lengths by increasing the cone angle without compromising the Purcell factor and reducing the antenna efficiency. Another important practical aspect in favor of nanocones is the in- creased robustness and stability for vertical orientation because the base is larger than for nanorods. 62–68 Moreover, we have shown that an optimal angle for the Purcell factor exists, which is di- rectly related to the field enhancement studied in previous works. 49–51 We have also studied the antenna efficiency, showing that quenching does not necessarily occur in correspondence with the LSPP. 57 While F increases and then decreases with increasing cone angle, η$_{a}$ ex- hibits a monotonic growths. Therefore, because the fluorescence signal depends on the local electric field and on the quantum efficiency, the maximum field and fluorescence enhance- ments do not occur for the same cone angle. Nevertheless, we found that this difference is small if the Purcell factor is much larger than 1, which is easy to achieve using realistic nanocone parameters. The strong fluorescence and field enhancement as well as the electric field localization at the nanocone tip holds great promise for high-resolution fluorescence, Raman, and other non- linear nanoscopies. 41–44 In fact, finite nanocones could be very efficiently butt-coupled to a dielectric nanofiber or interfaced with a weakly-focused radially-polarized beam for the real- ization of a high-throughput and large-bandwidth scanning near-field optical microscope $^{73,74}$. Considering the recent progress in the fabrication of single and double gold nanocone on substrates, 62–65 cantilevers, 67,68 and on the facet of optical fibers, 66 we anticipate a great deal of activity on this antenna system. The authors would like to thank X.-W. Chen, E. Di Fabrizio, F. De Angelis, H. M. Eghlidi, K.-G. Lee and S. G¨ otzinger for fruitful discussions. A. Mohammadi acknowledges support from the Persian Gulf University Research Council. This work was performed in the framework of the ETH Zurich initiative on Composite Doped Metamaterials (CDM). ∗ Niels Bohr Institute, 2100 Copenhagen, Denmark † Electronic address: mario.agio@phys.chem.ethz.ch 1 K. H. Drexhage, M. Fleck, , F. P. Sch¨ afer, and W. Sperling, Ber. Bunsenges. Phys. Chem. 20 , 1179 (1966). 12