Patent ID: 7203001

Claim:
An article, comprising: a plurality of walls configured to form a grating, each of the plurality of walls comprising: a layer of a first material; a layer of a second material different from the first material; and a layer of a third material different from the first material, wherein the layer of the third material of each wall is positioned between the layer of the first material of that wall and the layer of the first material of an adjacent wall, the layer of the third material is spaced apart from the layer of the first material of the adjacent wall, and the article is birefringent at wavelengths λ 1 and λ 2 and retards incident radiation at λ 1 and λ 2 by amounts Γ 1 and Γ 2 , respectively, where |λ 1 −λ 2 | is about 15 nm or more and Γ 1 is substantially equal to Γ 2 .