Surface Waves on SHDB Boundary

SHDB boundary conditions were recently introduced as a generalization of soft-and-hard (SH) and DB boundary conditions. In this letter, it is shown that the SHDB boundary conditions are self-dual-invariant in any duality transformation. Also, it is shown that any problem involving an isotropic medium and SHDB boundaries can be reduced to two uncoupled problems by decomposing the fields in two wavefields (self-dual fields). As an example, it is shown that circularly polarized surface waves above a planar SHDB boundary are possible for certain boundary parameter values. Some properties of such waves are discussed.