The strong interaction of individual quantum emitters with resonant cavities is of fundamental interest for understanding light matter interactions. Recent experiments revealed strong coupling between individual plasmonic structures and multiple organic molecules, but so far strong coupling at the limit of a single quantum emitter has not been reported. Here we demonstrate vacuum Rabi splitting, a manifestation of strong coupling, using silver bowtie plasmonic cavities loaded with semiconductor quantum dots (QDs). We have successfully integrated lithographically fabricated silver bowties with one to a few QDs that reside exactly within the plasmonic cavity. Dark field micro-spectroscopy was used in order to characterize the plasmonic behavior of every single bowtie by measuring its scattering spectrum. The very small plasmon mode volumes of the bowties allowed us to demonstrate vacuum Rabi splitting in the limit of an individual quantum emitter. Rabi splitting as high as ~180 meV was observed with a single QD. These observations are verified by polarization-dependent experiments and validated by electromagnetic calculations. Using 50nm SiN suspended films as the substrate enables us upgrading the lithography results and achieving controllably sub 10nm gap and therefore significantly increase the coupling between the QD and the bowties. Transmission Electron Microscopy (TEM) enables us spatially mapping the QDs within the gap and therefore understanding better the light matter interaction in this system.
