Strong coupling regime and quantum material response*

The functionality of localized quantum emitters (QEs) is limited by their slow operation; making them susceptible to dephasing and decoherence effects through the interaction of the sourses with their environment. Plasmons in metal nanostructures (NSs) are used to accelarate the relaxation of the QEs. When the QE/NS separation distance is small their interaction can be within the strong coupling regime, where coherent energy exchange is observed between QE and NS. While the nonclassical behavior of such quantum emitters (QEs) is well-understood in this context, the role of quantum and surface effects in the plasmonic NS is usually neglected. The relaxation dynamics of a two-level QE placed nearby metal-dielectric layered NSs is investigated, combining the Green's tensor approach with the Feibelman $d$-parameter formalism to theoretically explore the influence of quantum surface effects in the metal. Having identified electron spill-out as the dominant source of quantum effects in jellium-like metals, we focus our study on sodium. The results presented reveal a clear splitting in the  emission spectrum, indicative of having reached the strong-coupling regime, and non-Markovian relaxation dynamics of the emitter. Our findings establish that strong light--matter coupling is not suppressed by the emergence of nonclassical surface effects in the optical response of the metal.

[1] V. Karanikolas et al., arXiv:2102.10832 (2021)