  Coherent energy exchange between plasmons and excitons is a phenomenon that
arises in the strong coupling regime resulting in distinct hybrid states. The
DNA-origami technique provides an ideal framework to custom-tune
plasmon-exciton nanostructures. By employing this well controlled self-assembly
process, we realized hybrid states by precisely positioning metallic
nanoparticles in a defined spatial arrangement with fixed nanometer-sized
interparticle spacing. Varying the nanoparticle diameter between 30 nm and 60
nm while keeping their separation distance constant allowed us to precisely
adjust the plasmon resonance of the structure to accurately match the energy
frequency of a J-aggregate exciton. With this system we obtained strong
plasmon-exciton coupling and studied far-field scattering at the
single-structure level. The individual structures displayed normal mode
splitting up to 170 meV. The plasmon tunability and the strong field
confinement attained with nanodimers on DNA-origami renders an ideal tool to
bottom-up assembly plasmon-exciton systems operating at room temperature.
