Axion fields in Plasmonic Vortices*

The vector E×H and pseudoscalar E·H products of electric and magnetic fields are separately finite in vacuum transverse electric and magnetic plane waves, and angular momentum structured light. Current theories of interactions beyond the standard model of particle physics invoke E·H≠0 as the source term in the axion law that describes interactions with the cosmological dark matter axion particles outside of the quartet of Maxwell's equations. E·H≠0 also drives relativistic spin-charge magnetoelectric excitations of axion quasiparticles at a distinctively higher condensed matter scale in magnetic and topological materials. Yet, how to drive coherent E·H responses is unknown. By analytical theory and ultrafast coherent photoemission electron microscopy, we focus on E·H fields in surface plasmon polariton vortex cores at subwavelength scales, where the magnetoelectric relative to dipole density is intensified on a ~10 nm diameter scale. The generation and nanoscale localization of E·H fields introduces the magnetoelectric symmetry class, having the parity and time-reversal symmetry broken, and the joint parity-time symmetry preserved opening the electromagnetic interactions with axion quasiparticles on femtosecond scale. Our work opens a unique research field on electromagnetic probing of dark matter as well as topological dressing of quantum materials.