World's First Determination of X-Ray Nonlinear Susceptibility Using Diamonds – Establishment of Foundation for Applying Nonlinear Optics to Frontier Area of X Rays - (Press Release)
- Release Date
- 18 Dec, 2009
- BL19LXU (RIKEN SR Physics)
RIKEN
Key research achievements
• World's first success in actual measurement of nonlinear susceptibility, which determines the intensity of nonlinear optical response in X-ray region
• Clarification of characteristics of diamond, which increases X-ray nonlinear optical response by 100-fold
• Great expectations on development of nonlinear materials having high nonlinear susceptibility in X-ray region
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RIKEN (Ryoji Noyori, President) succeeded in determining nonlinear susceptibility*1 in the X-ray region for the first time in the world. The researchers at RIKEN found that the nonlinear susceptibility in the X-ray region is lower than that in the visible light region by ten orders, but can be increased by ten times using the core-excited resonance*2 of diamond carbon atoms. This was achieved by Kenji Tamasaku, a senior research scientist, and Tetsuya Ishikawa, Chief Scientist, of the Coherent X-ray Optics Laboratory, RIKEN SPring-8 Center. Nonlinear optics*3 has been under development for over 50 years. The research achievements have been applied to the fields of basic research, such as wavelength conversion, and also to ultrahigh-speed optical communication, which is a great contribution to the real world. However, the subject of nonlinear optics has been limited to mainly visible light with long wavelengths of 400-760 nm (a nanometer is one-billionth of a meter) in the low-energy region. There has been little experimental data in the (high-energy) region of X-rays with wavelengths 10,000 times shorter than those of visible light. Nevertheless, researchers have assumed that the nonlinear optical response in the X-ray region is completely different from that in the known visible light region. The clarification and formulation of the theory of nonlinear optics in the X-ray region have long been desired. In this study, by accurately measuring the radiation of X-rays to artificial diamond at SPring-8, it was demonstrated that parametric down-conversion*4, a nonlinear optical phenomenon, interferes with inelastic scattering*5, which is an independent optical process, in a quantum-mechanical*6 manner. Moreover, by analyzing the results, they estimated the magnitude of the nonlinear susceptibility that determines the intensity of nonlinear optical phenomena. The value obtained was smaller than that in the visible light region by ten orders but can be increased by tenfold (100 times in efficiency) by resonating it with the core excitation of the carbon atoms in the diamond. The X-ray nonlinear optical elements with high nonlinear susceptibility can be developed by applying this mechanism of increasing the susceptibility using the resonance. These research achievements will be the first solid foundation for pioneering the frontier of X-ray nonlinear optics and broadening the subject of nonlinear optics to the X-ray region. Moreover, by combining these achievements with the use of the X-ray free electron laser*7 now under construction, X-ray nonlinear optics will rapidly progress in the near future and will be applied widely to basic research in materials science and to the development of highly efficient nonlinear materials. These research achievements were published in the American scientific journal, Physical Review Letters (online version), on 18 December 2009. Publication: |
<Figure>
Fig. 1 Schematic of observed X-ray nonlinear optical phenomenon (parametric down-conversion) and inelastic scattering
In the parametric down-conversion shown in (a), photon 1 is annihilated and photons 2 and 3 are generated. Then, photon 3 excites the electron by normal light absorption. On the other hand, in the inelastic scattering process shown in (b), part of the energy of photon 1 excites the electron when photon 1 is scattered by photon 2. By correctly dealing with the interference effect (Fano effect) between these two processes, the research group succeeded in determining nonlinear susceptibility, which corresponds to the intensity of X-ray nonlinear optical phenomena, for the first time in the world.
Fig. 2 Energy dependence of nonlinear susceptibility
Nonlinear susceptibility, which determines the intensity of X-ray nonlinear optical phenomena, significantly changes according to the energy of photon 3(the photon that excites the electron among the two photons generated by parametric down-conversion). It increases by nearly 100 times with the energy of approximately 290 eV (4.27 nm in wavelength) when a core-excited resonance occurs.
<Glossary>
*1 Nonlinear susceptibility
Nonlinear susceptibility is a quantity that defines the intensity of a nonlinear optical response. The higher the nonlinear susceptibility, the greater the nonlinearity. The intensity of the nonlinear optical phenomena observed is proportional to the square of the nonlinear susceptibility. Generally, since nonlinear susceptibility is significantly lower than the (linear) susceptibility related to observing an object, we are not aware of its presence in our daily life.
*2 Core-excited resonance
Core-excited resonance indicates a case in which the energy an electron needs to move from an orbit close to the nucleus to an outer orbit corresponds to the photon energy. This correspondence often significantly affects the nonlinear susceptibility.
*3 Nonlinear optics
Nonlinear optics deals with the optical phenomenon that the response of a material to light is not proportional to the amplitude of the light wave. Since these effects are significantly weaker than the linear responses, a laser is usually needed to observe them.
*4 Parametric down-conversion
A nonlinear optical phenomenon in which one photon (a particle of light) is converted to two photons through its interaction with the electron in a material.
*5 Inelastic scattering
A phenomenon in which the light incident on a material gives part of its energy to electrons and the electrons are scattered. Scattering like that in the collision of billiard balls is called elastic scattering.
*6 Quantum mechanics
A theory where light and electrons are considered to behave as both particles and waves. In this theory, the future is discussed not deterministically but stochastically.
*7 X-ray free electron laser
A laser in the X-ray region. The X-ray free electron laser project has been planned as a national critical technology in Japan since fiscal 2006. RIKEN and the Japan Synchrotron Radiation Research Institute have jointly established the "X-ray Free Electron Laser Project Head Office" and are constructing the facility beside SPring-8, with the aim of starting laser oscillation in fiscal year 2010.
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