0.0 0.2 0.4 0.6 0.8 1.0 Antenna Efficiency 0 300 600 900 1200 1500 1800 2100 Purcell Factor 600 700 800 900 1000 1100 Vacuum Wavelength [nm] 0.0 0.2 0.4 0.6 0.8 1.0 Antenna Efficiency 0 300 600 900 1200 1500 1800 2100 Purcell Factor b 40 60 80 b  (nm) b (a) (b) FIG. 4: Effect of a supporting substrate on nanocone optical antennas in air. Purcell factor F (solid curves) and antenna efficiency η$_{a}$ (dashed curves) without (a) and with (b) a glass substrate (refractive index 1.5). The nanocone is 140 nm long, it has a base diameter of 40, 60 or 80 nm and a fixed tip diameter of 20 nm. The insets in (a) and (b) sketch the antenna cross section, indicating the emitter position and orientation, and the nanocone parameter that is varied. base diameter. The last important aspect that we would like to address is the effect of the tip termi- nation. Previous studies on the field enhancement in triangular nanoparticles have shown that changing the tip termination can affect the results. 70,71 Here, we compare the case of a flat tip, shown in 5(a), with the case of a rounded tip having the same diameter, as shown in 5(b). We see that the resonance frequency and the antenna efficiency η$_{a}$ are almost the same. The only noticeable difference occurs for the Purcell factor F , which is larger for the case of a flat tip. This holds also if the flat and rounded ends have a diameter of 10 E. Effect of the tip termination 10