A key phenomenon in quantum electrodynamics is the so-called “strong coupling” regime, which takes place when the interaction between light and matter excitations is so strong that they mix to create hybrid states, called polaritons. In the search for nonclassical light sources, much effort has focused lately on realizing strong coupling between a single electromagnetic (EM) mode and a single emitter [1]. Plasmonic nanostructures have become promising candidates for polaritonic cavities as they enable light-matter coupling strengths well beyond those provided by semiconductor devices. They sustain EM modes with effective volumes approaching the sub-nanometric scale, which open the way to the formation of polaritons at the single-emitter level at room temperature. Recently, far-field signatures of plasmon-exciton strong coupling in ensembles of very few emitters have been reported in cavities presenting nanometric gaps [2].

In this talk, I will present our theoretical work in this context. I will introduce our analytical approach able to describe the full richness of the EM spectrum supported by plasmonic cavities, and will reveal the geometric and material conditions most convenient for strong coupling [3]. I will also show the remarkable robustness of far-field photon correlations in these systems against the increasing size of the emitter ensembles, which opens the way to realizing single-photon sources beyond the single emitter regime [4]. Finally, I will also discuss polaritonic effects that originate from the confined nature of surface plasmons, such as the coupling to light-forbidden excitons [5] or the emergence of quasi-chiral interactions in circularly polarized emitters [6].

[1] A. I. Fernández-Domínguez et al. ACS Photonics 5, 3447−3451 (2018).
[2] R. Chikkaraddy et al., Nature 535, 127 (2016).
[3] R.-Q. Li et al., Phys. Rev. Lett. 117, 107401 (2016).
[4] R. Sáez-Blázquez et al., Optica 4, 1363 (2017)
[5] A. Cuartero-González et al., ACS Photonics 5, 3415 (2018).
[6] C. A. Downing et al., Phys. Rev. Let. 122, 057401 (2019).