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Ultrastrong light-matter coupling has recently been achieved in several experimental platforms at different optical frequencies, leveraging on the collective enhancement of the interaction with the number of excitations and the simultaneous sub-wavelength electromagnetic field localization obtainable employing metallic resonators. 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. In this work we will discuss two results that are relevant with respect to the limits of achievable light-matter coupling strength and to the measurement of such coupling in single, strongly subwavelength resonators. In the first experiment 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 discrete-to-continuum strong coupling.
In the second part of the work, we show that by combining an asymmetric immersion lens setup and complementary design of metasurfaces we are able to perform THz time domain spectroscopy of an individual, strongly subwavelength meta-atom. We unravel the linewidth dependence of planar metamaterials as a function of the meta-atom number indicating quenching of the Dicke superradiance. We investigate as well ultrastrongly coupled Landau polaritons at the single resonator level, measuring a normalized coupling ratio of 0.6 resulting from coupling of the fundamental mode of a single, deeply subwavelength LC resonatorto a few thousand electrons.
Our findings pave the way towards the control of light-matter interaction in the ultrastrong coupling regime at the single electron/single resonator level. The proposed technique is way more general and can be useful to characterize the complex conductivity of micron-sized samples in the THz and sub-THz domain.
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