FDTD analysis of metal-insulator-metal waveguide resonant cavity with metallic nano-cylinders

Surface plasmon polaritons (SPPs), which carry the electromagnetic energy along the metal-dielectric interface, have received much attention in the field of the photonic devices because they have potential to realize nanoscale photonic devices with the dimension smaller than the diffraction limit. There are mainly two types of configurations that are composed of metal-insulator-metal (MIM) and insulator-metal-insulator (IMI). Among of these, MIM configurations have been attracting much interest because they can realize strong field localization and their fabrication process is simpler and easier compared with the IMI ones. Many types of MIM plasmonic devices have been proposed so far, such as ring resonators, Bragg gratings, splitters and demultiplexers, filters utilizing cascaded transverse cavities. Analytical modeling of resonant cavities for MIM waveguide junctions has also been investigated. In this study, MIM waveguide resonant cavities that include metallic nano-cylinders in them are investigated through FDTD analysis. In the FDTD method, special handling of the metallic material is required because the permittivity is dispersive and has a negative value in the optical frequency. To overcome this difficulty, the motion equations are introduced into the FDTD method in order to evaluate the conducting current. The MIM waveguide consists of silver and air. The fundamental mode of the MIM waveguide is excited at the input port and the frequency characteristics of the power transmittance through the output port are investigated. It is shown that the drastic change of the power transmittance characteristics occur by slightly changing the location of the nano-cylinders inside the cavity. The field pattern inside the cavity is analyzed and the association with the power transmittance characteristics is demonstrated. It is shown that a wide variety of filtering characteristics can be obtained by arranging the nano-cylinders properly.