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Exceeding Limit to Observation Wavelength Using “Sister” Photons (Press Release)

Release Date
18 Jul, 2011
  • BL19LXU (RIKEN SR Physics)
- Developing a method to separate a wave used to observe a substance and a wave used to determine the spatial resolution

Nagoya University
Japan Science and Technology Agency (JST)

Key research achievements
•Achieving an ultrahigh spatial resolution of 1/380 of an optical-wave wavelength (λ/380) using "sister" photons
•A "big sister" photon was found to observe a "little sister" photon while a substance is responding to the little sister photon
•Showing the possibility of optical microscopes with an ultrahigh spatial resolution that can directly observe the optical response of a substance

Scientists of RIKEN (President, Ryoji Noyori) and Nagoya University (President, Michinari Hamaguchi) have developed a microscopic technique with an ultrahigh spatial resolution of as fine as 0.54 angstrom (Å)*1 using a nonlinear optical phenomenon*2 in the X-ray region. This resolution corresponds to 1/380 of a wavelength (206 Å), which cannot be achieved by conventional methods. The achievement of this study was obtained by the group of Kenji Tamasaku (Senior Research Scientist) and Tetsuya Ishikawa (Chief Scientist) of the Coherent X-Ray Optics Laboratory of RIKEN SPring-8 Center (Director, Tetsuya Ishikawa) and Eiji Nishibori (Associate Professor) of School of the Engineering, Nagoya University.

Since the optical microscope was invented at the end of the 16th century, improving its resolution has been a top priority. However, Dr. Abbe of University of Jena (Germany) demonstrated in 1878 that the maximum spatial resolution is in principal approximately a half of the wavelength and that a substance smaller than this cannot be observed. Although researchers worldwide have tried various methods to overcome this constraint, a spatial resolution of approximately a tenth of the wavelength is still the limit even today. At this spatial resolution, it is impossible to directly observe how electrons in a substance respond to light (the optical response), in other words, the reason why a substance has a particular color.

To solve this problem, the scientists of the above research group developed a new microscopic technique with an ultrahigh spatial resolution based on a nonlinear optical phenomenon in the X-ray region, in which one "parent" photon*3 is divided into a big sister photon and a little sister photon, using the RIKEN Synchrotron Radiation Physics Beamline (BL19LXU) at SPring-8, which can radiate extremely intense X-rays. In this method, the little sister photon with a long wavelength acts on a substance and the big sister photon observes the response. In this way, the sister photons work cooperatively. The big sister photon can observe the phenomenon at an ultrahigh spatial resolution because it is an X-ray photon. In this study, X-rays (parent photons) were irradiated on a diamond sample, and they succeeded in observing the behavior of each internal electron in the extreme-ultraviolet region*4 where the little sister photon exists.

The results of this study indicate the possibility of microscopes with a spatial resolution that is far beyond the limit expressed in terms of the wavelength. It is hoped that we will be able to directly observe the optical response of a substance to obtain a deeper understanding of substances and to contribute to the sophisticated use of microscopes through further advances in measurement technology in the future.

This research was supported by the JST Basic Research Programs for individual researchers [Precursory Research for Embryonic Science and Technology (PRESTO)] project "Innovative Use of Light and Materials/Life" (Research Supervisor: Hiroshi Masuhara, Specially Appointed Professor at Nara Institute of Science and Technology and Chair Professor of National Chiao Tung University in Taiwan) under the title "Investigation of local optical response using X-ray nonlinear diffraction" (Scientist: Kenji Tamasaku). The results were published online in the scientific journal Nature Physics on 17 June 2011.

"Visualizing the local optical response to extreme-ultraviolet radiation with a resolution of λ/380"
Kenji Tamasaku, Kei Sawada, Eiji Nishibori and Tetsuya Ishikawa
Nature Physics (2011), published online 17 July 2011

*1 Angstrom (Å)

An angstrom is one-ten millionth of a millimeter and is used as a unit to represent atomic-scale dimensions. For example, the radius of a carbon atom is approximately 0.7 Å.

*2 Nonlinear optics
In nonlinear optics, optical phenomena with an optical response that is not proportional to the amplitude of the optical wave are focused on. In general, a laser is required to observe such phenomena because a nonlinear response is much weaker than a linear response.

*3 Photon
A photon is an elementary particle in the consideration of light as particles.

*4 Extreme-ultraviolet region
Among the light in the ultraviolet region with wavelengths shorter than that of visible light (4,000-7,000 Å), the extreme-ultraviolet region refers to light with wavelengths of several hundreds of angstrom.


Fig. 1	Relationship between wavelength and spatial resolution
Fig. 1 Relationship between wavelength and spatial resolution

In general, the wavelength of light used for observation determines the lower limit of the size of the sample that can be observed. A sample cannot be observed with light in the shaded region in the figure. In this study, an ultrahigh spatial resolution of 1/380 of the wavelength of the extreme-ultraviolet light was realized using the characteristics of a big sister photon, which is an X-ray photon in the extreme-ultraviolet region, where the little sister photon exists.

Fig. 2 Macroscopic diamond (left) and diamond responding to a little sister photon while being observed by a big sister photon (right)
Fig. 2 Macroscopic diamond (left) and diamond responding to a little sister photon
while being observed by a big sister photon (right)

In the right figure, the blue spheres, corresponding to core electrons that are strongly bound to a carbon atom, move in the same direction as the electric field of the light. In contrast, the red disks, corresponding to valence electrons that are involved in bonding between atoms, move in the opposite direction. Each side of the cube is approximately 3.6 Å, which is much smaller than the wavelength of 206 Å.

For more information, please contact:
 Dr. Kenji Tamasaku (RIKEN)

 Dr. Eiji Nishibori (Nagoya University)