Patent ID: 6947215

Claim:
An optical element comprising: a substrate; and a plurality of periodic structures laminated to each other and provided on the substrate, the periodic structures each having a period smaller than a predetermined wavelength of usable light; wherein, when the plurality of periodic structures comprises k periodic structures (k is an integer of one or more) each having a period in a first direction and l periodic structures (l is an integer of zero or more) each having a period in a second direction perpendicular to the first direction, the following equation is satisfied for two designed wavelengths λ 1 and λ 2 in a visible light region: ( ∑ i = 1 k ⁢ ⁢ Δ ⁢ ⁢ n i ⁢ d i - ∑ j = 0 I ⁢ ⁢ Δ ⁢ ⁢ n j ⁢ d j ) / λ I = ( ∑ i = 1 k ⁢ ⁢ Δ ⁢ ⁢ n i ′ ⁢ d i - ∑ j = 0 I ⁢ ⁢ Δ ⁢ ⁢ n j ′ ⁢ d j ) / λ 2 where Δn i indicates the difference of the refractive index of an ith periodic structure, having the period in the first direction, for TE polarized light from that for TM polarized light with respect to designed wavelength λ 1 ; d i indicates the thickness of the ith periodic structure having the period in the first direction; Δn j indicates the difference of the refractive index of a jth periodic structure, having the period in the second direction, for TE polarized light from that for TM polarized light with respect to designed wavelength λ 1 ; d j indicates the thickness of the jth periodic structure having the period in the second direction; Δn′ i indicates the difference of the refractive index of the ith periodic structure, having the period in the first direction, for TE polarized light from that for TM polarized light with respect to designed wavelength λ 2 ; and Δn′ j indicates the difference of the refractive index of the jth periodic structure, having the period in the second direction, for TE polarized light from that for TM polarized light with respect to designed wavelength λ 2 , the TE polarized light being polarized light having a polarization direction perpendicular to the periodic direction of the periodic structure, the TM polarized light being polarized light having a polarization direction parallel to the periodic direction of the periodic structure.