Plasmonic Nanoantennas Enable Forbidden Forster Dipole-Dipole Energy Transfer and Enhance the FRET Efficiency

Forster resonance energy transfer (FRET) plays a key role in biochemistry, organic photovoltaic$, and lighting sources. FRET is commonly used as a nanorulet for the short (nanometer) distance between donor and acceptor dyes, yet FRET is equally sensitive to the mutual dipole orientation. The orientation dependence complicates the FRET analysis in biological samples and may even lead to the absence of FRET for perpendicularly oriented, donor and acceptor dipoles. Her, we exploit;the strongly inhomogeneous -and localized` fields, in plasmonic nanoantennas to open new energy transfer routes, overcoming the limitations,from the mutual dipole orientation to ultimately enhance the, FRET efficiency, We demonstrate that the simultaneous presence, of perpendicular near field components in the nanoantelma sets favorable energy transfer routes that increase the FRET efficiency up to 50% for nearly perpendicular-donor and acceptor dipoles. This new facet of plasmonic nanoantennas enables dipole dipole energy transfer that would otherwise be forbidden in a homogeneous environment: As such) our approach further increases the applicability of single molecule FRET over diffraction-limited approaches, with the additional benefits of higher sensitivities and higher concentration ranges toward physiological levels.