Structured Light, a Portal to Dark Matter?

Weakly interacting axion pseudoscalar particles are leading candidates for cold dark matter. Proposed as a solution to the strong Charge-Parity problem, they possess Parity-Time (𝓟𝒯) symmetry and primarily interact through the Lagrangian ℒ ~ E ∙ B term, which makes them unresponsive to transverse electromagnetic fields where E ∙ B = 0. Interactions involving E ∙ B ≠ 0 can occur in a condensed phase, but such 𝓟𝒯 symmetry magnetoelectric fields cannot propagate in vacuum. At metal-vacuum interfaces, we generate and image evanescent fields propagating as surface plasmon polaritons at a velocity of 15 nm/50 as, using ultrafast nonlinear photoelectron microscopy. We observe that structured light, evolving through spin-orbit interaction, forms plasmonic vortices that serve as sources of an evanescent E ∙ B ≠ 0 term at their phase-singularity cores. Within the IQ Initiative, we consider the topological dressing of matter by 𝓟𝒯-symmetric light as a portal for interaction with synthetic axion quasiparticles and potentially with cosmological axion dark matter.