We describe an ultra-small resonant structure that produces electromagnetic radiation (e.g., visible light) at selected frequencies. The resonant structure can be produced from any conducting material (e.g., metal such as silver or gold). In one example, a number of rows of posts are etched or plated on a substrate, with each row having a particular geometry associated with the posts and cavities between the posts. A charged particle beam is selectively directed close by one of the rows of posts, causing them to resonate and produce radiation (e.g., in the visible spectrum at a predominant frequency). Directing the charged particle beam at a different row yields radiation at a different predominant frequency.