An optical device comprises a support having a reference plane, and a dielectric periodic structure including a plurality of periodic elements which are arranged, on the reference plane, in a two-dimensional lattice having a sub-wavelength spacing and are either projections projecting from the reference plane or recesses depressed from the reference plane. The optical device further includes a metal layer, which is positioned on a surface of the periodic structure including a region of the reference plane surrounding the individual periodic elements and the surfaces of the periodic elements and which has a shape that follows the surface profile of the periodic structure.