Most experiments of polaritonic systems involve many molecules coupled to many photonic modes in optical cavities. Although there have been exciting works demonstrating the possibility of strongly coupling a single molecule to a plasmonic cavity mode, it is understandably difficult (if not impossible) to achieve strong coupling in a Fabry-Pérot micro-cavity in the single molecule limit. This is because the relatively large cavity mode quantization volume in Fabry-Pérot micro-cavities leads to a negligible coupling for a single molecule.