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

Authors: 
Ilawe, N. V., University of California, Riverside
Wong, B. M., University of California, Riverside
Oviedo, M. B., University of California, Riverside
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.