Describing optical fields in terms of their topology is becoming an important concept, since the complex behavior of electromagnetic fields in any system is dictated by an invariant topology. Obtaining the topological properties of electromagnetic fields at surfaces is a complex experimental endeavor. It requires measuring the electromagnetic field vectors with nanometer spatial and subfemtosecond temporal resolution. The invariant topology can be found either in the electromagnetic fields directly, or in derived quantities, such as the spin angular momentum.

The cover of Advanced Photonics Volume 6 Issue 6 showcases a topological quasiparticle, a meron pair, formed in the spin angular momentum vector field of surface plasmon polaritons. The spin vectors were measured using a cutting-edge technique that combines pump–probe photoemission electron microscopy with polarimetry. Further information is presented in the original research article by Pascal Dreher, Alexander Neuhaus, David Janoschka, Alexandra Rödl, Tim Colin Meiler, Bettina Frank, Timothy J. Davis, Harald Giessen, and Frank Meyer zu Heringdorf, “Spatiotemporal topology of plasmonic spin meron pairs revealed by polarimetric photo-emission microscopy” Adv. Photon.6(6) 066007 (2024), doi: 10.1117/1.AP.6.6.066007.