  Metallic nanoparticles can localize the incident light to hotspots as plasmon
oscillations, where the intensity can be increased up to four orders of
magnitude. Even though the lifetime of plasmons are typically short, it can be
increased via interactions with quantum emitters, e.g. spaser nano-lasers.
However, molecules can bleach in days. Here, we study the lifetime enhancement
of plasmon excitations due to the coupling with longer lifetime dark plasmon
modes. Exact solutions of the 3D Maxwell equations, i.e. FDTD, demonstrates
that the lifetime of the coupled system increases, as also predicted by a basic
oscillator model. We report an optimum bright-dark plasmon mode coupling, where
lifetime enhancement becomes maximum, and show that no precise positioning of
the nanostructures is required to obtain enhanced lifetime.
