Japanese researchers Kenji Tamasaku and Tetsuya Ishikawa, writing in the journal Physical Review Letters, present a study that looks at a potential use of an optical effect known as ‘parametric down-conversion’ to make x-rays “colorful”:
“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. 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.