstrate on simulated and megahertz imaging data, measured at the European XFEL, the possibility of overcoming such a limitation by using two different methods based on principal component analysis and deep learning. These methods retrieve flat-field corrected images from individual frames by separating the sample and flat-field signal contributions; thus, enabling advanced phase-retrieval reconstructions. We anticipate that the proposed methods can be implemented in a real-time processing pipeline, which will enable online data analysis and phase reconstructions of coherent full-field imaging techniques such as in-line holography at XFELs.

High-repetition-rate seeded free-electron laser with direct-amplification of an external coherent laser

Various scientific and industrial researches such as spectroscopy and advanced nano-technologies have been demanding high flux and fully coherent extreme ultraviolet (EUV) and x-ray radiation. These demands can be commendably satisfied with a MHz-level repetition-rate seeded free-electron laser (FEL). Dictated by the seed laser system, seeded FELs have faced obstacles for the realization of MHz repetition rate. Reducing the required peak power of an external coherent seed laser can effectively increase the repetition rate of a seeded FEL. This paper presents a novel technique that employs a long modulator as a carrier for laser amplification and electron modulation, which is quite different from nominal seeded FELs. Applications of the proposed technique into high-gain harmonic generation (HGHG) and echo-enabled harmonic generation (EEHG) are investigated. Simulation results demonstrate that seed laser power is reduced by about three orders of magnitude and the FEL radiation possesses consistently high coherence and power stability with respect to the nominal HGHG or EEHG. The proposed technique paves the way for the realization of fully coherent EUV and soft x-ray FELs with a repetition rate of MHz and an average power of about 100 W.

SPring-8におけるコヒーレントX線回折イメージングの展開

SPring-8におけるコヒーレントX線回折イメージングの展開

Wave-optics simulation software for synchrotron radiation from 4th generation storage rings based on a coherent modes model.

The evolution from 3rd to 4th generation of storage rings significantly enhanced the coherence of synchrotron radiation sources, making coherent scattering techniques such as coherent X-ray diffraction imaging (CXDI) and X-ray photon correlation spectroscopy more accessible. In conformance with the design requirements of coherent beamlines at the High Energy Photon Source (HEPS), we have developed wave optics simulation software, the Coherence Analysis Toolbox, based on coherent modes decomposition and a wavefront propagation model. Simulations of beamline performance and a CXDI experiment on the hard X-ray coherent scattering beamline at HEPS were carried out. This software is open source and now available on GitHub.

Online single-shot characterization of ultrafast pulses from high-gain free-electron lasers

X-ray free-electron lasers (FELs) provide cutting-edge tools for fundamental researches to study nature down to the atomic level at a time-scale that fits this resolution. A precise knowledge of temporal information of FEL pulses is the central issue for its applications. Here we proposed and demonstrated a novel method to determine the FEL temporal profiles online. This robust method, designed for ultrafast FELs, allows researchers to acquire real-time longitudinal profiles of FEL pulses as well as their arrive times with respect to the external optical laser with a resolution better than 6 fs. Based on this method, we can also directly measure various properties of FEL pulses and correlations between them online. This helps us to further understand the FEL lasing processes and realize the generation of stable, nearly fully coherent soft X-ray laser pulses at the Shanghai Soft X-ray FEL facility. This method will enhance the experimental opportunities for ultrafast science in various areas.

Research Spotlights

Research Spotlights

New tools for membrane protein structural biology at X-ray free electron lasers

By utilizing their ultra-fast, intense, and highly coherent X-ray pulses, serial femtosecond crystallography (SFX) at X-ray free electron lasers (XFELs) has proven a powerful tool for structural characterization of membrane proteins, which are challenging to crystallize due to solubility and stability obstacles during expression and purification. However, the limitations of both detergents to solubilize membrane proteins in native-like environments, and delivery of the microcrystalline sample into the X-ray beam path while maintaining low X-ray scatter background and with minimal sample consumption, remain technical challenges, especially where this methodology is applied to relatively low-ordered samples or those difficult to purify and crystallize in large quantities.

Sizes of pure and doped helium droplets from single shot x-ray imaging.

Advancements in x-ray free-electron lasers on producing ultrashort, ultrabright, and coherent x-ray pulses enable single-shot imaging of fragile nanostructures, such as superfluid helium droplets. This imaging technique gives unique access to the sizes and shapes of individual droplets. In the past, such droplet characteristics have only been indirectly inferred by ensemble averaging techniques. Here, we report on the size distributions of both pure and doped droplets collected from single-shot x-ray imaging and produced from the free-jet expansion of helium through a 5μm diameter nozzle at 20 bars and nozzle temperatures ranging from 4.2 to 9K. This work extends the measurement of large helium nanodroplets containing 109-1011 atoms, which are shown to follow an exponential size distribution. Additionally, we demonstrate that the size distributions of the doped droplets follow those of the pure droplets at the same stagnation condition but with smaller average sizes.

Applicability of coherent x-ray diffractive imaging to ferroelectric, ferromagnetic, and phase change materials

Rapid development in the field of ferroelectric and magnetic materials has attracted much interest in the past decade. The underlying mechanisms of the fundamental phenomenon of phase transitions in these materials are extremely important in understanding their physical properties and their potential technological applications. Therefore, it is vital for the advancement of high-resolution versatile imaging techniques that enable high-throughput and nano-scale characterization in the nano-crystals and electronic devices. X-ray based imaging techniques such as Bragg coherent x-ray diffractive imaging (CXDI) has been one of the dominant nondestructive imaging tools with high-resolution and refraction sensitivities that provide quantitative information in bulk and nano-scale crystals and their associated nano-devices. In this review, we will focus on the recent developments of using Bragg CXDI at the state-of-art endstations in synchrotron radiation facilities worldwide to understand the structures of ferroelectric and magnetic materials and the structural phase transition of complex materials. We will also present our perspectives on the opportunities and challenges in using Bragg CXDI techniques for materials characterization.

Generating coherent and ultrashort X-ray pulses via HHG-seeding in storage rings.

The generation of fully coherent and femtosecond time-scale radiation pulses in the X-ray regime is one of the most common demands of ring-based synchrotron light source users. In this paper, a method that utilizes the recent proposed angular dispersion induced microbunching technique to convert external light from high-harmonic generation (HHG) to coherent light at shorter wavelength is proposed. Numerical simulations using the practical parameters of a diffraction-limited storage ring demonstrate the generation of coherent pulse trains with photon energy as high as 2 keV, pulse duration as short as ∼10 fs and high peak brightness directly from an HHG source at 13 nm.

Three-dimensional nano-coherent diffraction imaging technology based on high order harmonic X-ray sources

Coherent diffractive imaging (CDI) using ultra-short wavelength light source has become an three-dimensional(3D) nanoimaging technique. In CDI, a target sample is first illuminated by a coherent EUV and soft X-ray light, then the diffraction pattern is recorded by using a charge coupled device (CCD), and finally the image of the sample is obtained based on the pattern by using a phase retrieval algorithm. Of the many currently available coherent EUV and soft X-ray light sources, the high-order harmonic generation (HHG) is the simplest in structure, the lowest in cost, and most compact in size. Therefore, it has become the most promising light source for CDI. Through years of development, HHG based CDI technique(HHG-CDI) has become an outstanding 3D nano-imaging technique with the advantages of no aberration, no damage, and no contact either, and it also possesses the extra-capabilities of probing the dynamics, chemical composition and quantum information in various semiconductor and quantum devices. We believe that the HHG-CDI will soon become a generic nano-imaging tool that can complement or even replace the matured nanoimaging techniques, such as atomic force, near field, X-ray, electron, or scanning tunneling microscopes.

Collagen Structure Orientation at the Bone–Cartilage Interface

Abstract The articular cartilage overlying subchondral bone consists of variously oriented collagen fibres, the detailed organisation of which is essential for cartilage integrity that has suffered damage in degenerative joint disease such as osteoarthritis (OA). A technique attracting particular interest is that of coherent small-angle X-ray scattering mapping; the present investigations are supported by PILATUS, a highly pixelated two-dimensional detector system developed at the Paul Scherrer Institute (PSI). The system has yielded information on the anatomical features that correspond to the large-scale organisation of collagen and the mineralised phase contained within the collagen fibres in the deep cartilage zone. Results obtained from a decalcified cartilage–bone sample have been plotted in terms of the orientation of cartilage and bone components for particular intervals of k-space, showing the organisation of collagen-II within superficial layers and collagen-X in the chondrocyte-rich deeper layers of cartilage. It is apparent in undamaged cartilage that there is a gradual reorientation of the collagen-II fibres of the cartilage, from parallel to the surface of the joint, to normal to the cartilage–bone interface. A similar pattern of orientation is seen below the cement line (the surface at which cartilage anchors to the bone), for the collagen type-I prevalent in bone. Changing the interval of k-space allows subchondral features such as the microvasculature to be seen.

QED effects on high-order harmonic generation from ultraintense laser-irradiated solid-plasmas

High-order harmonic generation (HHG) from relativistically intense laser irradiation of solid-density plasmas provides novel coherent light sources in the extreme-ultraviolet (XUV) to x-ray spectral region with attosecond duration and high intensity even toward the Schwinger limit. However, whether the HHG process and mechanism are effective at extremely high laser intensities in the QED-plasma regime remains unclear, since QED processes, such as discrete synchrotron radiation, quantum radiation reaction, and pair creation, would change the energy partition and particle dynamics. In this work, through particle-in-cell simulations, we numerically demonstrate that the HHG process with the relativistically oscillating mirror mechanism can be scaled to an ultrahigh intensity level of 1024 W/cm2. The high harmonic spectrum, attosecond pulses, and conversion efficiency are not significantly affected by the QED processes. This result is of great benefit to the generation of coherent XUV and x-ray light sources with higher intensity, higher cutoff frequency, and shorter duration.

Lensless X-Ray Nanoimaging: Revolutions and opportunities

Lensless X-ray nanoimaging provides 3D views of a wide range of materials with a spatial resolution better than 10 nm. These advances are enabled in part by dramatic gains in coherent X-ray flux, but they also rely on advances in signal processing to obtain images from coherent diffraction data. We outline the scientific problems that can be addressed by X-ray nanoimaging, the various imaging approaches and their associated reconstruction methods, and highlight opportunities for future advances.

Generating Isolated Attosecond X-Ray Pulses by Wavefront Control in a Seeded Free-Electron Laser

We proposed a simple method based on the seeded free-electron laser (FEL) to generate fully coherent X-ray pulses with durations at dozens of attosecond level. The echo-enabled harmonic generation technique is utilized to generate the fully coherent laser pulse covering the water-window range. A wavefront rotation laser is adopted as the seed to tailor the longitudinal contour of the radiation pulse. Due to the sensitivity of seeded FEL to external lasers, this method can effectively inhibit the bunching of the adjacent regions while preserving an isolated bunching in the middle. Sending such an electron beam into a short undulator, simulation results show that ultrashort X-ray pulses with peak power of GW level and pulse duration as short as 86 attoseconds can be generated. The proposed scheme can make it possible to study the electronic dynamic of the valence electrons of which the time scale is about 100 attoseconds and may open up a new frontier of ultrafast science.

Using coherent X-rays to follow dynamics in amorphous ices.

Amorphous solid water plays an important role in our overall understanding of water's phase diagram. X-ray scattering is an important tool for characterising the different states of water, and modern storage ring and XFEL facilities have opened up new pathways to simultaneously study structure and dynamics. Here, X-ray photon correlation spectroscopy (XPCS) was used to study the dynamics of high-density amorphous (HDA) ice upon heating. We follow the structural transition from HDA to low-density amorphous (LDA) ice, by using wide-angle X-ray scattering (WAXS), for different heating rates. We used a new type of sample preparation, which allowed us to study μm-sized ice layers rather than powdered bulk samples. The study focuses on the non-equilibrium dynamics during fast heating, spontaneous transformation and crystallization. Performing the XPCS study at ultra-small angle (USAXS) geometry allows us to characterize the transition dynamics at length scales ranging from 60 nm-800 nm. For the HDA-LDA transition we observe a clear separation in three dynamical regimes, which show different dynamical crossovers at different length scales. The crystallization from LDA, instead, is observed to appear homogenously throughout the studied length scales.

In situ Bragg coherent X-ray diffraction imaging of corrosion in a Co–Fe alloy microcrystal

Corrosion is a major concern for many industries, as corrosive environments can induce structural and morphological changes that lead to material dissolution and accelerate material failure. The progression of corrosion depends on nanoscale morphology, stress, and defects present. Experimentally monitoring this complex interplay is challenging. Here we implement in situ Bragg coherent X-ray diffraction imaging (BCDI) to probe the dissolution of a Co–Fe alloy microcrystal exposed to hydrochloric acid (HCl). By measuring five Bragg reflections from a single isolated microcrystal at ambient conditions, we compare the full three-dimensional (3D) strain state before corrosion and the strain along the [111] direction throughout the corrosion process. We find that the strained surface layer of the crystal dissolves to leave a progressively less strained surface. Interestingly, the average strain closer to the centre of the crystal increases during the corrosion process. We determine the localised corrosion rate from BCDI data, revealing the preferential dissolution of facets more exposed to the acid stream, highlighting an experimental geometry effect. These results bring new perspectives to understanding the interplay between crystal strain, morphology, and corrosion; a prerequisite for the design of more corrosion-resistant materials.