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

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 (Q _NF) around spherical gold nanoparticles (Au NPs) using Discrete Dipole Approximation (DDA). The reliability of the method was tested by comparing Q _NF 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 Q _NF enhancement, due to coupling in longitudinal mode, is much greater than the enhancement in Q _ext. There is a linear correlation between the Q _NF and Q _ext peak positions, with the Q _NF peak redshifted from the Q _ext peak by an average of approximately 12 nm. In the case of the multimers, Q _NF 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 Q _NF spectra. For the transverse mode, we differentiate two different scenario, transverse-Y where both electric field (E) and light propagation vector (k) are perpendicular the chain axis, and transverse-X where k is parallel to the chain axis. In transverse-Y mode, coupling leads to reduced Q _NF spectra and the center sphere has the lowest Q _NF intensity. In transverse-X mode, there is retardation effect from the front sphere to the back sphere. The Q _NF from the front sphere is stronger than from the back sphere. In addition, due to the phase lag in k-direction, the Q _NF 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.