(562c) Unraveling Excitation Energy Transfer Mechanisms in Plasmonic Nanoantennas

Plasmonic systems composed of nanoparticle arrays have fascinated researchers over the last couple decades, prompting comprehensive studies on plasmon-mediated excitation energy transfer (EET) processes. A theoretical understanding of these processes is crucial not only for the development of nanoscale photonic circuitry, but also for the design of highly efficient energy-guiding nanoantennas. Although theoretical methods based on Forsters resonance energy transfer (FRET) have been successfully applied to single donor/acceptor systems, these methods in their standard form fail for large multi-donor/acceptor assemblies in complex configurations. While ab-initio quantum-mechanical methods like density functional theory (DFT) go beyond the simple point-dipole and spectral overlap approximations of FRET, they are computationally intensive and limited to a few hundred atoms. Here, we describe our use of the density functional tight binding (DFTB) approach and its real-time time-dependent counterpart, RT-TDDFTB, to probe in detail the EET dynamics of plasmonic nanoantenna systems without recourse to the point-dipole or spectral overlap approximations. The computational efficiency of DFTB is due to additional integral approximations arising from the tight-binding approach, resulting in a linear scaling computational cost. In particular, we discuss the results obtained by the RT-TDDFTB calculations for a large plasmonic nanoantenna composed of 220 sodium atoms. We reveal a complex interplay of many-body interactions that govern the EET mechanism in the plasmonic nanoantenna that go beyond the single donor/acceptor interactions considered in traditional theories. We attribute these effects in part to the exceedingly long-range electrodynamic couplings, which are a result of the coherent nature of oscillating electrons in plasmonic nanoparticles. We also corroborate our findings via an analytical two-level system that captures many of the intricacies of the full quantum dynamical method. Most importantly, our time-domain studies provide an intuitive approach to probe in microscopic detail the real-time electron dynamics in large plasmonic nanoantennas.