FRET is highly sensitive to the mutual orientation of donor and acceptor dipoles, and can be strongly prohibited for perpendicularly oriented dipoles. However, all previous studies combining FRET with nanophotonics largely overlooked this orientation effect. In a recent Nano Letters article "Plasmonic Nanoantennas Enable Forbidden Förster Dipole–Dipole Energy Transfer and Enhance the FRET Efficiency", we show for the first time how to exploit the orientation dependence in FRET using plasmonic nanoantennas.

Significance:

- Nanoantennas can allow FRET even for perpendicularly oriented donor and acceptor pairs. This provides a new strategy using nanophotonics to reveal FRET interactions that would otherwise be impossible to probe with diffraction-limited microscopy.

- While all previous work were dominated by the plasmonic losses which compete with FRET and decrease the FRET efficiency, our approach exploiting the dipolar orientation effect overcomes the losses. We report the first gain in FRET efficiency using nanophotonics, which even reaches up to 50%.

- Numerical simulations elucidate this new FRET enhancement and reveal the crucial role of the simultaneous presence of electric field components along all three directions of space inside the nanogap. This bridges the gap between FRET orientation effects and near-field optics.

Plasmonic optical antennas enhance light-matter interactions at the nanoscale, yet this phenomenon is currently limited by the ohmic losses in the metal. Silicon-based nanophotonics is an appealing alternative approach to implement cost-effective CMOS-compatible molecular sensors. So far, the fluorescence experiments with silicon-based antennas did not reach the single molecule level to demonstrate clearly the phenomenon and explore its physical origins. We bridge this gap in a recent Nano Letters publication “All-Dielectric Silicon Nanogap Antennas to Enhance the Fluorescence of Single Molecules”.

Significance:

- This report provides the first experimental evidence that silicon nanoantennas achieve single molecule fluorescence enhancements above 200-fold together with a detection volume of 140 e-21 L that allows the detection of individual molecules at micromolar concentration using dielectric materials only.

- The fluorescence enhancement results from a combination of excitation intensity and radiative rate enhancement within the nanogap region. These effects are quantified in excellent agreement with numerical simulations.

- These results open new routes to implement high sensitivity molecular (bio)sensors with on-chip photonic devices that are CMOS compatible.