Tadayuki Ohchi

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.

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%.

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.

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.

Mo/Si and MoSi2/Si nanostructures for multilayer Laue lens

To develop a multilayer Laue lens (MLL), we fabricated depth-graded Mo/Si and MoSi2/Si multilayers with each boundary according to the Fresnel zone configuration. The multilayers were deposited by magnetron sputtering. From the result of SEM image analysis of the multilayer cross sections, MoSi2/Si multilayer had smaller layer-thickness errors than Mo/Si multilayer. In addition, from the result of the focusing test by using 20-keV X-rays, the measured beam size of MoSi2/Si MLL had a small blurring from the diffraction limited beam size. These results suggest that MoSi2/Si multilayer is better suited than Mo/Si multilayer for use as an MLL in hard x-ray nanofocusing.

Dotted-Array Plasma Production by Using a Line-Focusing Lens System with Segmented Prism Array for Compact X-ray Laser Experiments

A long narrow plasma originated from an array of microdots has been produced using a new line-focusing lens system. The lens system consists of a five-segmented prism array, a beam expander and a cylindrical lens assembly that was designed for compact X-ray laser experiments using a small Nd:YAG (yttrium aluminum garnet) laser. The irradiation pattern on a slab target has a small-scale intensity modulation due to interference among the segmented beams. The focused line has an array of microdots with a 50 µm diameter, a 140 µm spacing and an 11 mm length. Spectroscopic observation has been carried out in the expanding plasma produced by irradiating the YAG laser via this lens system onto an Al target. Near the target surface, highly ionized plasma was produced to have a spatial distribution corresponding to the microdot array in the irradiation pattern. It has been demonstrated that soft X-ray amplification takes place in the plasma produced by this irradiation method with very low input energy.

Point spread function measurement of an X-ray beam focused by a multilayer zone plate with narrow annular aperture.

The experimental procedure for obtaining the point spread function (PSF) of a focusing beam generated using an X-ray multilayer zone plate (MZP) with a narrow annular aperture has been developed. It was possible to reconstruct the PSF by applying the tomographic process to the measured dataset consisting of line spread functions (LSFs) in every radial direction on the focal plane. The LSFs were measured by a knife-edge scanning method of detecting scattered intensity. In the experimental work, quasi-monochromatic undulator radiation with a first harmonic energy of 20 keV was directly focused without a monochromator by the MZP, and the PSF was measured using this procedure. As a result, a near diffraction-limited focused beam size of 46 nm full width at half-maximum was obtained.

Formation of microbeam using tabletop soft X-ray laser

Abstract An X-ray microprobe with a sub-micron size beam and high intensity can provide X-ray analyses with a remarkable spatial resolution. We have performed focusing of an X-ray laser output into a sub-micron beam for the first time. In our experiment, an X-ray laser of Li-like Al 3d–4f transition at 15.47 nm was delivered from an unstable cavity consisting of a concave mirror and a flat mirror with a square orifice of 100×100 μm in size. The beam from the orifice was then focused by using a Schwarzschild mirror coated with a Mo/Si multilayer. An X-ray beam size with a diameter of about 0.45 μm and an estimated photon number of about 2×106 photons per shot was achieved. Such sources could be well suited for the realization of X-ray microprobes.

DEMONSTRATION OF X-RAY AMPLIFICATION IN AN X-RAY LASER CAVITY PUMPED BY A PULSE-TRAIN YTTRIUM ALUMINIUM GARNET LASER

X-ray lasings of a Li-like Al transition line at 15.47 nm in the recombination scheme have been investigated using a tabletop pulse-train yttrium aluminium garnet (YAG) laser system. A cavity experiment has been carried out with a resonator consisting of a flat and a concave Mo/Si multilayer mirrors. Clear enhancement of the X-ray output from the cavity has been demonstrated. It was found that the cavity output beam has a divergence of about 3 mrad and an absolute intensity of about 1×108 photons/shot. The temporal feature of the observed cavity output can be reproduced by a simple ray-trace calculation with multiple times reflection and the presence of a lasing medium.

Performance test of Mo/Si and MoSi2/Si multilayer Laue lenses

A multilayer Laue lens (MLL) as a hard X-ray focusing device has been designed and fabricated. Mo/Si and MoSi2/Si were chosen to form the multilayers owing to their superior properties of high diffraction efficiency and relatively sharp interface between layers. DC magnetron sputtering system was used for deposition of the multilayers. The optical properties of the MLL were measured at BL24XU of SPring-8 with 20-keV X-rays. To confirm the dynamical diffraction effect, far-field diffraction images were taken at various incidence angles and depths. The resultant intensity distributions showed similar structure to those expected by calculations. Further, an almost diffraction-limited beam size of around 30 nm was obtained.