Recent research revealed that in resonators with deep subwavelength gaps coupled to two-dimensional electron gases, propagating plasmons lead to energy leakage, hindering polaritonic resonance. This study introduces plasmonic reflectors to create an artificial energy stopband, confining terahertz-range plasmons and recovering polaritonic resonances. Using this approach demonstrates a normalized coupling ratio of 0.36, enabling the observation of polaritonic resonances not seen without plasmonic reflectors.

It was recently demonstrated that, in deep subwavelength gap resonators coupled to two-dimensional electron gases, propagating plasmons can lead to energy leakage and prevent the formation of polaritonic resonances. This process, akin to Landau damping, limits the achievable field confinement and thus the value of light-matter coupling strength. In this work, we show how plasmonic reflectors can be used to create an artificial energy stopband in the plasmon dispersion, confining them and enabling the recovery of the polaritonic resonances. Using this approach we demonstrate a normalized light-matter coupling ratio of ^Ω_ω^R₀ = 0.36 employing a single doped quantum well with a resonator’s gap size of 250 nm equivalent to λ/3000 in vacuum, a geometry in which the polaritonic resonances would not be observable in the absence of the plasmonic reflectors.

Subwavelength electromagnetic field localization has been central to photonic research in the last decade, allowing us to enhance sensing capabilities as well as increase the coupling between photons and material excitations. The strong and ultrastrong light–matter coupling regime in the terahertz range using 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 us to approach the regime of few-electron 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 subwavelength 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 the regime of discrete-to-continuum strong coupling.

We report a physical limit for reducing the modal volume of a cavity and ultimately increasing the light-matter coupling strength. Extremely confined photonic mode below a critical length-scale can introduce a large in-plane wave vector and excite a continuum of high momentum matter resonances. This excitations act as loss channels and reduce the field confinement by smearing the distribution of surface charges, thus consequently limiting the achievable field enhancement.

In this work, we theoretically and experimentally show that the confinement of an electromagnetic field below critical length-scales can excite high momentum matter resonances and can ultimately limit the light-matter coupling enhancement in an ultrastrong coupling regime.

In this work, we theoretically and experimentally show that the confinement of an electromagnetic field below critical length-scales can excite high momentum matter resonances and can ultimately limit the light-matter coupling enhancement in an ultrastrong coupling regime.