Ultracompact mirror device for forming 20-nm achromatic soft-X-ray focus toward multimodal and multicolor nanoanalyses.

Nanoscale soft-X-ray microscopy is a powerful analysis tool in biological, chemical, and physical sciences. To enhance its probe sensitivity and leverage multimodal soft-X-ray microscopy, precise achromatic focusing devices, which are challenging to fabricate, are essential. Here, we develop an ultracompact Kirkpatrick-Baez (ucKB) mirror, which is ideal for the high-performance nanofocusing of broadband-energy X-rays. We apply our advanced fabrication techniques and short-focal-length strategy to realize diffraction-limited focusing over the entire soft-X-ray range. We achieve a focus size of 20.4 nm at 2 keV, which represents a significant improvement in achromatic soft-X-ray focusing. The ucKB mirror extends soft-X-ray fluorescence microscopy by producing a bicolor nanoprobe with a 1- or 2-keV photon energy. We propose a subcellular chemical mapping method that allows a comprehensive analysis of specimen morphology and the distribution of light elements and metal elements. ucKB mirrors will improve soft-X-ray nanoanalyses by facilitating photon-hungry, multimodal, and polychromatic methods, even with table-top X-ray sources.

In situ high-pressure pair distribution function measurement of liquid and glass by using 100 keV pink beam.

Understanding the pressure-induced structural changes in liquids and amorphous materials is fundamental in a wide range of scientific fields. However, experimental investigation of the structure of liquid and amorphous material under in situ high-pressure conditions is still limited due to the experimental difficulties. In particular, the range of the momentum transfer (Q) in the structure factor [S(Q)] measurement under high-pressure conditions has been limited at relatively low Q, which makes it difficult to conduct detailed structural analysis of liquid and amorphous material. Here, we show the in situ high-pressure pair distribution function measurement of liquid and glass by using the 100 keV pink beam. Structures of liquids and glasses are measured under in situ high-pressure conditions in the Paris-Edinburgh press by high-energy x-ray diffraction measurement using a double-slit collimation setup with a point detector. The experiment enables us to measure S(Q) of GeO2 and SiO2 glasses and liquid Ge at a wide range of Q up to 20-29 Å-1 under in situ high-pressure and high-temperature conditions, which is almost two times larger than that of the conventional high-pressure angle-dispersive x-ray diffraction measurement. The high-pressure experimental S(Q) precisely determined at a wide range of Q opens the way to investigate detailed structural features of liquids and amorphous materials under in situ high-pressure and high-temperature conditions, as well as ambient pressure study.

Fabrication of ultrashort sub-meter-radius x-ray mirrors using dynamic stencil deposition with figure correction.

This paper presents nanometer-scale production and metrology methods for elliptic-cylindrical x-ray mirrors with an unprecedentedly small tangential radius of curvature of 160 mm. Sub-millimeter-scale figure correction is conducted based on dynamic stencil deposition. The deposition flux through one or two shadow masks is examined by a comparison to a simple model. The masked deposition flux distribution is improved, leading to film thickness profiles that are 50 times sharper in terms of aspect ratio than those obtained using existing differential deposition approaches. Surface roughness deterioration is also effectively suppressed. A 2-mm-long 160-mm-radius mirror is produced with a width of 10 mm and measured using simple interferometry. The results are confirmed by conventional mirror metrology, contact profilometry, and x-ray ptychography. The x-ray focusing profile is diffraction-limited with a 142-nm focus size at a photon energy of 300 eV. The proposed methods have the potential to enhance the ultraprecise fabrication of highly curved mirrors, thus benefiting nanoscale photon-hungry x-ray techniques.

Pressure-induced reversal of Peierls-like distortions elicits the polyamorphic transition in GeTe and GeSe.

While polymorphism is prevalent in crystalline solids, polyamorphism draws increasing interest in various types of amorphous solids. Recent studies suggested that supercooling of liquid phase-change materials (PCMs) induces Peierls-like distortions in their local structures, underlying their liquid-liquid transitions before vitrification. However, the mechanism of how the vitrified phases undergo a possible polyamorphic transition remains elusive. Here, using high-energy synchrotron X-rays, we can access the precise pair distribution functions under high pressure and provide clear evidence that pressure can reverse the Peierls-like distortions, eliciting a polyamorphic transition in GeTe and GeSe. Combined with simulations based on machine-learned-neural-network potential, our structural analysis reveals a high-pressure state characterized by diminished Peierls-like distortion, greater coherence length, reduced compressibility, and a narrowing bandgap. Our finding underscores the crucial role of Peierls-like distortions in amorphous octahedral systems including PCMs. These distortions can be controlled through pressure and composition, offering potentials for designing properties in PCM-based devices.

Soft-X-ray nanobeams formed by aberration-reduced elliptical mirrors with large numerical aperture.

X-ray focusing mirrors often employ the Kirkpatrick-Baez (KB) geometry, which sequentially crosses two elliptic-cylindrical mirrors in grazing-incidence configurations. However, KB mirrors do not satisfy the Abbe sine condition and thus potentially expand the focus size with severe coma aberration. Satisfying the Abbe sine condition complicates mirror shapes or increases the number of ultraprecision mirrors required. The present study shows that the focal length and mirror length of KB mirrors have to be shortened to simultaneously achieve a large numerical aperture and reduced aberration. Such ultracompact KB (ucKB) mirrors are examined using a simulation that combines ray tracing and wave propagation. The focus intensity distributions show that ucKB mirrors suppress the aberration produced by their rotation errors and that they robustly achieve diffraction-limited focusing. The simulation results are confirmed in a synchrotron radiation experiment. ucKB mirrors can be advantageous for soft-X-ray nanoprobes, which require focusing devices to achieve a large numerical aperture.

BL09XU: an advanced hard X-ray photoelectron spectroscopy beamline of SPring-8.

The BL09XU beamline of SPring-8 has been reorganized into a beamline dedicated for hard X-ray photoelectron spectroscopy (HAXPES) to provide advanced capabilities with upgraded optical instruments. The beamline has two HAXPES analyzers to cover a wide range of applications. Two sets of double channel-cut crystal monochromators with the Si(220) and (311) reflections were installed to perform resonant HAXPES analyses with a total energy resolution of less than 300 meV over a wide energy range (4.9-12 keV) while achieving a fixed-exit condition. A double-crystal X-ray phase retarder using diamond crystals controls the polarization state with a high degree of polarization over 0.9 in the wide energy range 5.9-9.5 keV. Each HAXPES analyzer is equipped with a focusing mirror to provide a high-flux microbeam. The design and performance of the upgraded instruments are presented.

Abrasive slurry jet machining system using polyurethane@silica core-shell particles for internal surfaces of axisymmetric x-ray mirrors.

Abrasive machining has been used for inner surface processing of various hollow components. In this study, we applied an in-air fluid jet as a precision machining method for the inner surface of an axisymmetric x-ray mirror whose inner diameter was less than 10 mm. We employed an abrasive with a polyurethane@silica core-shell structure, which has a low density of about 1.2 g/cm3 and a relatively large particle size of about 15 µm. By using this abrasive, a practical removal rate and a smooth machined surface were simultaneously obtained. We performed figure corrections for an axisymmetric mirror and improved the circumferential figure accuracy to a sub-10 nm root mean square level. To evaluate the machining performance in the longitudinal direction of the ellipsoidal surface, we also performed periodic figure fabrication on the inner surface of a 114 mm-long nickel ellipsoidal mirror. X-ray ptychography, an optical phase retrieval method, was also employed as a three-dimensional figure measurement technique of the mirror. The wavefield of the x-ray beam focused by the processed ellipsoidal mirror was observed with the ptychographic system at SPring-8, a synchrotron radiation facility. The retrieval calculations for the wavefront error confirmed that a sinusoidal waveform with a period of 12 mm was fabricated on the mirror surface. These experimental results suggest that a nanoscale figure fabrication cycle for the inner surface consisting of jet machining and wavefront measurement has been successfully constructed. We expect this technique to be utilized in the fabrication of error-free optical mirrors and various parts having hollow shapes.

Double-multilayer monochromators for high-energy and large-field X-ray imaging applications with intense pink beams at SPring-8 BL20B2.

In this study, double-multilayer monochromators that generate intense, high-energy, pink X-ray beams are designed, installed and evaluated at the SPring-8 medium-length (215 m) bending-magnet beamline BL20B2 for imaging applications. Two pairs of W/B4C multilayer mirrors are designed to utilize photon energies of 110 keV and 40 keV with bandwidths of 0.8% and 4.8%, respectively, which are more than 100 times larger when compared with the Si double-crystal monochromator (DCM) with a bandwidth of less than 0.01%. At an experimental hutch located 210 m away from the source, a large and uniform beam of size 14 mm (V) × 300 mm (H) [21 mm (V) × 300 mm (H)] was generated with a high flux density of 1.6 × 109 photons s-1 mm-2 (6.9 × 1010 photons s-1 mm-2) at 110 keV (40 keV), which marked a 300 (190) times increase in the photon flux when compared with a DCM with Si 511 (111) diffraction. The intense pink beams facilitate advanced X-ray imaging for large-sized objects such as fossils, rocks, organs and electronic devices with high speed and high spatial resolution.

Fabrication of soft x-ray monolithic Wolter mirror based on surface scanning measurement using touch probe.

The monolithic Wolter mirror is an ideal optical device for focusing soft x rays to a submicron-sized spot, with the advantages of high efficiency, large acceptance, achromaticity, and robustness to alignment error. The fabrication process for this type of mirror has not been established because of the difficulty in highly accurate figure measurement of free-form surfaces with small radii of curvature and steep profiles. In this study, we employed tactile scanning measurement for surface characterization to fabricate a high-precision Wolter mirror. First, it was demonstrated that the touch probe measurement did not leave scratches on the raw surface of the mirror substrate. Next, the measurement capability of the surface profiler was assessed, and the data analysis conditions were determined. Finally, the Wolter mirror was fabricated through repeated figure correction based on the tactile measurement, and the figure error of the final surface was evaluated. Wave-optical simulations that used this error as reference suggested that the size of the beam focused by the mirror was equivalent to the theoretical value at 1000 eV. The reflected image with uniform intensity distribution obtained at SPring-8 also revealed the effectiveness of the present fabrication approach based on tactile measurement.

Experimental evidence of tetrahedral symmetry breaking in SiO2 glass under pressure.

Bimodal behavior in the translational order of silicon's second shell in SiO2 liquid at high temperatures and high pressures has been recognized in theoretical studies, and the fraction of the S state with high tetrahedrality is considered as structural origin of the anomalous properties. However, it has not been well identified in experiment. Here we show experimental evidence of a bimodal behavior in the translational order of silicon's second shell in SiO2 glass under pressure. SiO2 glass shows tetrahedral symmetry structure with separation between the first and second shells of silicon at low pressures, which corresponds to the S state structure reported in SiO2 liquid. On the other hand, at high pressures, the silicon's second shell collapses onto the first shell, and more silicon atoms locate in the first shell. These observations indicate breaking of local tetrahedral symmetry in SiO2 glass under pressure, as well as SiO2 liquid.

Soft X-ray ARPES for three-dimensional crystals in the micrometre region.

An endstation dedicated to angle-resolved photoemission spectroscopy (ARPES) using a soft X-ray microbeam has been developed at the beamline BL25SU of SPring-8. To obtain a high photoemission intensity, this endstation is optimized for measurements under the condition of grazing beam incidence to a sample surface, where the glancing angle is 5° or smaller. A Wolter mirror is used for focusing the soft X-rays. Even at the glancing angle of 5°, the smallest beam spot still having a sufficient photon flux for ARPES is almost round on the sample surface and the FWHM diameter is ∼5 µm. There is no need to change the sample orientation for performing kx - ky mapping by virtue of the electron lens with a deflector of the photoelectron analyzer, which makes it possible to keep the irradiation area unchanged. A partially cleaved surface area as small as ∼20 µm was made on an Si(111) wafer and ARPES measurements were performed. The results are presented.

Stable sub-micrometre high-flux probe for soft X-ray ARPES using a monolithic Wolter mirror.

A focusing optics that can provide a sub-micrometre high-flux probe for soft X-ray micrometre-scale angle-resolved photoemission spectroscopy (ARPES) is proposed. A monolithic Wolter-type mirror with a large acceptance, achromatism and small comatic aberration was designed and evaluated. A focused beam size of 0.4 µm (vertical) × 4 µm (horizontal), a high throughput of 59% and a high tolerance of 1.6 mrad to the pitching error were realized at a photon energy of 1000 eV. A Wolter-type mirror can be practically employed as a stable sub-micrometre focusing mirror with high throughput in ARPES applications.

Probing the spatial coherence of wide X-ray beams with Fresnel mirrors at BL25SU of SPring-8.

Probing the spatial coherence of X-rays has become increasingly important when designing advanced optical systems for beamlines at synchrotron radiation sources and free-electron lasers. Double-slit experiments at various slit widths are a typical method of quantitatively measuring the spatial coherence over a wide wavelength range including the X-ray region. However, this method cannot be used for the analysis of spatial coherence when the two evaluation points are separated by a large distance of the order of millimetres owing to the extremely narrow spacing between the interference fringes. A Fresnel-mirror-based optical system can produce interference patterns by crossing two beams from two small mirrors separated in the transverse direction to the X-ray beam. The fringe spacing can be controlled via the incidence angles on the mirrors. In this study, a Fresnel-mirror-based optical system was constructed at the soft X-ray beamline (BL25SU) of SPring-8. The relationship between the coherence and size of the virtual source was quantitatively measured at 300 eV in both the vertical and horizontal directions using the beam. The results obtained indicate that this is a valuable method for the optimization of optical systems along beamlines.

Effective protocol for realizing contamination-free X-ray reflective optics.

The surface contamination of reflective X-ray optics has long been a serious problem that degrades beam quality. We evaluated the total organic content at the surface by gas chromatography to clarify the source of contamination. We found that various materials that can become contamination sources are used around the optical elements. After covering the optics with cleaned materials and applying synchrotron radiation cleaning during commissioning, the observed reflected intensity at the beamline has not reduced for 2.5 years.

Fabrication of a precise ellipsoidal mirror for soft X-ray nanofocusing.

In X-ray focusing, grazing incidence mirrors offer advantages of no chromatic aberration and high focusing efficiency. Although nanofocusing mirrors have been developed for the hard X-ray region, there is no mirror with nanofocusing performance in the soft X-ray region. Designing a system with the ability to focus to a beam size smaller than 100 nm at an X-ray energy of less than 1 keV requires a numerical aperture larger than 0.01. This leads to difficulties in the fabrication of a soft X-ray focusing mirror with high accuracy. Ellipsoidal mirrors enable soft X-ray focusing with a high numerical aperture. In this study, we report a production process for ellipsoidal mirrors involving mandrel fabrication and replication processes. The fabricated ellipsoidal mirror was assessed under partial illumination conditions at the soft X-ray beamline (BL25SU) of SPring-8. A focal spot size of less than 250 nm was confirmed at 300 eV. The focusing tests indicated that the proposed fabrication process is promising for X-ray mirrors that have the form of a solid of revolution, including Wolter mirrors.

Realization of a scanning soft X-ray microscope for magnetic imaging under high magnetic fields.

For the purpose of imaging element- and shell-specific magnetic distributions under high magnetic fields, a scanning soft X-ray microscope has been developed at beamline BL25SU, SPring-8, Japan. The scanning X-ray microscope utilizes total electron yield detection of absorbed circularly polarized soft X-rays in order to observe magnetic domains through the X-ray magnetic circular dichroism effect. Crucially, this system is equipped with an 8 T superconducting magnet. The performance and features of the present system are demonstrated by magnetic domain observations of the fractured surface of a Nd14.0Fe79.7Cu0.1B6.2 sintered magnet.

A soft X-ray free-electron laser beamline at SACLA: the light source, photon beamline and experimental station.

The design and performance of a soft X-ray free-electron laser (FEL) beamline of the SPring-8 Compact free-electron LAser (SACLA) are described. The SPring-8 Compact SASE Source test accelerator, a prototype machine of SACLA, was relocated to the SACLA undulator hall for dedicated use for the soft X-ray FEL beamline. Since the accelerator is operated independently of the SACLA main linac that drives the two hard X-ray beamlines, it is possible to produce both soft and hard X-ray FEL simultaneously. The FEL pulse energy reached 110 µJ at a wavelength of 12.4 nm (i.e. photon energy of 100 eV) with an electron beam energy of 780 MeV.

X-ray collimation by the parabolic cylinder mirror in SPring-8/BL29XUL.

A combination of plane and threefold-shape X-ray mirrors was installed in SPring-8 BL29XUL. The second mirror has parabolic cylinder surfaces that collimate X-rays in the vertical direction. A performance test was conducted, yielding highly collimated 8 keV photon beams with an effective angular divergence of 0.4 µrad, below only 5% of that of the original beams. The double-mirror system preserved 70% of the total incident flux and nearly tripled the flux density at 988 m from the light source. The values of the observations were almost similar to those of our ray-tracing simulation. Based on the results a discussion of future prospects of the mirror system is included.

New soft X-ray beamline BL07LSU at SPring-8.

A new soft X-ray beamline, BL07LSU, has been constructed at SPring-8 to perform advanced soft X-ray spectroscopy for materials science. The beamline is designed to achieve high energy resolution (E/ΔE> 10000) and high photon flux [>10(12) photons s(-1) (0.01% bandwidth)(-1)] in the photon energy range 250-2000 eV with controllable polarization. To realise this state-of-the-art performance, a novel segmented cross undulator was developed and adopted as a light source. The details of the undulator light source and beamline monochromator design are described. The achieved performance of the beamline, such as the photon flux, energy resolution and the state of polarization, is reported.