  Optical response of silicon (SiV) and nitrogen (NV) vacancy diamond color
centers coupled to silver and gold nanorod dimers was numerically inspected.
Optimization of the coupled emitter - nanorod dimer configurations was
performed to attain the highest possible fluorescence enhancement by enhancing
the excitation and emission of color centers simultaneously through plasmonic
antenna resonances. To minimize losses conditional optimization was realized by
setting a criterion regarding the minimum quantum efficiency of the coupled
system (cQE). Restricted symmetric and allowed asymmetric antenna designs were
also inspected to prove the potential advantages of asymmetric configurations
tuneability. Among all inspected systems the highest 2.59*10^8 fluorescence
enhancement with 46.08% cQE was achieved in case of NV color center coupled to
asymmetric silver nanoantenna dimer. This is 3.17-times larger than the
enhancement in corresponding symmetric configuration, which has larger 68.52%
cQE. In case of SiV color center the highest 1.04*10^8 fluorescence enhancement
with 37.83% cQE was achieved via asymmetric silver nanoantenna dimer. This is
1.06-times larger than the enhancement in the corresponding symmetric
configuration, which has larger 57.46% cQE. The highest fluorescence
enhancement achieved by gold nanorods is 4.75*10^4 with 21.8% cQE, which was
shown in case of SiV color center coupled to asymmetric dimer. The attained
enhancement is 8.48- (92.42-) times larger than the fluorescence enhancement
achievable via symmetric (asymmetric) gold nanorod dimer coupled to SiV (NV)
color center.
