Realization of a spherical boundary by a layer of wave-guiding medium
  In this paper the concept of wave-guiding medium, previously introduced for planar structures, is defined for the spherically symmetric case. It is shown that a quarter-wavelength layer of such a medium serves as a transformer of boundary conditions between two spherical interfaces. As an application, the D'B'-boundary condition, requiring vanishing of normal derivatives of the normal components of D and B field vectors, is realized by transforming the DB-boundary conditions. To test the theory, scattering from a spherical DB object covered by a layer of wave-guiding material is compared to the corresponding scattering from an ideal D'B' sphere, for varying medium parameters of the layer. 
I.V. Lindell, J. Markkanen, A. Sihvola and P. Yl\"a-Oijala
physics.class-ph