Soichiro Handa

Single-nanometer focusing of hard x-rays by Kirkpatrick-Baez mirrors.

We have constructed an extremely precise optical system for hard-x-ray nanofocusing in a synchrotron radiation beamline. Precision multilayer mirrors were fabricated, tested, and employed as Kirkpatrick-Baez mirrors with a novel phase error compensator. In the phase compensator, an at-wavelength wavefront error sensing method based on x-ray interferometry and an in situ phase compensator mirror, which adaptively deforms with nanometer precision, were developed to satisfy the Rayleigh criterion to achieve diffraction-limited focusing in a single-nanometer range. The performance of the optics was tested at BL29XUL of SPring-8 and was confirmed to realize a spot size of approximately 7 nm.

At-wavelength figure metrology of hard x-ray focusing mirrors

We have developed an at-wavelength wave-front metrology of a grazing-incidence focusing optical systems in the hard x-ray region. The metrology is based on numerical retrieval from the intensity profile around the focal point. We demonstrated the at-wavelength metrology and estimated the surface figure error on a test mirror. An experiment for measuring the focusing intensity profile was performed at the 1-km-long beamline (BL29XUL) of SPring-8. The obtained results were compared with the profile measured using an optical interferometer and were confirmed to be in good agreement with it. This technique is a potential method of characterizing wave-front aberrations on elliptical mirrors for sub-10-nm focusing.

Wavefront Control System for Phase Compensation in Hard X-ray Optics

A highly precise adaptive optical system that can be used in the hard X-ray region was developed. To achieve highly precise control of the wavefront shape, we discussed an optical system with a bendable mirror of deformation accuracy better than 0.4 nm RMS. Using the system, we demonstrated the controllability of the wavefront of a 15 nm hard X-ray nanobeam. The intensity profile of the wavefront-modified beam was in good agreement with the wave-optically calculated profile.

Wavefield characterization of nearly diffraction-limited focused hard x-ray beam with size less than 10 nm.

In situ wavefront compensation is a promising method to realize a focus size of only a few nanometers for x-ray beams. However, precise compensation requires evaluation of the wavefront with an accuracy much shorter than the wavelength. Here, we characterized a one-dimensionally focused beam with a width of 7 nm at 20 keV using a multilayer mirror. We demonstrate that the wavefront can be determined precisely from multiple intensity profiles measured around the beamwaist. We compare the phase profiles recovered from intensity profiles measured under the same mirror condition but with three different aperture sizes and find that the accuracy of phase retrieval is as small as λ∕12.

Development of adaptive mirror for wavefront correction of hard x-ray nanobeam

We present the development of a phase compensator for wavefront control of X-rays. The optical device is a 150 mm-long total reflection mirror, the shape of which can be curved by adjusting the bias voltages of 36 piezoelectric ceramic plates attached to the mirror. The mirror surface was smoothed and made flat by elastic emission machining. To achieve a high degree of the accuracy in the controllability of a curved line, a Fizeau interferometer is placed in front of the mirror surface to monitor its shape in the experiment. We will apply this device to the optical system for the realization of sub-10-nm hard X-ray focusing.

Reflective optics for sub-10nm hard X-ray focusing

Nanofocused X-rays are indispensable because they can provide high spatial resolution and high sensitivity for X-ray nanoscopy/spectroscopy. A focusing system with reflective optics is one of the most promising methods for producing nanofocused X-rays due to its high efficiency and beams size. So, far we realize efficient hard X-ray focusing with a beam size of 25nm. Our next project is realization of sub-10nm hard X-ray focusing. Here, we describe the design of the graded multilayer mirror and evaluation method for hard X-ray focused beam.

Aberrations in curved x-ray multilayers

Aberration effects in curved multilayers for hard X rays are studied using a simple analytical approach. The method is based on geometrical ray tracing including refraction effects up to the first order of the refractive index decrement δ. The interpretation of the underlying equations provides fundamental insight into the focusing properties of these devices. Using realistic values for the multilayer parameters the impact on spot broadening and chromaticity is evaluated. The work is complemented by a comparison with experimental focusing results obtained with a W/B4C multilayer mirror.

Hard X‐ray Focusing less than 50nm for Nanoscopy/spectroscopy

X‐ray focusing using a Kirkpatrick‐Baez (KB) setup with two total reflection mirrors is a promising method, allowing highly efficient and energy‐tunable focusing. Fabricated mirrors having a figure accuracy of 1 nm peak‐to‐valley height gave ideal diffraction‐limited focusing of hard X‐rays. The focal size, defined as the full width at half maximum of the intensity profile, was 36 nm × 48 nm at an X‐ray energy of 15 keV. Fluorescence X‐ray microscopy with KB mirrors was also developed, targeting cell biological applications. The distribution of various elements in a single cell was successfully observed with high resolution. The developed microscopy is already used for various applications in the medical field. Our next main project is the realization of sub‐10‐nm‐level hard X‐ray focusing. At‐wavelength metrology is being developed, in which a phase‐retrieval simulator is coded for the determination of phase errors on mirror surfaces from only the intensity profiles of a focused beam.

High-spatial-resolution scanning x-ray fluorescence microscope with Kirkpatrick-Baez mirrors

We developed a high-spatial-resolution scanning X-ray fluorescence microscope (SXFM) with Kirkpatrick-Baez mirrors. As a result of focusing tests at 15 keV, the focused beam having a FWHM of 30 x 50 nm2 was achieved. Additionally, the size was controllable within the wide range of 30 ~ 1400 nm merely by adjusting the X-ray source size. The observation of a fine test chart suggests that SXFM enables us to visualize the element distribution inside the pattern at a spatial resolution better than 30 nm. We applied the SXFM to observe intracellular elemental distributions at a single-cell level, so that we could acquire element distribution maps with a spatial resolution of sub-100 nm and lower detection limit of 0.01 fg.

Development of Hard X‐ray Imaging Optics with Two Pairs of Elliptical and Hyperbolic Mirrors

To form a magnified hard X‐ray image with a 50 nm resolution, we have studied total reflection mirror optics with two pairs of elliptical and hyperbolic mirrors, which is called “Advanced Kirkpatrick‐Baez system”. A designed optical system has 200x and 300x magnifications in vertical and horizontal directions. Also diffraction limit size in the optical system is 40 nm×45 nm. We fabricated a pair of elliptical and hyperbolic mirrors for horizontal imaging with a figure accuracy of 2 nm using elastic emission machining (EEM), microstitching interferometry (MSI) and relative‐angle‐determinable stitching interferometry (RADSI). One‐dimensional tests for forming a demagnified image of a slit were carried out at an X‐ray energy of 11.5 keV at BL29XUL (EH2) of SPring‐8. As a result, a shape beam with a FWHM of 78 nm was observed. This demonstrates that we realized one‐dimensional Wolter optics that has a spatial resolution of 78 nm.