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When converting light into electricity, for example in solar cells, much of the light energy absorbed is lost. This is due to the behavior of electrons in materials.
If the light falls on the material, it stimulates the electrons for a fraction of a second before they return energy in the environment. Given the extremely short duration of this event in the interval of several femtosekúnd – femtosekunda is a quarter of a second – these processes have so far been difficult to examine. The team of the Institute of Experimental and Applied Physics of the University of Kiel (CAU), led by Professors Michael Bauer and Kai Roßnagl, was able to examine the exchange of information. Electron energy with their environment in real time, thus distinguishing the different phases of the process. In their experiment, graphite irradiated with an intense and ultra-short pulse of light and influenced the behavior of the electrons. A global understanding of fundamental processes could be important in the future for applications in high-speed optoelectronic components. The research team published these findings in the latest issue of Physical Review Letters.
The properties of the material depend on the behavior of its electrons and atoms. The basic model describing the behavior of electrons is the concept of so-called Fermi gas, named after Nobel laureate Enric Fermi. In this model, the electrons of the material are considered a gaseous system. In this way, it is possible to describe their interaction. To monitor the behavior of electrons based on this real-time description, Kiel's experimental team has developed an extreme time-based experiment: if a sample of material is irradiated with ultra-slow pulsed light, the electrons are stimulated for a short time. The second delayed light pulse releases some of these electrons from the solid. A detailed analysis allows you to draw conclusions about the electronic properties of the material after the first light stimulation. The dedicated camera films the distribution of light energy established through an electronic system.
With the high-speed systems of the CAU Physics Center, the behavior of electrons can be real-time.
Developed in Kiel: one of the fastest systems in the world
The peculiarity of Kiel's system is its extremely high resolution of 13 femtosekúnd. This makes it one of the fastest electronic devices in the world. "Thanks to the very short duration of use of the light pulses used, we are able to film ultra-fast processes alive," said Michael Bauer, professor of high-speed dynamics at CAU. This system was developed jointly with the working group of Professor Kaio Roßnagel, who studies synchrotron radiation solids.
In the current experiment, the research team irradiated the graphite sample with an intense pulse of intense short duration, lasting only seven femtoseconds. Graphite is characterized by a simple structure. As a result, fundamental processes can be observed very closely. In this experiment, light particles – photons – struck the graph and disrupted the thermal equilibrium of the electrons. This equilibrium describes a state in which electrons dominate a precisely defined temperature. The Kieli research team then defined the behavior of the electrons until equilibrium was restored after about 50 femtoseconds.
Many interactions in an extremely short time
Scientists have thus observed many processes of interaction of excited electrons with impact photons as well as atoms and other electrons of the material. On the basis of the digitized film, they could even distinguish the different phases of this extremely short period: the first irradiated electrons absorbed the photon energy in the graphite and converted it into electricity. Then, the energy was distributed to other electrons before being transmitted to the surrounding atoms. In this last process, the electricity is constantly changing to heat; the graphite is warming up.
Experiments by the Kieli research team also confirm theoretical predictions for the first time. They offer a new perspective on a research topic that has been difficult to study at these short time scales. "With our new technical capabilities, these simple and complex processes can be observed for the first time," Bauer said. This approach could also be used in the future to study and optimize the ultra-fast movement of light-induced electrons in materials with optical properties in perspective.
The study was funded by the German Research Foundation (DFG). Emphasized by the editors of Physics, the most famous magazine of the American Physical Society (https://physics.aps.org/synopsis-for/10.1103/PhysRevLett 121,256401).
Original Publication: Ultra-fast formation of an electronic gas distributed by Fermi-Dirac, G. Rohde, A. Stange, A. Müller, M. Behrendt, L.-P. Oloff, K. Hanff, T. J. Albert, P. Hein, K. Rossnagel, and M. Bauer, Phys. Rev. Lett. 121, 256401 (2018), published December 19, 2018. DOI: 10.1103 / PhysRevLett.121.256401 doi.org/10.1103/PhysRevLett.121.256401
Source: www.uni-kiel.de
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