Can nanophotonics control the Förster resonance energy transfer efficiency?

Summary form only given. Förster resonance energy transfer (FRET) is the dominant nonradiative energy transfer mechanism between a donor and acceptor fluorophore in nanometer proximity. FRET plays a pivotal role in the photosynthetic apparatus of plants and bacteria and many applications, ranging from photovoltaics and lighting, to probing molecular distances and interactions.It is an intriguing open question whether the FRET rate γFRET and the energy transfer efficiency ηFRET can also be controlled by the nanoscale optical environment, characterized by the local density of optical states (LDOS) [1]. Pioneering work suggested that the transfer rate depends linearly on the LDOS at the donor emission frequency [2], while later work suggested a dependence on the LDOS squared [3], or even a transfer rate independent of the LDOS [4]. We study the influence of the LDOS on Förster transfer, using precisely-defined, isolated, and efficient donor-acceptor pairs. The FRET pairs are dye molecules that covalently bound to the opposite ends of a 15 basepair long double-stranded with a precisely defined distance of 6.8 nm. Control over the LDOS is realized by positioning the FRET systems at well-defined distances (ranging from 60 nm to 270 nm) from a metallic mirror. The energy transfer rate γFRET and efficiency ηFRET are obtained by measuring the donor emission rate γDA in presence and the rate γD in absence of the acceptor using time-correlated single-photon counting based lifetime imaging. Our data unequivocally show that the FRET rate is independent of the LDOS at donor emission frequencies, consistent with quantum-optical theory. The FRET efficiency clearly changes with LDOS [5], since the LDOS alters the competition between the different decay processes. By controlling the radiative decay rate of the energy donor by the LDOS, the energy transfer efficiency can be enhanced or reduced. If a donor with unit qua- tum efficiency is placed in a 3D photonic bandgap, the energy transfer efficiency will approach 100 %, independent of the acceptor, and of the distances and orientations between the FRET partners.