Brighter means darker: Counterintuitive X-ray lasers
The SACLA free-electron laser facility, where the experiment on the diffraction of ultra-short X-ray pulses on crystalline silicon samples was carried out. (Source: SACLA)
The counterintuitive effect, in which the brighter the laser light beam, the weaker the diffraction image has been explained by physicists from Japanese, Polish and German scientific institutions. Understanding this phenomenon gives hope for the production of laser pulses that have significantly shorter pulse duration than those currently available, reports the Institute of Nuclear Physics of the Polish Academy of Sciences, involved in the research project.
Research shows that before the sample disintegrates under the influence of an avalanche of high-energy photons, first a rapid electronic damage occurs. As a result, the final part of the pulse practically no longer ionises the material, because further excitation of electrons by X-ray photons is no longer energetically possible. The processes responsible for the fading of the image therefore occur in the first femtoseconds of the interaction of light with matter.
'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. 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. But when the intensity of the X-ray beam starts to exceed a certain critical value, the diffraction images unexpectedly weaken,’ Polish representatives of the research team say.
A PHOTON AVALANCHE KNOCKS ELECTRONS FROM ATOMIC SHELLS
Professor Beata Ziaja-Motyka from the Institute of Nuclear Physics PAS in Kraków and the DESY Center for Free-Electron Laser Science in Hamburg explains that X-ray free-electron lasers (XFELs) generate very powerful X-ray pulses with durations of femtoseconds, i.e., quadrillionths of a second. Machines of this type, currently operating at only a few locations in the world, are used to analyse structure of matter by means of X-ray diffraction. With this technique, a sample is illuminated by an X-ray pulse and the diffracted radiation is recorded. The obtained diffraction image is used to reconstruct the original crystal structure of the studied material.
'Intuition tells us that the more photons we have, the clearer the diffraction image of the sample should be. This is indeed the case, but only up to a certain X-ray intensity, of the order of tens of trillions of watts per square centimetre. When this value is exceeded - and we have been only recently capable of doing this - the diffraction signal suddenly starts to weaken. Our research is the first attempt to explain this unexpected effect,’ says Professor Beata Ziaja-Motyka, co-author of the paper published in Physical Review Letters.
Theoretical research undertaken to explain the results of the experiment with XFEL laser firing on crystalline silicon samples at XFEL facility SACLA in Hyogo, Japan, has been supported by computer simulations.
ALL IS DECIDED IN THE FIRST FEMTOSECONDS OF INTERACTION
'When an avalanche of high-energy photons hits a material, there is a massive knockout of electrons from various atomic shells, resulting in a rapid ionisation of atoms in the material. Last year, our group showed that the first movements of ionised atoms in the crystal lattice, initiating the process of structural self-destruction of the sample, occurred with a delay of approximately 20 femtoseconds after the light pulse hit the sample. We are now convinced that the reason for the recently observed weakening of the diffraction signal is due to phenomena occurring earlier, in the first six femtoseconds of the interaction,’ says Dr. Ichiro Inoue from the RIKEN SPring-8 Centre, responsible for the experimental study.
During the initial phase of X-ray-matter interaction, high-energy photons rapidly excite not only 'surface' (valence) electrons from atoms, but also the electrons occupying deep atomic shells, located close to the atomic nucleus. It turns out that the presence of deep shell holes in atoms strongly reduces their atomic scattering factors, i.e., the quantities determining the intensity of the observed diffraction signal.
'Our research shows that before any structural damage to the material occurs and the sample disintegrates, first a rapid electronic damage occurs. As a result, the final part of the pulse practically no longer ionises the material, because further excitation of electrons by X-ray photons is no longer energetically possible,’ says Professor Ziaja-Motyka.
At first glance, the observed effect appears to be exclusively unfavourable, as it results in a decreased brightness of the recorded diffraction images. However, scientists believe that thanks to a deep understanding of the nature of the phenomenon, it will be possible to exploit it. The key is the observation that different atoms respond differently to ultrafast X-ray pulses. This knowledge may help to more accurately reconstruct three-dimensional complex atomic structures.
Another area of potential application is connected to the production of laser pulses with extremely short pulse durations. Since the material through which the high-intensity X-ray pulse passes cuts off a significant part of the already ultra-short pulse, it can be deliberately used as 'scissors' to generate pulses that are effectively shorter than those produced so far. According to the study authors, this could stimulate another breakthrough in imaging of quantum world. (PAP)
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