Simulation and characterization of the bulk refractive index sensitivity of coupled plasmonic nanostructures with the enhancement factor

The bulk refractive index (RI) sensitivity of coupled plasmonic nanostructures, namely gold and silver nanospheres and nanocubes in two-particle (dimer) arrangements, were simulated with the MNPBEM Matlab toolbox. The size of the nanoparticles (10-90 nm) and their separation distance were the running parameters. The enhancement factor (which characterizes the increased RI sensitivity of multi-particle arrangements compared to single particles) was introduced and used to evaluate the effect of particle coupling on the bulk RI sensitivity of the dimer arrangements. The enhancement factor is an exponential function of (D/D0), where D is the separation between the particles and D0 is the diameter of the spheres or the edge length of the cubes, and it was found that significant plasmonic coupling effects (e.g. EF > 1.5) starts below 0.5-0.3 D/D0 for the investigated dimers, depending on their size. It was also found for the four investigated dimer arrangements (gold/silver nanospheres/nanocubes) that the absolute peak shift and the enhancement factor values are inversely proportional: the dimers which have a larger absolute extinction peak shift (at the same particle separation) have smaller relative enhancement compared to single particles.