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Plasmonic optical antennas enhance and control the emission of quantum sources in the far-field. Interestingly, the antenna concept can also be applied to enhance the electric field produced by a quantum emitter in the near-field and increase the rate of Förster resonance energy transfer (FRET) between two nearby donor and acceptor dipole emitters. However, plasmonic antennas also influence numerous other photophysical processes such as the donor excitation intensity and decay dynamics and the acceptor emission yield, which compete with the observation of FRET. Understanding the balance between FRET and these processes and monitoring FRET under intense resonant optical confinement in plasmonic nanoantennas have remained challenging open questions. Here, we use DNA-driven self-assembly to accurately produce 40 and 60 nm gold nanoparticle dimer antennas containing a single FRET pair located in the center of a 14 nm gap. The spontaneous donor decay rate constants are increased by 2 orders of magnitude, creating high local densities of optical states (LDOS) to explore the link between LDOS and FRET. The antennas induce a 5-fold increase of Förster energy transfer rate constants associated with reduced transfer efficiencies, in good agreement with numerical simulations. The strong antenna–emitter interaction leads to the surprising association of an enhanced acceptor emission with a weak transfer efficiency. Our measurements exemplify the competition between radiative and nonradiative processes in complex nanophotonic systems and highlight geometrical parameters and design rules to optimize nanoantennas for nonradiative energy harvesting.
Bidault et al. (2016) studied this question.