An apparatus including a three dimensional sub-wavelength structure for electromagnetic applications including a surface layer formed of a metal or semiconductor film, one or more additional layers stacked to the surface layer, one or more sub-wavelength apertures in the surface layer, and one or more cavities nearby the one or more sub-wavelength apertures. The first slot extends along the shank to the distal end thereof. The base and the seat are joined by a cylindrical filter element made of a magnetically susceptible mesh screen material.