Conversion of x-ray beams into longer wavelengths creates a new spectroscopic tool
X-rays—beams of light with short wavelengths and high energy—are a commonly used diagnostic tool, not only in medicine, but also in materials science. Two researchers from RIKEN’s SPring-8 Center in Harima have demonstrated the conversion of x-rays into longer wavelengths that preserves some of their useful properties, such as high spatial resolution.
Normally, image resolution is limited by the wavelength of the light being used. Light with a very short wavelength, such as x-rays, therefore yields higher imaging resolution than, for example, visible light. On the other hand, since x-rays have high energies (equivalent to shorter wavelengths) they are not always suitable to study low energy (or long-wavelength) properties of materials.
Writing in the journal Physical Review Letters1, Kenji Tamasaku and Tetsuya Ishikawa, present an experimental demonstration of how some of the advantages of x-rays can be brought towards other wavelengths. They used an optical effect, known as ‘parametric down-conversion’ that can be realized with high-intensity x-rays. “The important point of x-ray nonlinear optical effects, such as parametric down-conversion, is that one can generate an arbitrary wavelength from the visible to the x-ray region for further study,” says Tamasaku.
In this process, an x-ray beam, the ‘pump’, is split into two beams, the ‘signal’ and ‘idler’, by scattering off a regular crystal such as a diamond (Fig. 1). The wavelength of the signal beam is in the x-ray region, whereas the wavelength of the idler beam is much longer, in the extreme ultraviolet light region. The precise relationship between signal and idler wavelengths is given by physical laws such as the conservation of energy. The idler beam in particular is of interest for high-resolution spectroscopy applications.
In their experiments, Tamasaku and Ishikawa have clearly and successfully demonstrated this effect for the first time. Their achievement is rooted in the very high x-ray intensities that can be achieved with the SPring-8 synchrotron radiation facility—necessary for an efficient conversion of the pump beam.
The extreme ultraviolet wavelength region is of particular interest to probe the nature of atomic bonds. As the creation of the idler is directly related to the local environment of the x-ray pump, a high spatial resolution can be achieved in experiments. Therefore, “it will be possible to obtain information about the bonds between individual atoms,” explains Tamasaku. The parametric down-conversion pioneered by the RIKEN researchers might therefore become an important analytical tool.
1. Tamasaku, K. & Ishikawa, T. Interference between Compton scattering and x-ray parametric down-conversion. Physical Review Letters 98, 244801 (2007).
