Special Camera Films Energy Exchange of Electrons with Their Environment in Real Time

When light is converted into electricity, for example, in solar cells, a major part of the input light energy is lost. This is because of the behavior of electrons inside the materials.

With the ultrafast system in the Physics Centre at the CAU, the behavior of electrons can be filmed live (Image credit: Jürgen Haacks, CAU)

On striking a material, the light stimulates electrons energetically for a fraction of a second, before they return back the energy into the environment. These processes have hardly been investigated so far due to their very short duration of a few femtoseconds (a femtosecond is one-quadrillionth of 1 second). Currently, a research group from the Institute of Experimental and Applied Physics at Kiel University (CAU), under the guidance of Professor Michael Bauer and Professor Kai Roßnagel, has been successful in exploring the energy exchange of the electrons with their environment in real time, and thus differentiating individual phases. In their experiment, they used an intense, ultrashort light pulse to irradiate graphite and filmed the effect on the behavior of electrons. Complete knowledge of the basic processes involved could be essential in the future for applications in ultrafast optoelectronic components. The researchers have published the study outcomes in the current edition of the journal Physical Review Letters.

A material’s properties are dependent on the behavior of electrons and atoms of which it is made up of. The so-called Fermi gas concept, named after the Nobel Prize winner Enrico Fermi, is a basic model for describing the behavior of electrons. This model considers the electrons in the material to be a gaseous system. This way, their interactions with each other can be explained. To be able to trace the behavior of electrons based on this description in real time, the Kiel researchers devised an experiment for analyses with great temporal resolution: on irradiating a material sample with an ultrafast light pulse, the electrons are stimulated for a short period. A second, delayed pulse of light releases a portion of these electrons from the solid. In-depth investigation of these enables conclusions to be drawn with respect to the electronic properties of the material following the first stimulation with light. A special camera captures how the introduced light energy is distributed through the electron system.

Developed in Kiel: One of the world’s fastest systems

The very high temporal resolution of 13 femtoseconds is the special feature of the Kiel system. Due to this, it is 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.

Michael Bauer, Professor of Ultrafast Dynamics, CAU.

He created the system in collaboration with the working group of Kai Roßnagel, professor of solid state research with synchrotron radiation.

In their present experiment, a graphite sample was irradiated with a short, intense light pulse with a duration of just 7 femtoseconds. Since graphite has a simple electronic structure, it is possible to particularly observe the basic processes clearly. During the experiment, the thermal equilibrium of the electrons was interrupted by the impacting light particles, also known as photons. This equilibrium describes a state in which a precisely definable temperature prevails among the electrons. After that, the Kiel researchers filmed the behavior of the electrons until a balance was restored after about 50 femtoseconds.

Numerous interactions within an extremely short period

Through this process, the researchers observed several interaction processes of excited electrons with the impacting photons, and also with the atoms and other electrons in the material. Based on the film footage, they could even differentiate various phases within this ultrashort period: firstly, the light energy of the photons in the graphite was absorbed by the irradiated electrons, and thus converted into electrical energy. Then, the energy was distributed to other electrons, before transferring it to the surrounding atoms. In the last process, the electrical energy is eventually converted permanently into heat; the graphite warms up.

Furthermore, for the first time, the experiments of the Kiel researchers also confirm theoretical predictions. They offer a fresh view on a research topic that has barely been investigated on this short timescale.

Through our new technical possibilities, these fundamental, complex processes can be observed directly for the first time.

Michael Bauer, Professor of Ultrafast Dynamics, CAU.

In future, this method could also be applied to study and optimize ultrafast motions of light-agitated electrons in materials with promising optical properties.

The study was funded by the German Research Foundation (DFG).

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