  We present a theoretical study of the radiative and nonradiative decay rates
of an optical emitter in close proximity to a prolate-shaped metal
nanoparticle. We use the model developed by Gersten and Nitzan, that we correct
for radiative reaction and dynamic depolarization and extend for prolate
particle shapes. We show that prolate-shaped metal nanoparticles can lead to
much higher quantum efficiency enhancements than corresponding spherical
nanoparticles. For properly engineered emitter-nanoparticle geometries, quantum
efficiency enhancements from an initial value of 1% (in absence of the
nanoparticle) to 70% are feasible. We describe the anisotropy-induced
enhancement effects in terms of large field enhancements near the metal tips
that cause strong coupling to the (radiative) dipolar modes. For increasing
particle anisotropy, a strong spectral separation between radiative dipolar and
dark higher-order modes occurs, which leads to higher radiative efficiencies
for anisotropic particles. In addition, we demonstrate that for large (> 100
nm) nanoparticles, the influence of Ohmic losses on plasmon-enhanced
luminescence is substantially reduced, which implies that, if prolate shaped,
even lossy metals such as Al and Cu are suitable materials for optical
nano-antennas.
