0.0 0.2 0.4 0.6 0.8 1.0 Antenna Efficiency 0 1000 2000 3000 4000 5000 6000 7000 Purcell Factor 20,10 20,6 10,10 10,6 a,d  (nm) 600 700 800 900 1000 1100 Vacuum Wavelength [nm] 0.0 0.2 0.4 0.6 0.8 1.0 Antenna Efficiency 0 1000 2000 3000 4000 5000 6000 7000 Purcell Factor (a) (b) a d d a FIG. 5: Effect of the tip termination on nanocone optical antennas in air. Purcell factor F (solid curves) and antenna efficiency η$_{a}$ (dashed curves) for a flat (a) and rounded (b) tip. The nanocone is 140 nm long, it has a fixed base diameter of 60 nm and a tip diameter of 10 or 20 nm. The emitter is at a distance of 6 or 10 nm from the end of the tip. The insets in (a) and (b) sketch the antenna cross section, indicating the emitter position and orientation, and the varied nanocone parameters. nm in place of 20 nm. Nevertheless, the difference between the two situations is not huge. Indeed, when the emitter is only 6 nm from the metal surface, the 20 nm rounded tip yields a stronger field enhancement in comparison to the flat one. These results stem essentially from the complex behavior of the electric field close to a metal tip $^{72}$. We investigated the performance of gold nanocones as optical antennas for enhancing molecular fluorescence. Compared to nanorods and nanospheroids, the great advantage of nanocones is that the spectral position of the LSPP can be tuned towards shorter wave- III. CONCLUSION 11