During the conversion of light into electricity, such as in solar cells, a large part of the input light energy is lost. This is due to the behavior of electrons inside of materials. If light hits a material, it stimulates electrons energetically for a fraction of a second, before they pass the energy back into the environment. Because of their extremely short duration of a few femtoseconds – a femtosecond is one quadrillionth of a second – these processes have hardly been explored to date. A team from the Institute of Experimental and Applied Physics at Kiel University (CAU), under the direction of Professor Michael Bauer and Kai RoBnagel, has now succeeded in investigating the energy exchange of the electrons with their environment in real time, and thereby distinguishing individual phases. In their experiment, they irradiated graphite with an intense, ultrashort light pulse and filmed the impact on the behavior of electrons. A comprehensive understanding of the fundamental processes involved could be important in future for applications in ultrafast optoelectronic components. The research team has published these findings in the current edition of the journal Physical Review Letters.
The properties of a material depend on the behavior of its constituent electrons and atoms. A basic model to describe the behavior of electrons is the concept of the so-called Fermi gas, named after the Nobel Prize winner Enrico Fermi. In this model, the electrons in the material are considered to be a gaseous system. In this way, it is possible to describe their interactions with each other. In order to follow the behavior of electrons on the basis of this description in real time, the Kiel research team developed an experiment for investigations with extreme temporal resolution: If a material sample is irradiated with an ultrafast pulse of light, the electrons are stimulated for a short period. A second, delayed light pulse releases some of these electrons from the solid. A detailed analysis of these allows conclusions to be drawn regarding the electronic properties of the material after the first stimulation with light. A special camera films how the light energy introduced is distributed through the electron system.
