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Friday, December 21, 2018

Camera films motion of electrons

By Nick Flaherty

A team from the Institute of Experimental and Applied Physics at Kiel University (CAU), under the direction of Professor Michael Bauer and Professor Kai Ro├čnagel, are 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, 7 femtosecond light pulse and filmed the impact on the behaviour of electrons.  A second, delayed light pulse releases some of these electrons from the solid.  

A comprehensive understanding of the fundamental processes involved could be important in future for applications in ultrafast optoelectronic components. 

The properties of a material depend on the behaviour of its constituent electrons and atoms. A basic model to describe the behaviour of electrons is the concept of the so-called Fermi gas, where 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. 

The resolution of 13 femtoseconds makes it one of the fastest electron cameras in the world. "Thanks to the extremely short duration of the light pulses used, we are able to film ultrafast processes live. Our investigations have shown that there is a surprising amount of stuff happening here," said Michael Bauer, professor of ultrafast dynamics at the CAU. He developed the system, together with the working group of Kai Ro├čnagel, professor of solid state research with synchrotron radiation.

In their current experiment, the research team irradiated a graphite sample with a short, intense light pulse of only seven femtoseconds duration. Graphite is characterised by a simple electronic structure. Thus, fundamental processes can be observed particularly clearly. In the experiment, the impacting light particles - also called photons - disturbed the thermal equilibrium of the electrons. This equilibrium describes a condition in which a precisely-definable temperature prevails amongst the electrons. The Kiel research team then filmed the behaviour of the electrons, until a balance was restored after about 50 femtoseconds.

In doing so, the scientists observed numerous interaction processes of excited electrons with the impacting photons, as well as atoms and other electrons in the material. On the basis of the film footage, they could even distinguish different phases within this ultrashort period: first of all, the irradiated electrons absorbed the light energy of the photons in the graphite, and thereby transformed it into electrical energy. Then the energy was distributed to other electrons, before they passed it on to the surrounding atoms. In this last process, the electrical energy is ultimately permanently converted into heat; the graphite warms up.

The experiments of the Kiel research team also confirm theoretical predictions for the first time. They enable a new perspective on a research topic which has hardly been investigated on this short time scale. "Through our new technical possibilities, these fundamental, complex processes can be observed directly for the first time," said Bauer. This approach could also be applied in the future to investigate and optimise ultrafast motions of light-agitated electrons in materials with promising optical properties.

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