Sub-wavelength electromagnetic field localization is a central theme in photonic research, as it allows  sensing capabilities as well as increasing the light-matter coupling strength.  Recently, the strong and ultrastrong light-matter coupling regime [1] in the THz range with split-ring resonators coupled to magnetoplasmons [2] 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. 

We will discuss, 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 [3]. This leads to peculiar nonlocal polaritonic effects, as certain polaritonic features disappear and the system enters in the regime of discrete-to-continuum strong coupling.

We will as well discuss experiments reporting spectroscopy of a single, ultrastrongly coupled, highly  subwavelength resonator operating at 300 GHz. By using a combination of immersion lenses we unravel the linewidth dependence of planar metamaterials as a function of the meta-atom number indicating quenching of the superradiance. On these grounds, we investigate ultrastrongly coupled Landau polaritons at the single resonator level [4], measuring a normalized coupling ratio Omega/omega=0.6.

Lastly, we will present magnetotransport measurements, showing that enhanced vacuum field fluctuations in subwavelength split-ring resonators dramatically affect the quantum Hall electron transport in high-mobility 2D electron gases. The observed breakdown of the topological protection of the integer quantum Hall effect is interpreted in terms of a long-range cavity-mediated electron hopping where the anti-resonant terms of the light-matter coupling finally result into a finite resistivity induced by the vacuum fluctuations [5].