Quantum plasmonic and electromagnetic coupling in plasmon rulers: new opportunities for imaging and sensing at the nanoscale

Gold and silver nanoparticles exhibit unique optical properties in the visible range of the electromagnetic spectrum where the incident light excites coherent collective oscillations (plasmons) of conduction band electrons. The distance dependent near-field coupling between the nanoparticles leads to spectral shifts in the far-field, which makes plasmonic molecules unique distance sensors on the nanoscale. In general, two distinct coupling regimes can be differentiated in the near-field: the classical electromagnetic coupling regime and the quantum plasmonic coupling regime at very short interparticle separations. The plasmon driven charge transfer between nanoparticles in the quantum plasmonic regime is currently of high interest for developing new non-linear spectroscopies and sensors, but the role of molecules in the gap between the nanoparticles remains insufficiently understood. We investigated the impact of DNA on the transition from the classical coupling regime to the quantum plasmonic coupling regime. We found that for separations beyond approx. 2.8 nm, classical electromagnetic coupling dominates the spectral response of coupled nanoparticles. At shorter separations, the recorded spectra are significantly blue-shifted when compared to the classical prediction, indicative of quantum plasmonic effects. The presence of the DNA was crucial to sustain the spectral blue-shifts, indicating coherent charge transfer across the DNA as main cause for the effective depolarization of the Plasmon Ruler.