Simulating electromagnetic response in coupled metallic nanoparticles for nanoscale optical microscopy and spectroscopy: nanorod-end effects

Collective oscillations of valence electrons in metallic materials determine their optical response. The energy and strength of these surface oscillations are a function of the shape, size and coupling of the nanoparticles. With the use of a boundary element method (BEM), we solve Maxwell's equations to calculate light scattering and surface modes in nanorods that are commonly used as hosts and/or samples in different field-enhanced scanning-probe microscopies and spectroscopies. We calculate the near-field and far-field response of nanorods and show that different geometrical terminations of the rods give different optical response in the far field for short rod lengths. For longer lengths, the response of rods with different terminations becomes more similar. The near field features of the ends become most evident close to the rod structural features that define the end capping. We identify four regimes for the separation between nanorod pairs that provide different coupling between nanorods. We also show that the size dependence of the nanorod response is characterized by a rod radius that gives a minimum wavelength for the dipolar response. For thicker and thinner rods, the response redshifts.