Effects of the Substrate Refractive Index, the Exciting Light Propagation Direction, and the Relative Cube Orientation on the Plasmonic Coupling Behavior of Two Silver Nanocubes at Different Separations

In this paper, we presented a detailed theoretical investigation on the effects of changing the refractive index of the substrate and the interparticle separation distance on the electromagnetic field distribution around the face-to-face (FF) and the edge-to-edge (EE) oriented silver nanocube (Ag NC) dimers. We found that the contribution of the scattering to the absorption components is markedly decreased with increasing the substrate refractive index. This suggests that for surface-enhanced Raman spectroscopic (SERS) applications, the use of a substrate of very low refractive index is highly recommended. However, for photothermal applications, the use of substrate of high refractive index is recommended. Furthermore, we found that the wavelengths corresponding to the dipolar and multipolar modes are nearly unaffected by increasing the interparticle separation distances above 40 nm on the high refractive index substrate. However, on the low refractive index substrate, the dipolar peaks consistently red-shifted as the separation distance decreased. We also observed that on a glass substrate, after a separation distance above 8 nm, the value corresponding to the maximum field in the FF oriented Ag NC dimer gradually increased with increasing separation distance. On the contrary, the field gradually decreased on the higher refractive index substrate (AlGaSb) above 8 nm separation. This substrate showed a larger effect on the redistribution of the dipoles and subsequent decrease in the intensities of both hot spots and cold spots. Additionally, studies on the effects of substrate location with respect to the propagation direction of the exciting light revealed that the high refractive index substrate is likely to absorb the incident light to a larger extent, thus, greatly diminishes the plasmonic enhancement of the exciting light if it propagates through the substrate first. This study is pointing toward the fact that the refractive index of the substrate has a strong effect on the intensity ratio of the multipole to the dipole modes. Substrate having high refractive index seems to relax the selection rules of light interaction with the different plasmonic multipolar characters.