When we illuminate something, we usually expect that the brighter the source we use, the brighter the resulting image will be. This rule also works for ultra-short pulses of laser light but only up to a certain intensity. The answer to the question why an X-ray diffraction image ‘darkens’ at very high X-ray intensities not only deepens fundamental understanding of the light-matter interaction but also offers a unique perspective for the production of laser pulses that have significantly shorter pulse duration than those currently available.
The more light, the brighter? This observation might sound trivial, were it not for the fact that it is not always true. When silicon crystals are illuminated with ultrafast laser pulses of X-ray light, the resulting diffraction images are indeed initially brighter the more photons fall on the sample, i.e., the higher the beam intensity. Recently, however, a counterintuitive effect has been observed: when the intensity of the X-ray beam starts to exceed a certain critical value, the diffraction images unexpectedly weaken.
This puzzling phenomenon has just been explained, thanks to the efforts of experimental and theoretical physicists from Japanese, Polish, and German research institutions. The researchers from 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 collaborated to shed light on this phenomenon.
X-ray free-electron lasers (XFELs) generate very powerful X-ray pulses with durations of femtoseconds. Machines of this type, currently operating at only a few locations worldwide, are used to analyze the structure of matter through X-ray diffraction. In this technique, a sample is illuminated by an X-ray pulse, and the diffracted radiation is recorded to reconstruct the original crystal structure of the material under examination.
Theoretical research supported by computer simulations was undertaken to explain the results of the experiment with XFEL laser firing on crystalline silicon samples at Japan’s XFEL facility, called SACLA, Hyogo. The researchers found that an avalanche of high-energy photons hitting a material leads to a massive knockout of electrons from various atomic shells, resulting in rapid ionization of atoms in the material.
According to the researchers, the recently observed weakening of the diffraction signal is due to phenomena occurring in the first six femtoseconds of the interaction. During this initial phase of X-ray-matter interaction, incoming high-energy photons rapidly excite not only ‘surface’ electrons from atoms but also electrons occupying deep atomic shells near the atomic nucleus. The presence of deep shell holes in atoms strongly reduces their atomic scattering factors, which determine the intensity of the observed diffraction signal.
Though the observed effect may seem unfavorable as it results in a decreased brightness of the diffraction images recorded, it holds potential for exploitation. The fact that different atoms respond differently to ultrafast X-ray pulses may help to more accurately reconstruct three-dimensional complex atomic structures from the recorded diffraction images.
Additionally, the phenomenon has implications for the production of laser pulses with extremely short pulse durations. By utilizing the material through which the high-intensity X-ray pulse passes as a ‘scissor’, pulses that are effectively shorter than those produced so far can be generated. This finding holds promise for further breakthroughs in imaging the quantum world.
The understanding gained from the investigation of the darkening of X-ray diffraction images at high intensities provides valuable insights into the complicated dynamics of light-matter interactions. By unraveling the mysteries behind this counterintuitive phenomenon, researchers have not only expanded their knowledge of fundamental science but have also discovered new opportunities for enhancing diffraction imaging and advancing laser pulse technology.