The structural symmetry required for long-range surface-plasmon-polariton modes to take place is examined and mapped to asymmetric plasmonic structures. This study leads to a design methodology that facilitates the realization through systematic design of long-range modes in any asymmetric hybrid plasmonic waveguide (AHPW). Examining the modal behavior of an AHPW reveals that field symmetry on either side of the metal is the only necessary condition for plasmonic structures to support long-range propagation. We report that this field symmetry condition can be satisfied irrespective of asymmetry in a waveguide structure, material, or even field profile. The structure is analyzed using the coupled mode theory, transfer matrix method, and finite-difference time-domain method. The AHPW supports high-loss antisymmetric and long-range symmetric supermodes. Dispersion of these supermodes with respect to waveguide dimensions display similar anticrossing characteristics to those obtained in two coupled harmonic oscillators, where the propagation losses display peaks and troughs in the vicinity of the anticrossing region. To place the work in perspective, an AHPW with a width of 200 nm was found to support a long-range supermode with a subwavelength mode area of and propagation loss of at the wavelength of 1550 nm, providing a radically improved attenuation confinement trade-off compared with other common types of plasmonic waveguides.
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