In the realm of optics, it is generally expected that the brighter the light source used, the brighter the resulting image will be. This principle holds true for ultra-short pulses of laser light as well, but only up to a certain intensity. Recently, a puzzling phenomenon was observed in X-ray diffraction images, where the images unexpectedly weakened at very high X-ray intensities. This phenomenon not only deepens our fundamental understanding of light-matter interaction but also offers unique possibilities for the production of laser pulses with significantly shorter durations. In this article, we will explore the recent findings of experimental and theoretical physicists from Japanese, Polish, and German research institutions who have shed light on this intriguing darkening effect.
When silicon crystals are illuminated with ultrafast laser pulses of X-ray light, the resulting diffraction images initially become brighter as more photons fall on the sample, indicating higher beam intensity. However, when the intensity of the X-ray beam surpasses a critical value, the diffraction images unexpectedly weaken. This counterintuitive effect has puzzled scientists for some time.
Through collaborative efforts and a combination of experimental studies and theoretical modeling, researchers from institutions such as the RIKEN SPring-8 Centre in Hyogo, the Institute of Nuclear Physics of the Polish Academy of Sciences (IFJ PAN) in Cracow, and the Center for Free-Electron Laser Science (CFEL) at the DESY laboratory in Hamburg have made significant progress in explaining this unexpected phenomenon.
X-ray free-electron lasers (XFELs) play a crucial role in this field of study. These machines generate extremely powerful X-ray pulses with durations of femtoseconds. By using X-ray diffraction techniques, the crystal structure of a material can be analyzed. It was long believed that the more photons present in the X-ray pulse, the clearer the resulting diffraction image would be. However, the researchers discovered that when the intensity of the X-ray beam exceeds an order of tens trillions of watts per square centimeter, the diffraction signal starts to weaken.
To account for this unexpected darkening effect, researchers turned their attention to the ionization of atoms in the crystal lattice. When high-energy photons from the X-ray pulse interact with the material, they knock out electrons from various atomic shells, initiating rapid ionization of the atoms. Previous research by the group showed that the ionized atoms movements, which eventually lead to structural self-destruction of the sample, occurred roughly 20 femtoseconds after the X-ray pulse hit the sample.
However, the recent findings suggest that the weakening of the diffraction signal occurs even earlier, within the first six femtoseconds of the X-ray-matter interaction. During this initial phase, the incoming high-energy photons rapidly excite not only surface electrons but also deep atomic shell electrons close to the atomic nucleus. Surprisingly, it was discovered that the presence of these deep shell holes in atoms greatly reduces their atomic scattering factors, affecting the intensity of the observed diffraction signal.
The research indicates that before any structural damage occurs to the material, rapid electronic damage takes place. This explains the darkening effect observed in the diffraction images. Although this darkening effect may initially seem unfavorable, it opens up new possibilities for applications. By exploiting the fact that different atoms respond differently to ultrafast X-ray pulses, three-dimensional complex atomic structures can be more accurately reconstructed from the recorded diffraction images. Additionally, the material can act as a “scissors,” effectively cutting off a part of the already ultra-short pulse and potentially generating even shorter laser pulses.
The explanation for the darkening of X-ray diffraction images at high intensities provides valuable insights into the dynamics of light-matter interaction. This unexpected phenomenon not only expands our understanding of optics but also offers exciting prospects for applications in material analysis and the production of laser pulses. Further research in this field could lead to breakthroughs in imaging the quantum world and advancing the capabilities of XFELs.
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