Flat-Top Reflection Characteristics in Metal-Dielectric-Metal Plasmonic Waveguide Structure Side Coupled with Cascaded Double Cavities

Based on a metal-dielectric-metal (MDM) plasmonic waveguide side coupled with a single cavity, we rebuild such resonator system by cascading double side-coupled cavities to obtain flat-top reflection response over a frequency bandwidth. The increased coherent scattering path provides an additional freedom to engineer the complex interference between the cavity modes and the waveguide mode. By decomposing the compound cavity modes into two decoupled resonances, we analyze the conditions to realize flat-top reflection response. The physics behind the flat-top reflection characteristics is found to be originated from the interference interaction between the two cavities through examining the cavity excitations and the reflected power response. Temporal coupled-mode theory and finite difference time domain method are utilized as theoretical and numerical tools which convince each other.