In a plasmonic structure, a first conductor layer, a dielectric layer, and a second conductor layer are stacked in this order. The second conductor layer includes a plurality of conductor patterns that is two-dimensionally and periodically arranged, each of the plurality of conductor patterns having a circular shape or a regular polygonal shape. A diameter D of a circle circumscribed on each of the plurality of conductor patterns satisfies 200 nm≤D≤800 nm. A thickness g of the dielectric layer and the diameter D satisfy 0.3≤g/D≤0.6.