Theoretical analysis of square surface plasmon-polariton waveguides for long-range polarization-independent waveguiding

Bound electromagnetic modes supported by a square metal rod embedded in a lossless dielectric medium are determined and characterized. Based on the symmetry of the waveguide configuration, a schematic overview of the supported fundamental modes is given. These fundamental modes are related to coupling between four corner modes associated with individual metal corners of the square rod waveguide. Utilizing the finite element method a numerical analysis of the waveguide is performed at telecom wavelengths (similar to 1550 nm). The mode dispersion with the rod size, mode field orientations, profiles, and widths are presented. Unlike thin metal stripe waveguides, the square metal waveguide is shown to support long-range surface plasmon-polariton modes for both polarizations. The configuration studied is a gold core embedded in a polymer (benzocyclobutene). Surface plasmon-polariton modes of both polarizations are shown to be guided with a propagation length of approximately 3 mm for a metal core size of 200x200 nm(2). The possibility of long-range polarization-independent transmission of surface plasmon-polariton modes makes the square surface plasmon-polariton waveguide unique and a promising optical component, from which future plasmonic interconnects and circuits can benefit.