Takahisa Koyama

Generation of 10 20 W cm −2 hard X-ray laser pulses with two-stage reflective focusing system

Intense X-ray fields produced with hard X-ray free-electron laser (XFEL) have made possible the study of nonlinear X-ray phenomena. However, the observable phenomena are still limited by the power density. Here, we present a two-stage focusing system consisting of ultra-precise mirrors, which can generate an extremely intense X-ray field. The XFEL beam, enlarged with upstream optics, is focused with downstream optics that have high numerical aperture. A grating interferometer is used to monitor the wavefront to achieve optimum focusing. Finally, we generate an extremely small spot of 30 × 55 nm with an extraordinary power density of over 1 × 10(20) W cm(-2) using 9.9 keV XFEL light. The achieved power density provides novel opportunities to elucidate unexplored nonlinear phenomena in the X-ray region, which will advance development on quantum X-ray optics, astronomical physics and high-energy density science.

Circular multilayer zone plate for high-energy x-ray nano-imaging

A circular multilayer zone plate (MZP) was fabricated and its focusing performance was evaluated using 20-keV x-rays. MoSi(2) and Si layers were alternately deposited by DC magnetron sputtering on a wire core; all the interfaces satisfied the Fresnel zone condition. The measured line spread function was converted to a point spread function by tomographic reconstruction. The results suggest that the MZP has the potential to realize the diffraction-limited resolving power, which is calculated to be 35 nm using the diffraction integral. Furthermore, scanning transmission microscopy using the MZP could resolve a 50-nm line-and-space pattern.

Investigation of ablation thresholds of optical materials using 1-µm-focusing beam at hard X-ray free electron laser

We evaluated the ablation thresholds of optical materials by using hard X-ray free electron laser. A 1-µm-focused beam with 10-keV of photon energy from SPring-8 Angstrom Compact free electron LAser (SACLA) was irradiated onto silicon and SiO2 substrates, as well as the platinum and rhodium thin films on these substrates, which are widely used for optical materials such as X-ray mirrors. We designed and installed a dedicated experimental chamber for the irradiation experiments. For the silicon substrate irradiated at a high fluence, we observed strong mechanical cracking at the surface and a deep ablation hole with a straight side wall. We confirmed that the ablation thresholds of uncoated silicon and SiO2 substrates agree with the melting doses of these materials, while those of the substrates under the metal coating layer are significantly reduced. The ablation thresholds obtained here are useful criteria in designing optics for hard X-ray free electron lasers.

Optical Properties of MoSi2/Si Multilayer Laue Lens as Nanometer X-ray Focusing Device

In this study, we designed and fabricated a multilayer Laue lens (MLL) as a hard X-ray focusing device. MoSi2 and Si were chosen to form the layers by DC magnetron sputtering owing to their superior properties. The optical properties of the MLL were measured at BL24XU of SPring-8 for 20-keV X-rays. In order to confirm the effect of dynamical diffraction, far-field diffraction images were captured at various incidence angles and depths. The resultant intensity distributions showed a similar structure to those derived through calculations. An almost diffraction-limited size of 28.2 nm was obtained. The maximum local diffraction efficiency was 64.7%.

Sub-15 nm Hard X-Ray Focusing with a New Total-Reflection Zone Plate

A new total-reflection zone plate that consists of a reflective zone pattern with varied-space on a flat substrate was fabricated for hard X-ray nanofocusing. This device is much easier to fabricate than other focusing devices. This is because its focusing size is much smaller than its finest constituent structure since it exploits the effect of glancing X-rays by having a small total reflection angle. Its focusing properties were evaluated using 10-keV X-rays and a focusing size of 14.4 nm was achieved.

Scanning Differential-Phase-Contrast Hard X-Ray Microscopy with Wedge Absorber Detector

A new and simple idea for scanning differential-phase-contrast (S-DPC) hard X-ray microscopy has been proposed. It only uses a wedge absorber coupled with two intensity detectors, and is much more sensitive to sample structures than absorption contrast. It can also extract pure quantitative one-dimensional phase gradient given by a sample without an effect of sample absorption. The S-DPC microscope has been constructed at BL24XU of SPring-8, and its feasibility has been successfully demonstrated at the photon energy of 10 keV by clearly visualizing structures of samples. Further, the experimental phase gradient profile agrees well with simulation. By integrating the resultant phase gradient, the corresponding phase shift distribution could be also imaged.

Development of Multilayer Laue Lenses; (1) Linear Type

A multilayer Laue lens (MLL) made from MoSi2/Si has been fabricated aiming at a sub‐10‐nm spatial resolution for hard x‐ray microscopy. A multilayer of 1000 alternating MoSi2 and Si layers with a layer thickness gradually increasing from 5 nm to 316 nm according to the Fresnel lens structure was deposited on a silicon substrate using DC magnetron sputtering, and then the substrate was diced, polished and thinned. The multilayer total thickness was about 10 μm. Optical properties were measured at the SPring‐8 beamline BL24XU using 20‐keV x‐rays. While observing changes in far‐field diffraction patterns by changing the tilt angle of the MLL, a dynamical diffraction effect of the MLL was visually confirmed. The achieved line focusing size was 13.1 nm, which is the best ever reported by using an MLL. The maximum local diffraction efficiency was measured to be 62%. Further, two MLLs were arranged in the KB configuration and applied to scanning transmission microscopy.

High-Spatial-Resolution Phase Measurement by Micro-Interferometry Using a Hard X-Ray Imaging Microscope

With the aim towards high-spatial-resolution phase measurement, a novel hard X-ray micro-interferometer using an imaging microscope has been proposed and constructed at Hyogo-BL of SPring-8. It is a wavefront-division-type interferometer consisting of a twin zone plate arranged in the same plane. We have succeeded in producing good interference fringes with a visibility of as high as 60% at the photon energy of 9 keV. The fringe scanning method was applied to retrieve phase-shift distribution of a sample. The phase-shift distribution of a 75-µm-thick kapton film and polystyrene microparticles could be imaged clearly with a spatial resolution of 160 nm and the obtained phase shift agreed well with the expected value.

RISING beamline (BL28XU) for rechargeable battery analysis

The BL28XU beamline, dedicated to rechargeable battery analysis, is described.

Development of Multilayer Laue Lenses; (2) Circular Type

A circular type multilayer Laue lens (MLL) has been designed and fabricated. A graded‐thickness multilayer was deposited on a tapered wire core by using a magnetron sputtering system. A combination of MoSi2 and Si was chosen as the multilayer materials owing to its superior properties of high diffraction efficiency and relatively sharp interfaces between layers. Optical properties of the circular type MLL were measured at BL24XU of SPring‐8 with 20‐keV x‐rays. It was confirmed that only the +first‐order diffraction was focused in the focal point owing to the wedged zone structure. Measured +first‐order diffraction efficiency of the multilayer part was as high as 52%. Applying the circular type MLL to scanning transmission microscopy, a 50‐nm line and space of a test chart was resolved.