Optimization of nanocavity field enhancement using two-dimensional plasmonic photonic crystals

We have investigated the influence of Ag nanorod radius (r) on the resonant modes of a two-dimensional plasmonic photonic crystal (PPC) with dipole sources embedded into the central vacancy area, using finite-difference time-domain methods. Both the localized surface plasmon (LSP) mode of individual Ag nanorods and the resonant cavity mode of PPC are found to vary as a function of r. The resonant cavity mode is strongly enhanced as r is increased, while the LSP signal will eventually become no longer discernable in the Fourier spectrum of the time-evolved field. An optimized condition for the nanocavity field enhancement is found for a given PPC periodicity (e.g. d = 375 nm) with the critical nanorod radius r _c = d/3. At this point the resonant cavity mode has the strongest field enhancement, best field confinement and largest Q-factor. We attribute this to competition between the blocking of cavity confined light to radiate out when the cavity resonant frequency falls inside the opened photonic stopband as r reaches r _c, and the transfer of cavity mode energy to inter-particle plasmons when r is further increased.