Modeling and measuring plasmonic excitations in hollow spherical gold nanoparticles.

We investigate molecular plasmonic excitations sustained in hollow spherical gold nanoparticles using time-dependent density functional theory (TD-DFT). Specifically, we consider Au60 spherical, hollow molecules as a toy model for single-shell plasmonic molecules. To quantify the plasmonic character of the excitations obtained from TD-DFT, the energy-based plasmonicity index is generalized to the framework of DFT, validated on simple systems such as the sodium Na20 chain and the silver Ag20 compound, and subsequently successfully applied to more complex molecules. We also compare the quantum mechanical TD-DFT simulations to those obtained from a classical Mie theory that relies on macroscopic electrodynamics to model the light-matter interaction. This comparison allows us to distinguish those features that can be explained classically from those that require a quantum-mechanical treatment. Finally, a double-shell system obtained by placing a C60 buckyball inside the hollow spherical gold particle is further considered. It is found that the double-shell, while increasing the overall plasmonic character of the excitations, leads to significantly lowered absorption cross sections.

Multiscale Modeling of Broadband Perfect Absorbers Based on Gold Metallic Molecules.

The modeling of functional photonic devices that rely on molecular materials continues to be a major contemporary challenge. It is a challenge because, in the Maxwell equations, which govern the light-matter interaction, material properties are primarily introduced on phenomenological grounds and not from first principles. To overcome such limitations, we outline a multiscale modeling approach that bridges multiple length scales. We can predict with our approach the optical response of a photonic device that exploits in its design molecular materials whose properties were determined using time-dependent density functional theory. The specifically considered device is a broadband perfect absorber that uses in part a thin film comprising gold molecules made from 144 atoms. Our methodology discloses various chemical and physical effects that define such a device's response. Our methodology is versatile, and a larger number of applications will profit from this development.