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World’s First Observation of Saturable Absorption of Hard X-rays -Opening a promising path using the world’s most intense X-rays of SACLA- (Press Release)

Release Date
02 Oct, 2014

The University of Electro-Communications
Japan Synchrotron Radiation Research Institute (JASRI)
Osaka University
The University of Tokyo
Kyoto University

Key points
• The transparency of materials increases with increasing X-ray intensity.
• This observation was realized for the first time in the world using the most intense X-rays.
• This finding opens a promising path for future innovations of attosecond X-ray optics.

   Scientists from RIKEN (President, Ryoji Noyori) and the University of Electro-Communications (President, Takashi Fukuda) succeeded in observing saturable absorption of X-rays*1 using the SPring-8 Angstrom Compact Free Electron Laser (SACLA)*2, an X-ray free electron laser (XFEL) facility. This was achieved by a joint research group led by Hitoki Yoneda (professor) of the University of Electro-Communications; Makina Yabashi (group director) of the Beam Line Research and Development Group of RIKEN SPring-8 Center (Director, Tetsuya Ishikawa).

   The amount of light absorbed by a material irradiated with light depends on the characteristics of the material. With increasing intensity of light, the material becomes unable to absorb the light and transparent, which is known as saturable absorption. Since its discovery more than half a century ago, saturable absorption has been widely used in advanced technologies including optical communication, for example, producing an optical waveguide*3 by making a material transparent. It has been theoretically predicted that saturable absorption is induced for X-rays with a short wavelength with increasing X-ray intensity. The saturable absorption of X-rays is expected to be applied to various X-ray optical devices, such as optical waveguides and ultrahigh-speed X-ray switching devices because the saturable absorption of X-rays is induced selectively at the region irradiated with high-intensity X-rays. However, no researchers to date have experimentally succeeded in inducing saturable absorption of X-rays because the intensity of X-rays has to be extremely high.

   The joint research group succeeded in generating X-rays with world’s highest intensity of 1020 W/cm2 using a two-stage focusing system*4 that was developed by the group for the high-brilliance X-ray lasers of SACLA (see the press release “100-fold increase in X-ray laser power density” dated 28 April 2014). The joint research group observed the absorption spectra of an X-ray laser transmitted through an iron foil and found that the transparency of X-rays increases by a factor of 10 or more compared with the case of using X-rays from conventional sources. It was also clarified that optical waveguides can be formed in the X-ray absorber because only the region irradiated with X-rays with a high intensity becomes transparent. This finding provides a first step for realizing next-generation attosecond (one-quintillionth of a second) X-ray optics*5 and dynamic X-ray optics*6 and is expected to be a technology leading to the development of new X-ray optical devices.

   The achievements of this study were published online in the British scientific journal Nature Communications on 1 October 2014.

Nature Communications
Title: "Saturable Absorption of Intense Hard X-rays in Iron"
Authors: Hitoki Yoneda, Yuichi Inubushi, Makina Yabashi, Tetsuo Katayama, Tetsuya Ishikawa, Haruhiko Ohashi, Hirokatsu Yumoto, Kazuto Yamauchi, Hidekazu Mimura, and Hikaru Kitamura
doi: 10.1038/ncomms6080


Fig.1 Saturable absorption
Fig.1 Saturable absorption

 Materials that do not allow X-rays with a low intensity to be transmitted become transparent when most electrons absorbing X-rays are ionized using X-rays with a high intensity. Optical waveguides of X-rays and high-speed switching mechanisms using X-rays can be structured because only the regions irradiated with X-rays with a high intensity become transparent.

Fig.2 Experiment of saturable absorption using X-rays
Fig.2 Experiment of saturable absorption using X-rays

 An iron foil (thickness, 20 µm) is irradiated with X-rays focused to a diameter of 50 nm using a two-stage focusing system. The transmitted X-rays are detected using a spectroscope with a high energy resolution.

Fig.3 Dependence of transparency on intensity of X-rays applied to iron foil
Fig.3 Dependence of transparency on intensity of X-rays applied to iron foil

 The red dots denote experimental values. Blue and green curves denote calculated values obtained by computer simulation assuming that the times required for the holes in the K shell (the electron orbital nearest to the atomic core), emptied after X-ray irradiation, to be occupied again are 0.5 and 2 fs (femtosecond is one-quadrillionth of a second), respectively. As theoretically predicted, the iron foil becomes significantly transparent at X-ray intensities higher than 1019 W/cm2.

※1 Saturable absorption

Saturable absorption is a phenomenon of decreased absorption and increased transparency in materials when strong absorption is induced in the materials and the elements responsible for absorption (mainly electrons in some level) are depleted. A excitation absorption process sometimes follows after the fundamental absorption. Thus a simple system is generally considered to be appropriate for saturable absorption.

※2 SPring-8 Angstrom Compact Free Electron Laser (SACLA), an X-ray free electron laser (XFEL) facility
Japan’s first XFEL facility constructed jointly by RIKEN and JASRI. As one of the five national key technologies in the Basic Program for Science and Technology in Japan, the facility was constructed and developed in a five-year project starting from FY 2006. It was completed in March 2011 and named SACLA after the initial letters of SPring-8 Angstrom Compact Free Electron LAser. The first successful generation of an X-ray laser was in June 2011. Public operations started in March 2012. Since then, SACLA has been used in various experiments. Although the facility is smaller than those in other countries, SACLA can produce lasers with the world’s shortest wavelength of 0.1 nm or shorter.

※3 Optical waveguide
When a waveguide with a locally high reflective index is formed in a material, light propagates within the waveguide. An optical fiber is a type of optical waveguide and enables light to propagate for several hundred kilometers within the waveguide with a small diameter.

※4 Two-stage focusing system
To simultaneously realize a small diameter of a focused X-ray laser and a long distance from a focusing optical system to a focal point (working distance), the diameter of the X-ray lasers themselves should be increased and the lasers should be focused using a large mirror. SACLA realizes this goal by adopting a two-stage focusing system with two optical systems in charge of the former and latter roles.

※5 Attosecond X-ray optics
An attosecond is one-quintillionth (10−18) of a second. An attosecond is so short that even electron’s motion in a material are freezed.

※6 Dynamic X-ray optics
In a visible to infrared wavelength region optical constants itself are changed by dynamically with optical wave and applied electric/magnetic field. They change the direction, wavelength, pulse waveform, spectral width and so on. This concept is applied to the light in an X-ray region.

For more information, please contact:
Director   Hitoki Yoneda
(Institute for Laser Science, The University of Electro-Communications)
TEL:81-42-443-5711 FAX:81-42-485-8960

Group Director   Makina Yabashi
(Group Director, Beam Line Research and Development Group,
XFEL Research and Development Division, RIKEN SPring-8 Center)

Professor   Kazuto Yamauchi
(Graduate School of Engineering, Osaka University)
TEL:81-6-6879-7285 FAX:81-6-6879-7286

Associate Professor   Hidekazu Mimura
(Graduate School of Engineering, The University of Tokyo)
TEL : 81-3-5841-6550 FAX : 81-3-5841-6550

Assistant Professor   Hikaru Kitamura
(Graduate School of Engineering, Kyoto University)
TEL:81-75-753-3750 FAX:81-75-753-3819

RIKEN Global Relations Office, RIKEN
TEL:81-48-467-9272 FAX:81-48-462-4715

Public Relations Division, Administration Office,
The University of Electro-Communications
TEL:81-42-443-5019  FAX:81-42-443-5887

Public Relations Office, SPring-8 Users Office, JASRI
TEL:81-791-58-2785 FAX:81-791-58-2786

Evaluation/Public Relations Division, Administration Office,
Graduate School of Engineering, Osaka University
TEL:81-6-6879-7231 FAX:81-6-6879-7210

Public Relations Office, Graduate School of Engineering,
The University of Tokyo
TEL:81-3-5841-1790 FAX:81-3-5841-0529

Public Relations Division, Kyoto University
TEL:81-75-753-2071 FAX:81-75-753-2094

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