Subnanometric control of the coherent coupling between a single molecule and a plasmonic nanocavity

The interaction between excitons and plasmons provides useful information about the coupling strength and the coherence of the hybrid system. A single molecule can be regarded as an ideal two-level system, and a plasmonic... [ view full abstract ]

The interaction between excitons and plasmons provides useful information about the coupling strength and the coherence of the hybrid system. A single molecule can be regarded as an ideal two-level system, and a plasmonic nanocavity in a tip-substrate configuration can sustain ultra-localized electric fields that serve as atomic-scale optical probes to study plasmon-exciton interactions. We have previously applied a plasmonic nanocavity formed by the tip-substrate configuration to enhance the Raman signal of a single molecule and retrieve the information of the dipole-dipole coupling regime at sub-molecular level. However, the intrinsic properties of the coherent coupling between a single molecule and the plasmon have not been studied yet, especially at sub-nanometer level. In this work, we use a scanning tunneling microscope (STM) tip/substrate nanocavity to generate a localized plasmonic field, and a zinc phthalocyanine(ZnPc) molecule as a quantum emitter to study the coherent coupling between the plasmon and a single molecule at the sub-nanometer scale (as depicted in the schematics of Fig. 1, left). If there is no molecule under the STM tip, an emission spectrum of the nanocavity plasmon is observed (blue spectrum in Fig.1, right). However, if the STM tip is placed in the proximity of the single molecule,a dramatic change in the fluorescence spectra is observed, evolving from a pure plasmonic peak to a Fano lineshape (red spectrum in Fig. 1, right), indicating a coherent coupling between the molecular excitation and the plasmonic resonance.The coupling strength can be tuned in a controlled manner by varying the lateral distance between the STM tip and the molecule, due to the spatial confinement of the local electric field. Furthermore, the detuning between the nanocavity plasmon resonance and the molecular transition can be carefully modified, with the strongest coupling strength obtained when both excitations are resonant. All these results can help to provide a better understanding of the exciton-plasmon coupling mechanism at the sub-nanometer level with implications in molecular sensing, optical modulators, and quantum information devices.