Analytical Modeling of Resonant Cavities for Plasmonic-Slot-Waveguide Junctions

Transmittance of a straight junction between two plasmonic slot waveguides can be enhanced by either coupling a perpendicular stub to the input waveguide or inserting an intermediate slot section between the waveguides. We derive analytic expressions for the transmission spectra in these two configuration schemes and analyze them to obtain simple formulas, relating the parameters of the stub and intermediate slot section that maximize the transmittance at a given frequency. A considerable advantage of our results over the recently reported approximations is in their broader applicability domain, which is the consequence of the correct accounting for the phase shift resulting from the reflection of the surface plasmon-polariton (SPP) at the end of the stub, and inclusion of ohmic losses. By the example of a 100-fs Gaussian pulse at the telecommunication wavelength of 1.55 m, we demonstrate that both transmission enhancement techniques can almost completely eliminate SPP reflection from the junction of 125- and 25-nm-gap silver-air-silver plasmonic waveguides, whereupon the preference can be given to either of the techniques, depending on the desired design features. Although, for concreteness, we studied a pair of aligned slot waveguides, the concepts of optics and transmission-line theory that we marry in our work are not specific for straight junctions and are well suited for analyzing more complex waveguide geometries. Our solutions to the problem of SPP transmission through the two types of junctions provide a simple tool for design optimization of power-efficient plasmonic interconnects for all-optical supercomputers.