High-precision ellipsoidal mirror achieved two-dimensional X-ray focusing with world’s minimum focus size of 100 nm at SPring-8 (Press Release)
- Release Date
- 01 Dec, 2017
- BL29XU (RIKEN Coherent X-ray Optics)
December 1, 2017
Japan Synchrotron Radiation Research Institute (JASRI)
Osaka University
RIKEN
Key research findings
・Fabricating ellipsoidal mirror with nanometer accuracy
・Achieving world’s minimum focus size of 100 nm for ellipsoidal mirror
・Establishing precision fabrication techniques for two-dimensional aspherical mirrors applicable to generation of nanofocusing beams for high-efficiency X-ray microscopy
JASRI (President, Yoshiharu Doi), Osaka University (President, Shojiro Nishio), and RIKEN (President, Hiroshi Matsumoto) have jointly succeeded in fabricating an ellipsoidal mirror having nanometer surface accuracy and in achieving the world’s minimum focus size of 100 nm for an ellipsoidal mirror at the large synchrotron radiation facility, SPring-8*1. In SPring-8 and the X-ray free electron laser facility SACLA*2, X-ray microscopy is expected to be applied to sensitive analysis and microelement analysis, in which X-ray beams generated by the light source must be fully collected to irradiate minute objects so that the signals from samples can be detected as efficiently as possible. To realize this, sophisticated techniques to fabricate X-ray focusing optics (lenses to focus X-ray beams) are required. Conventionally, two-dimensional nanofocusing beams have been generated by combining two X-ray mirrors, but there has been no technique for fabricating high-efficiency X-ray mirrors that can generate two-dimensional nanofocusing beams with a single reflection. The research group succeeded in developing single-reflecting two-dimensional X-ray focusing mirrors having an ellipsoidal surface. High precision is required for processing the highly sloped surface of ellipsoidal X-ray mirrors that can generate two-dimensional nanofocusing beams. However, with conventional techniques, it is difficult to fabricate the two-dimensional aspherical surface of ellipsoidal mirrors with nanometer accuracy. The research group fabricated an ellipsoidal mirror with a surface accuracy of 1 nm as a result of resolving issues related to aspheric machining*3 and surface profile measurements*4, which are essential techniques in the fabrication of X-ray mirrors. Also, they developed an X-ray microscope at SPring-8 using this ellipsoidal mirror and succeeded in generating a two-dimensional focusing beam of 125 nm in the vertical direction and 85 nm in the horizontal direction, thus achieving the world’s minimum focus size obtained using an ellipsoidal mirror. This success demonstrated the validity of the techniques for fabricating X-ray mirrors having a two-dimensional aspherical surface. The developments in X-ray focusing using an ellipsoidal mirror will contribute to the stable generation of high-efficiency nanobeams and the increased stability of optical systems that support a variety of cutting-edge X-ray microscopy techniques used at SPring-8 and SACLA. This accomplishment was achieved by a joint research group including Hirokatsu Yumoto (research scientist) and Haruhiko Ohashi (chief scientist) of JASRI, Professor Kazuto Yamauchi at Graduate School of Engineering, Osaka University, their research group, and Yoshiki Kohmura (unit leader) of SR Imaging Instrumentation Unit, RIKEN SPring-8 Center (Director, Tetsuya Ishikawa). The results were published online in Scientific Reports on November 27, 2017. (Article) |
Fig. 1 Optical layout for two-dimensional focusing with two reflective mirrors.
Each mirror has a one-dimensionally curved surface with an elliptical-cylinder shape to focus X-rays one-dimensionally. X-rays are focused two-dimensionally after two reflections. This optical layout is called the K‒B(Kirkpatrick‒Baez) mirrors arrangement.
Fig. 2 Optical layout for two-dimensional focusing with a single reflective mirror.
This mirror has a two-dimensionally curved surface with an ellipsoidal shape. Ellipsoidal mirrors can focus X-rays two-dimensionally with a single reflection. X-rays are reflected under grazing-incidence angle conditions.
Fig. 3 Photograph of developed ellipsoidal focusing mirror.
An ellipsoidal shape is formed on a glass (synthetic fused silica) substrate surface with dimensions of 100 mm × 50 mm × 30 mm. The surface was coated with a platinum film to obtain high reflectivity in the hard X-ray regions. The mirror surface was manufactured with a precision of 1 nanometer.
Fig. 4 Measured intensity distribution of X-ray focusing beam formed by developed ellipsoidal mirror.
X-rays were two-dimensionally focused with a beam size of 125 nm (vertical) × 85 nm (horizontal), which shows the world's smallest beam size focused by ellipsoidal mirrors.
(Glossary)
※1. SPring-8
The name “SPring-8” comes from “Super Photon ring-8 GeV”. It is owned by RIKEN and located in Harima Science Garden City, Hyogo Prefecture, Japan. SPring-8 generates the world's highest-performance synchrotron radiation and was opened in 1997 to users from universities, research institutes, and companies. Synchrotron radiation is an electromagnetic wave that is produced when electron beams, accelerated to nearly the speed of light, are bent in a high magnetic field. High-intensity synchrotron radiation in a wide range of energy from infrared to soft and hard X-ray regions is available at SPring-8.
※2. X-ray free electron laser facility, SACLA
The name “SACLA” comes from “SPring-8 Angstrom Compact free-electron Laser”. It is owned by RIKEN and located on the SPring-8 campus in Harima Science Garden City, Hyogo Prefecture, Japan. X-ray free electron lasers are available at SACLA, which was opened to users in 2012. The features of the X-ray free electron lasers generated at SACLA are very high peak brilliance (one billion times brighter than the X-rays at SPring-8) and ultrashort pulse duration as instantaneous as a camera flash (10 femtoseconds).
※3. Aspheric machining
The aspheric ultraprecision machining technique used to fabricate the ellipsoidal X-ray mirror in this research was based on the elastic emission machining (EEM) technique developed at Osaka University. This technique enables the removal of material from a workpiece surface with atomic-level precision by supplying fine particles to the workpiece surface. With precise control of the region of the workpiece surface in contact with the fine particles and of the amount of fine particles fed onto the workpiece surface, the surface can be machined into the desired shape with atomic-level precision. In the fabrication of the ellipsoidal X-ray mirror in this study, local areas with dimensions of 0.1 mm were machined by supplying fine-particle slurry through a nozzle with an aperture size as small as 0.05 mm.
※4. Surface profile measurements
To achieve the desired nanoscale accuracy on the two-dimensional highly sloped surface of an ellipsoidal mirror, it is necessary to know in advance the amount of materials to be removed from the surface, namely, the size of the gap between the current surface shape and the target surface shape of the mirror (the extent of irregularities). Surface profile measurements are therefore essential for fabricating ultraprecision aspherical mirrors. In this study, a microscopic interferometer (an instrument used to obtain phase information on the basis of optical coherence) was used to obtain the surface profile data from local areas with dimensions of 0.5 × 0.7 mm2 on an ellipsoidal mirror. Also, the surface profile data of the entire surface of the ellipsoidal mirror was obtained by combining the surface profile data from local areas by the special stitching method applicable to the ellipsoidal mirror fabricated in this study. This enabled the high-precision surface profile measurements of ellipsoidal mirrors with measurement reproducibility of 1 nm.
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