Physicist Martin Esmann has been awarded a "Carl von Ossietzky Young Researchers' Fellowship" by the university for his research. The researcher is investigating vibrations in solids.
It is an exotic and strange world in which Dr Martin Esmann is at home. Populated by strange particles that are half light, half matter, by waves that behave like particles and by curious materials whose properties are described by abstract mathematical concepts. Esmann, who has been supported by a University of Oldenburg "Carl von Ossietzky Young Researcher's Fellowship" since April, is interested in the realm of quantum physics, which he wants to explore using the simplest possible experimental means. "It fascinates me when you can transform an abstract question into a beautiful experiment and finally hold the answer visibly in your hands," he says.
Esmann experienced such an aha moment, for example, during his doctoral thesis, which he completed from 2012 to 2016 in the Oldenburg working group of Prof Dr Christoph Lienau: "I managed to confine light in a very small space and thus create a new type of optical microscope with extremely high resolution," he reports. He published the result in the renowned journal Nature Nanotechnology.
Acoustic vibrations and exotic particles
In his current research project, Esmann combines several phenomena that non-physicists have probably never heard of. Among experts, however, they are regarded as particularly exciting starting points for researching unknown phenomena and strange states of matter. Topology (an abstract concept from mathematics), exciton-polaritons (an exotic type of particle) and acoustic vibrations in solids play an important role.
Esmann studied these vibrations, which can also be considered particles from a physical point of view and are called phonons, in detail during his four-and-a-half-year stay as a postdoctoral researcher in France. From 2016 to the beginning of 2021, he conducted research at the Center for Nanoscience and Nanotechnology (C2N) in Palaiseau near Paris, one of the major centres of the French research organisation CNRS. "It has one of the largest scientific cleanrooms in the whole of France," he reports.
During this time, he developed the idea of viewing acoustic vibrations in solids, which are basically a form of heat, not as a disturbance but as an interesting ingredient. "Phonons can disrupt switching processes in computer chips, for example, and are therefore considered detrimental," he explains. In France, Esmann focussed intensively on how phonons can be controlled. "I built a system that spatially confines light and sound together," reports the researcher. He investigated how the interaction between light and sound can be promoted - a prerequisite for ultimately making acoustic vibrations usable.
New quantum materials
With this experience under his belt, Esmann has now returned to Oldenburg and wants to build up his own research area here that complements the existing specialisations at the Institute of Physics. Esmann is a member of the Quantum Materials team led by Prof Dr Christian Schneider. His project, funded by the Ossietzky Fellowship, does not initially deal directly with phonons, but with other strange physical objects: Exciton-polaritons, the favourite particles of the Quantum Materials working group. These states can be imagined as a coupling of light particles and excited electrons in solids. They consist of matter and light at the same time, so to speak - and also combine their properties.
"Exciton polaritons have a lot in common with a liquid," explains Esmann. For example, the particles are mobile like light particles and interact with each other like electrons. Under certain circumstances, the billionths of a metre small exciton polaritons merge to form a single object that becomes macroscopically visible (by the standards of quantum physics), for example on a scale of a fraction of a millimetre. "The highlight is that we obtain a platform with which quantum mechanics can be observed on the macroscale," says the researcher.
The idea is that such systems can be used to simulate and understand quantum mechanical processes that would otherwise remain hidden. In semiconductors, the required state of the exciton polaritons can only be generated at extremely cold temperatures, but the Oldenburg researchers led by Schneider are using thin protein layers in which the phenomenon already occurs at room temperature.
When waves only travel in one direction
Esmann wants to gain experience with exciton polaritons and then use this knowledge to develop new methods for his experiments with phonons. This is where topology comes into play, a mathematical concept that can be used to describe unusual symmetry properties of solids. These symmetries can, for example, cause wave-like excitations at interfaces to move in a certain direction. One of Esmann's many ideas for utilising phonons is to combine acoustic vibrations with light to control chemical reactions. Another is to use them to facilitate the separation of charges, for example in solar cells.
After four exciting and formative years in Paris, the researcher is delighted to have been able to return to Oldenburg with his family thanks to the Ossietzky Fellowship: "I am very grateful to receive support from the university for this challenging career step towards my own research group." The start has already been made: Esmann has already produced the first samples, got experiments up and running - and started to build up a small team.
