Abstract: A device for efficiently coupling electromagnetic radiation in the far-infrared and submillimeter microwave regions of the electromagnetic spectrum to surface electromagnetic waves comprising a base metal substrate formed by producing periodic alternating areas thereof in which the dielectric constants differ so as to provide periodic field modulation to, in turn, enhance coupling between the surface plasmons and free photons at the proper coupling angles. The different dielectric constant areas may be formed by selective overplating or evaporating or may alternately be formed by the selective processing of a uniformly deposited material such as by annealing of an amorphous material in strips. In an alternate embodiment of the invention the periodic modulation of the dielectric constant is of the overcoat material rather than the base metal substrate.