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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.

Access to scatterers that offer a specific multipolar response is of utmost importance to design metasurfaces and metamaterials—and, more general, to control light at the nanoscale. While isolated scatterers have been extensively considered, it is interesting to see more clearly whether higher order multipolar resonances can be induced by exploiting the interaction of multiple scatterers. Here, we study the appearance and deterministic control of higher order multipolar resonances in two coupled high permittivity dielectric scatterers by changing their spacing. To disentangle the effects from those of isolated scatterers, we concentrate on the multipolar expansion of only the field that emerges from the interaction. By expanding the contribution of a specific multipole into a series of Lorentzian oscillators, the resonances can be studied clearly and discussed in terms of resonance position, oscillator strength, and damping. The oscillator strength shows an oscillatory behavior depending on the spacing between the scatterers, and we can identify an optimal distance where the oscillator strength is maximal. With that, we disclose new approaches to tailor the optical response from photonic scatterers.

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.

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