No fewer than three research groups from the Institute for Photonic Quantum Systems (PhoQS) and the Department of Physics at Paderborn University have made progress in the field of modern photonics in their latest work. In two publications, the researchers investigated how light can be precisely manipulated through targeted structuring and coupling at the nanoscale, operating at the interface between classical optics and topological Physics.
In the first publication, researchers from the‘Ultrafast Nanophotonics’research group, led by Prof. Dr Thomas Zentgraf, and‘Theory of Functional Photonic Structures’led by Dr Stefan Schumacher, as well as PhD students Helene Wetter and Jan Wingenbach, collaborated on a sinusoidally modulated silicon metasurface. They utilised near-field couplings to induce unusual polarisation states. They were thus able to demonstrate that, along certain angles of incidence, linearly and circularly polarised light states form, originating from a common Dirac point. The results open up new possibilities for the realisation of angle-dependent polarisation filters and, at the same time, provide insights into the precision and limitations of the nanofabrication of such structures. The paper, published in *ACS Photonics*, can be viewed at: https://pubs.acs.org/doi/abs/10.1021/acsphotonics.5c02865.
In the second publication by the team led by Prof. Schumacher and members of Prof. Dr Jan Sperling’s‘Theoretical Quantum Systems’research group, the Paderborn-based researchers, in collaboration with scientists from the University of Arizona, investigated non-Hermitian optical resonators. Here, so-called exceptional rings (ERs) were detected. These ring-shaped singularities are characterised by mode merging and distinctive topological properties. The researchers demonstrated that these rings arise through circular dichroism and TE–TM splitting in planar optical resonators. In particular, the behaviour of these singularities in non-linear systems had not previously been investigated. The researchers demonstrated that, upon the introduction of Kerr non-linearity, a single ring splits into a closed surface of rings. These topological light states exhibit high sensitivity and are considered promising for future optical sensing and signal processing. The paper, published as a cover story in *ACS Photonics*, can be found at: https://pubs.acs.org/doi/abs/10.1021/acsphotonics.5c02049.
Both approaches demonstrate that controlled coupling and structural symmetry breaking can lead to extraordinary states of light and highly sensitive optical behaviour.
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