Quantum plasmonics: Symmetry-dependent plasmon-molecule coupling and quantized photoconductances

We investigate photoinduced electron transport through nanoparticle dimers connected by linear and ringlike molecules. The two model junctions exhibit similar absorption but distinct photoconductance. The linear-chain junction shows a constant conductance that is independent of photon energies. In contrast, the ring molecules show quantized conductance resonances at energies corresponding to the molecular excitations. These results can be understood from the symmetry and quantization of local molecular excitations, and have implications for plasmon-molecule interactions in general.

Figure 1

Absorption spectra of the ring (a) and chain (b) connected junctions of a nanoparticle dimer with a 2.54 nm diameter. The number of atoms in the molecules ranges from $N=1$ to 6. Three collective modes are visible: the CTP around 0.5 eV, ME around 1–3 eV, and SBDP around 4.6 eV.

Figure 2

Photocurrent (upper panels) and photoconductance (lower panels) spectra of the ring (left panels) and chain (right panels) junctions. The currents and conductances of rings increase in a steplike fashion with the increasing number of atoms. The insets in (a) and (b) show the distinct charge density excitations at the plasmon resonance peaks.

Figure 3

Absorption spectra (left panels) of isolated rings and chains as a function of the number of atoms. For rings, the A mode refers to polarization in the azimuthal plane of the rings and the P mode is for polarization perpendicular to the atomic plane. For the chains, the L mode is polarized along the chain while the T mode is polarized perpendicularly. The right panel shows the excitation energies of these modes. Only the two modes shown with the solid line can couple to the longitudinal dimer plasmons.

Figure 4

Local density of states of the ring and chain molecules for different $N$. The cyan background in the upper panel marks the final states involved in the P mode. In the lower panel, the cyan background is the corresponding region of the L mode for the chains. The insets show the wave functions of characteristic electron-hole pair states involved in these modes.