Understanding the Coupling Mechanism of Gold Nanostructures by Finite-Difference Time-Domain Method

Gold nanoparticle assemblies show a strong plasmonic response due to the combined effects of the plasmon modes of the individual nanoparticles. Increasing the number of nanoparticles in a structured assemblies lead to significant shifts in the optical and physical properties. We use Finite-Difference Time-Domain (FDTD) simulations to analyze the electromagnetic response of structurally ordered gold nanorods in monomer and dimer configurations. The plasmonic coupling between nanorods in monomers or dimers configurations provides a unique technique for tuning the spectrum intensity, spatial distribution, and polarization of local electric fields within and around the nanostructures. Our study shows that an exponential coupling behavior when two gold nanorods are assembled in end-to-end and side-by-side dimer configurations with a small separation distance. The maximum electric field in the hot spots in between adjacent nanoparticles in end-to-end dimer configuration describes of a higher enhancement factor relative to the individual gold nanorod. Our FDTD simulation on dimer in end-to-end assembly for small separation distance up to ~ 40 nm can well explain the observed experimental growth dynamics of gold nanorods.