In certain aspects, the disclosure relates to articles that include a plurality of walls configured to form a grating. Each of the plurality of walls can include a layer of a first material and a layer of a second material different from the first material. The articles can retard incident radiation at wavelengths .lamda..sub.1 and .lamda..sub.2 by amounts .GAMMA..sub.1 and .GAMMA..sub.2, respectively, where |.lamda..sub.1-.lamda..sub.2| is about 15 nm or more and .GAMMA..sub.1 is substantially equal to .GAMMA..sub.2.