  Sub-wavelength electromagnetic field localization has been central in
photonic research in the last decade, allowing to enhance sensing capabilities
as well as increasing the coupling between photons and material excitations.
The ultrastrong light-matter coupling regime in the THz range with split-ring
resonators coupled to magnetoplasmons has been widely investigated, achieving
successive world-records for the largest light-matter coupling ever achieved.
Ever shrinking resonators have allowed to approach the regime of few electrons
strong coupling, in which single-dipole properties can be modified by the
vacuum field. Here we demonstrate, theoretically and experimentally, the
existence of a limit to the possibility of arbitrarily increasing
electromagnetic confinement in polaritonic systems. Strongly sub-wavelength
fields can excite a continuum of high-momenta propagative magnetoplasmons. This
leads to peculiar nonlocal polaritonic effects, as certain polaritonic features
disappear and the system enters in the regime of bound-to-continuum strong
coupling. Emerging nonlinearities due to the local breaking of Kohn's theorem
are also reported.
