Hidden symmetry protection for surface plasmon polaritons

Abstract

In this study, we reveal the general mechanism of hidden symmetry protection for surface plasmon polaritons (SPPs). SPPs are protected at zero frequency when the dielectric-metal interface exhibits an exceptional symmetry. This symmetry is characterized by a combined symmetry operation of mirror reflection and internal transformation regarding the electric and displacement fields. Coalesced monopole- and dipolelike interface fields were identified as the full degrees of freedom of the zero modes. Because symmetry breaking produces conventional SPPs continuously from zero modes, the zero modes are considered the explicit origins of SPPs. Even for nonuniform permittivity distributions, the symmetry protection generally functions. Time-domain simulations demonstrate the excitation of analogous zero modes at a temporal boundary.