Spatiotemporal topology of plasmonic spin meron pairs revealed by polarimetric photo-emission microscopy

Topology is the study of geometrical properties and spatial relations\nunaffected by continuous changes, and has become an important tool for\nunderstanding complex physical systems. Although recent optical experiments\nhave inferred the existence of vector fields with the topologies of merons, the\ninability to extract the full three dimensional vectors misses a richer set of\ntopologies that have not yet been fully explored. In our work, we extend the\nstudy of the topology of electromagnetic fields on surfaces to a spin\nquasi-particle with the topology of a meron pair, formed by interfering surface\nplasmon polaritons, and show that the in-plane vectors are constrained by the\nembedding topology of the space as dictated by the Poincare-Hopf theorem. In\naddition we explore the time evolution of the three dimensional topology of the\nspin field formed by femtosecond laser pulses. These experiments are possible\nusing our here developed method called polarimetric photoemission electron\nmicroscopy (polarimetric PEEM) that combines an optical pump-probe technique\nand polarimetry with photo-emission electron microscopy. This method allows for\nthe accurate generation of surface plasmon polariton fields and their\nsubsequent measurement, revealing both the spatial distribution of the full\nthree-dimensional electromagnetic fields at deep sub-wavelength resolution and\ntheir time evolution.\n