Unraveling excitation energy transfer mechanisms in plasmonic nanoantennas

Plasmonic nanoantennas (PNA) have fascinated researchers over the last couple decades, prompting comprehensive theoretical studies on plasmon-mediated excitation energy transfer (EET) processes. While, Forsters resonance energy transfer (FRET) based methods fail for large multi-donor/acceptor assemblies in complex configurations, ab-initio quantum-mechanical methods that go beyond the point-dipole and spectral overlap approximations, are computationally expensive. 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 PNA systems without recourse to the above approximations. The computational efficiency of DFTB is due to integral approximations arising from the tight-binding approach. We discuss the results obtained by the RT-TDDFTB calculations for a large PNA and reveal a complex interplay of interactions that govern the EET mechanism beyond the single donor/acceptor interactions. We attribute these effects to the exceedingly long-range electrodynamic couplings in plasmonic NPs and corroborate our findings via an analytical system. Most importantly, we provide an intuitive approach to probe in microscopic detail the real-time electron dynamics in large PNAs