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.