Scanning-exciton optical nanoscopy using a single quantum dot

Mapping light fields and local density of optical states (LDOS) around nanostructured materials is instrumental for advancing both fundamentals and practical applications in nano-optics, nanomaterial science, and quantum technologies. In particular, LDOS governs key processes such as spontaneous emission, light scattering, van der Waals interactions, and nanoscale heat transfer, yet it remains inaccessible with conventional optical microscopy techniques. Here, we develop scanning-exciton optical nanoscopy to simultaneously map the light intensity and LDOS near plasmonic and dielectric photonic structures with a few-nanometer resolution. By grafting a rationally designed core–shell-shell quantum dot to a silica nanotip, we obtain an ultra-photostable and sensitive scanning quantum probe carrying excitons that are generated and decay at the rates proportional to the local light intensity and LDOS, respectively. We first demonstrate the correlative dual-parameter sensing capability using a model plasmonic structure, and subsequently apply it to uncover previously unobservable nanoscale coupling physics occurring in a plasmonic trimer. Furthermore, we report the first optical mapping of LDOS over a photonic-crystal nanocavity, visualizing the resonant nanocavity mode and its position-dependent coupling with a quantum emitter, essential for quantum photonic applications. Our work demonstrates a transformative technique towards bridging the gap between surface morphology and far-field optical response, and constitutes a platform for exploring light-matter interactions at deep-nanoscale and single-quanta level.