Insight on the Coupling of Plasmonic Nanoparticles from Near-Field Spectra Determined via Discrete Dipole Approximations.

Coupling between plasmonic nanoparticles (NPs) in nanoparticle assemblies has been investigated extensively via far-field properties, such as absorption and scattering, but very rarely via near-field properties, and a quantitative investigation of near-field properties should provide great insight into the nature of the coupling. We report a numerical procedure to obtain reliable near-field spectra ( ) around spherical gold nanoparticles (Au NPs) using Discrete Dipole Approximation (DDA). The reliability of the method was tested by comparing from DDA calculations with exact results from the Mie theory. We then applied the method to examine Au NPs assembled in dimer, trimer, and up to pentamer in a linear arrangement. For the well-studied dimer system, we show that the enhancement, due to coupling in longitudinal mode, is much greater than the enhancement in . There is a linear correlation between the and peak positions, with the peak redshifted from the peak by an average of approximately 12 nm. In the case of the multimers, spectra from individual spheres were not always identical and become dependent on the sphere location. In the longitudinal model, the center sphere has the strongest spectra. For the transverse mode, we differentiate two different scenario, transverse-Y where both electric field () and light propagation vector () are perpendicular the chain axis, and transverse-X where is parallel to the chain axis. In transverse-Y mode, coupling leads to reduced spectra and the center sphere has the lowest intensity. In transverse-X mode, there is retardation effect from the front sphere to the back sphere. The from the front sphere is stronger than from the back sphere. In addition, due to the phase lag in -direction, the in transverse-X can differ quite significantly from transverse-Y for large particles. All these results could be understood when one considers how electric field from induced dipoles on neighboring NPs add on or subtract from the incident E-field. These results provide new insight into the coupling properties of Au NPs.