Generation of radially polarized single photons with plasmonic bullseye antennas

Radially polarized optical beams are desirable in many applications because of their property to generate longitudinally polarized fields when being strongly focused. Single-photon sources are however normally limited to the generation of linearly polarized photons due to their dipolar nature. Here, we report on emission of radially polarized single photons from a nitrogen-vacancy (NV) center in a nanodiamond (ND) coupled to a bull's-eye plasmonic antenna. Design optimization and characterization of plasmonic bull's-eye antennas consisting of polymer circular nanoridges deterministically fabricated on a silica protected silver film around an NV-ND emitter are reported. We demonstrate with numerical simulations that the collection efficiency for photons emitted at the design wavelength can exceed 80% with optimized bull's-eye structures. Analysis of the emission angular distribution indicates that, even when limiting the detection to the main lobe by using an 0.2 NA objective, the detection efficiency can overreach 55%. A 3-fold enhancement in the total number of detected radially polarized photons is experimentally observed with an NV-ND single-photon emitter being coupled to a bull's-eye antenna. Generation of radially polarized single photons suggests new interesting possibilities for single-photon imaging and sensing.