Wireless power transfer guides

It is experimentally studied that the power transfer efficiency is improved by inserting a proper structure made of dielectrics, magnetic materials or metals between two spiral coils constituting a coupled-resonator WPT (Wireless power transfer) system. The structures can be hollow/solid circular cylinder, plate or rectangular parallelepiped. The power transfer is based on the coupling of evanescent fields, not propagating fields. The principle of power guide is temporarily considered to be the refraction of the electric and magnetic fields at the boundary of 2 different materials. But it is still not clear.