Tip-induced Strong Coupling in a Single Quantum Dot

Light-matter interaction in the strong coupling regime has long been reported using, e.g., quantum dots (QDs), dye molecules, or J-aggregates with metallic nano-photonic structures. However, most of the experiments were performed with multiple emitters and with devices of no or limited tunability or complicated control. Here we induce, image, and control strong coupling with single QDs at room temperature in a tunable plasmonic nano-resonator formed by an optical antenna nano-tip and a planar Au surface. From spatial and spectral variations of the splitting of the tip-enhanced photoluminescence (TEPL) signal, we induce and quantify the strong coupling between excitons and gap plasmons. Through variation of tip-sample distance and thickness of a dielectric capping layer we control the confinement of the gap plasmon and hence the coupling strength. Similarly, lateral variation of tip position allows for nanometer scale imaging of the spatial dependence of the plasmon-exciton coupling strength. This antenna-tip-controlled strong coupling approach opens forms of single emitter coherently controlled devices.