1. An apparatus, comprising: a mode converting structure with a volumetric distribution of dielectric constants configured to be positioned relative to an electromagnetic radiation (EMR) device, to modify a field pattern of the EMR device for a finite frequency range from a first mode to a second mode, wherein the mode converting structure is divided into a plurality of sub-wavelength voxels, wherein each voxel has a maximum dimension that is less than half of a wavelength of a frequency within the finite frequency range, and wherein each voxel is assigned one of a plurality of dielectric constants to approximate the distribution of dielectric constants of the mode converting structure.