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 rc=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 rc,and the transfer of cavity mode energy to inter-particle plasmons when r is further increased.
