Finite-Difference Time-Domain (FDTD) design of gold nanoparticle chains with specific surface plasmon resonance

We employ Finite-Difference Time-Domain (FDTD) simulations to analyze the electromagnetic far- and near-field response of gold nanoparticles (NPs) organized in chain-like structures as function of the number of particles and inter-particle distance in structures. As a result an empirical formula to predict the position of collective localized surface plasmon resonance (LSPR) as function of number of particles in the chain is devised. On the other hand the experimental LSPR spectrum recorded from a colloidal solution exhibiting a certain degree of aggregation has been effectively reconstructed by linear combination of individual LSPR contribution as calculated for NP ensembles of different size (monomers, dimers, trimers, etc.). Notably, we find that the maximum of electric field intensity (E2) in between adjacent NPs increases from dimeric to trimeric and tetrameric ensembles, followed by a steady state decrease as the number of NPs per chain further increases. The central gap in a long chain of NPs accommodate the highest field enhancement (‘hot-spots’). Our findings are relevant for designing effective substrates for Surface-Enhanced Raman Scattering (SERS) and plasmonic waveguides.