Numerical Model Studies of Energy Transfer Processes in Coupled Exciton-plasmon Systems

In this work we are investigating the optical response in 2D systems made of single and coupled nanoparticles in the presence of molecular assemblies. Our goal is to gain a further understanding of the way molecular relaxation and dephasing processes affect scattered field lineshapes, light propagation and heat development within metal-molecules composites, in both the linear and non-linear response regimes.

The study is carried out using the basic FDTD algorithm, together with an advanced numerical model for the solution of the coupled Maxwell-Bloch equations. This numerical solver allows us to account for the pure dephasing process which plays an important role in the interaction between light and matter.

We use a home-build code in which the FDTD grid is decomposed into several slices and the Maxwell equations in each slice are computed by a different processor. This parallel evaluation of the FDTD algorithm allows us to minimize time and computer memory costs, which is highly important in 3D FDTD calculations, but is also very efficient for our 2D simulations.

We display the effect of relaxation and pure dephasing on scattering and absorption lineshapes, for various coupled exciton-plasmon systems, as studied using this Maxwell-Louville numerical solver. We also show that in the linear regime, modeling the molecular parts by the Lorentz susceptibility function yields similar results.

In addition, we present results for more primitive systems, obtained using a simpler model, in which molecules are represented as 2-level systems and plasmonic excitations are described as harmonic oscillators. Although this kind of modeling cannot describe complex realistic systems, it provides a qualitative description and an intuitive understanding on the effect of relaxation and dephasing on energy transfer processes within coupled exciton-plasmon systems.