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Removing X-Ray Optics Helps Study Nonlinear Effects in Atoms

Researchers have now overcome the last hindrance in the filming and photography of processes that occur on the attosecond scale, where one attosecond is billionths of a billionth of a second. The solution to remove it is in the random nature of the processes that govern the formation of X-ray laser pulses.

Removing X-ray optics eliminates one of the last obstacles hindering the observations of ultra-fast changes of the electronic state of atoms and molecules. Image Credit: IFJ PAN/Anna Wach.

Currently, there are only a handful of X-ray lasers across the globe. It is possible to use these advanced devices to record fast processes including the variations in the electron states of atoms. The pulses produced by modern X-ray lasers are sufficiently short, so they can be used for taking attophotos or even attofilms. But the X-ray optic itself is the main problem.

Upon leaving the laser in which it was created, an ultra-short X-ray pulse can be extended in time more than a dozen-fold.

An international team of physicists under the guidance of Dr. Jakub Szlachetko and Dr. Joanna Czapla-Masztafiak from the Institute of Nuclear Physics of the Polish Academy of Sciences (IFJ PAN) in Cracow and Dr. Yves Kayser of the Physikalisch-Technische Bundesanstalt in Berlin have demonstrated that X-ray optics should not be a problem anymore. The study has been reported in Nature Communications.

The publication is the outcome of research performed at the Linac Coherent Light Source (LCLS) X-ray laser at the SLAC National Accelerator Laboratory in Menlo Park, California.

The best way to get rid of problems with X-ray optics was ... to get rid of X-ray optics. Instead of solving the problem, we found a way around it. It is interesting that we replaced the optics ... by chance. Literally! We have shown that much better parameters than the current X-ray laser pulses can be obtained by skilful use of processes of a stochastic nature.

Dr Jakub Szlachetko, Physicist, Institute of Nuclear Physics of the Polish Academy of Sciences

Physics itself has helped designers before in the history of X-ray lasers. The main element in classical lasers is the optical resonator, which is a system of mirrors that strengthens only photons of a specific wavelength, traveling in a specific direction. For a long time, it was regarded that X-ray lasers are impossible to develop, owing to the lack of mirrors with the ability to reflect X-rays.

This difficulty was overcome when it was observed that it was possible to replace the resonator with just relativistic physics. Upon moving along a system of several oriented magnets, an electron accelerated to a velocity close to that of light, does not pass in a straight line. By contrast, it moves around, losing energy at the same time.

Relativistic effects force the electron to release high-energy photons not just in any direction, but towards the original path of the electron beam (thus the name Free-Electron Laser—FEL).

The high expectations on X-ray lasers are due to the possibility to use in recording chemical reactions. Information related to the present electron state of the system under study (molecule or atom) can be obtained from each single laser pulse.

Meanwhile, the pulse energy is so high, that soon after the image is recorded, the illuminated objects do not exist. Luckily, it is possible to repeat the observation process several times. The images gathered as part of a longer session, allow researchers to precisely reconstruct all the stages of the chemical reaction under study.

The situation can be compared to attempts to photograph events of the same type with a flash camera. When we take enough photos of a sufficient number of the same events, we can use them to construct a film with high accuracy showing what happens during a single event. The problem is that the pulses generated in X-ray lasers arise in spontaneous self-reinforcing stimulated emission and cannot be fully controlled.

Dr Joanna Czapla-Masztafiak, Physicist, Institute of Nuclear Physics of the Polish Academy of Sciences

Since the pulses are spontaneous, in X-ray lasers, the parameters of successive pulses are not precisely the same. The pulses arise once later, once earlier, and also vary in their number energy of photons. In the analogy presented, this corresponds to a condition when successive photos are taken with varying flash units, also, triggered at random moments.

Due to the unavoidable randomness of X-ray pulses, the physicists were forced to add more optical diagnostic equipment in FEL lasers. Consequently, even if an original attosecond pulse was produced by the laser, X-ray optics extend it to femtoseconds.

At present, it has been found that to record impulses with accuracy, controlled parameters are not necessary in recording the electronic states of molecules or atoms. Therefore, chemical reactions can be reconstructed.

Removing X-ray optics also allowed us to use of extremely high-energy pulses to study non-linear effects. This means that atoms begin to be transparent to X-rays at some point, which in turn is associated with an increase in absorption in a different range of radiation.

Dr Jakub Szlachetko, Physicist, Institute of Nuclear Physics of the Polish Academy of Sciences

The new technique will be launched in collaboration with IFJ PAN in experiments performed using both the existing X-ray lasers: SwissFEL in Villigen (Switzerland) and European XFEL near Hamburg (Germany).

The study regarding testing of the new method within the framework of chemical experiments was performed in close collaboration with Dr. Jacinto Sa from the Institute of Physical Chemistry of the Polish Academy of Sciences in Warsaw and the University of Uppsala.

In classical optics, a few purely physical limitations exist that are associated with the resolution of the optical instruments, for instance, the famous diffraction limit.

The new technique does not have any physical limitations—since no optics are involved. Therefore, if X-ray lasers with much shorter pulses than those generated at present emerge, the new technique can be successfully applied to them.


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