An electromagnetic device includes: a substrate having an elongated aperture having an overall length, L, and an overall width, W, as observed in a plan view of the device, where L>W; a dielectric medium having a dielectric material other than air disposed on the substrate substantially covering the aperture, the dielectric medium having a cross sectional boundary as viewed in the plan view of the device, the cross sectional boundary having at least one recessed portion that recesses inward toward the aperture from a tangent line that partially bounds the cross sectional boundary; wherein the length L is not perpendicular to the tangent line.