The present disclosure relates to a three-dimensional dielectric structure comprising at least one input and at least one output configured to transmit electromagnetic waves of at least one predetermined wavelength, a metamaterial between the at least one input and the at least one output comprising a substrate and objects with a predetermined dielectric characteristic different to the dielectric characteristic of the substrate, the objects being distributed in the substrate according to a spatially varying distribution function that depends on the wavelength. The disclosure further relates to a method of forming a three-dimensional dielectric structure.