  Configuration of three different concave silver core-shell nanoresonators was
numerically optimized to enhance the excitation and emission of embedded
silicon vacancy (SiV) diamond color centers simultaneously. According to the
tradeoff between the radiative rate enhancement and quantum efficiency (QE)
conditional optimization was performed to ensure ~2-3-4 and 5-fold apparent cQE
enhancement of SiV color centers with ~10% intrinsic QE. The enhancement
spectra, as well as the near-field and charge distribution were inspected to
uncover the physics underlying behind the optical responses. The conditionally
optimized coupled systems were qualified by the product of the radiative rate
enhancements at the excitation and emission, which is nominated as Px factor.
The optimized spherical core-shell nanoresonator containing a centralized
emitter is capable of enhancing considerably the emission via bonding dipolar
resonance. The Px factor is 529-fold with 49.7% cQE at the emission.
Decentralization of the emitter leads to appearance of higher order multipolar
modes, which is not advantageous caused by their nonradiative nature.
Transversal and longitudinal dipolar resonances of the optimized ellipsoidal
core-shell resonator were tuned to the excitation and emission, respectively.
The simultaneous enhancements result in 6.2x10^5 Px factor with 50.6% cQE at
the emission. Rod-shaped concave core-shell nanoresonators exploit similarly
transversal and longitudinal dipolar resonances, moreover they enhance the
fluorescence more significantly due to their antenna-like geometry. Px factor
of 8.34x10^5 enhancement is achievable while the cQE is 50.3% at the emission.
The enhancement can result in 2.03x10^6-fold Px factor, when the criterion
regarding the minimum QE is set to 20%.
