Plasmon lifetime enhancement in a bright-dark mode coupled system

Metallic nanoparticles can localize the incident light to hot spots as plasmon oscillations, where the intensity can be enhanced by up to four orders of magnitude. Even though the lifetime of plasmons is typically short, it can be increased via interactions with quantum emitters, e.g., spaser nanolasers. 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. We apply an analytical model based on harmonic oscillators to demonstrate that a coupled system of bright and dark plasmon modes decays more slowly than the bright mode alone. Furthermore, exact solutions of the three-dimensional Maxwell equations, i.e., finite-difference time domain, demonstrate that the lifetime of the coupled system significantly increases at the hot spot, which is not predictable by far-field response. The decay of the overall energy of such a coupled system, which can be extracted from experimental absorption measurements, is substantially different from the decay of the hot spot field. This observation enlightens the plasmonic applications in which the hot spot intensity enables the detection of the optical responses.