We describe an ultra-small structure that produces visible light of varying frequency, from a single metallic layer. In one example, a row of metallic posts are etched or plated on a substrate according to a particular geometry. When a charged particle beam passed close by the row of posts, the posts and cavities between them cooperate to resonate and produce radiation in the visible spectrum (or even higher). A plurality of such rows of different geometries can be etched or plated from a single metal layer such that the charged particle beam will yield different visible light frequencies (i.e., different colors) using different ones of the rows.