The Sensitivity of the Distance Dependent Plasmonic Coupling between Two Nanocubes to their Orientation: Edge-to-Edge versus Face-to-Face

Using DDA calculations, the electromagnetic field distribution and polarization vector plots were calculated as functions of the separation distance between a pair of 42-nm Ag nanocube dimers oriented in either an edge-to-edge or face-to-face configuration. We found that, as the separation distance of the nanocube dimer is decreased, the orientation of the dimer dramatically affects its near field coupling behavior. The expected dipole-dipole exponential coupling behavior between the surfaces of the two nanocubes was found to fail at a separation distance of 6 nm for the face-to-face configuration and 14 nm for the edge-to-edge arrangement. The failure of the dipole-dipole coupling mechanism results from an increased contribution from the higher-order multipoles (e.g., quadrupole-dipole). Because this failure occurs at a larger separation distance for the edge-to-edge orientation, the relative ratio of the multipole to the dipole moment generated by the edge-to-edge dimer must be larger than the ratio for the face-to-face orientation. To form a multipole, two or more oscillating dipoles are required; thus, one would expect that an increase in the ratio of the multipole to the dipole moment would suggest a higher density of oscillating dipoles for the edge-to-edge dimer. This increased density is supported by the larger calculated maximum electromagnetic field for the edge-to-edge orientation (similar to 7000) compared to the calculated field for the face-to-face orientation (similar to 5000).