Symmetry in the elementary scattering of surface plasmon polaritons and a generalized symmetry principle

In the elementary scattering of surface plasmon polaritons (SPPs) at individual subwavelength (sub-lambda) objects on metallic surfaces, the in-plane transmission and reflection of SPPs are shown to be two related scattering processes and to satisfy some novel symmetry relations, provided that the objects are mirror symmetric and are narrow enough (<0.1lambda approximately). To interpret these symmetry relations, a new generalized symmetry principle for the scattered field is put forward, which is much less limited than the classical one and is shown to have wide applications for other sub-lambda scattering problems.