Patent ID: 7492519

Claim:
An optical element comprising: a first layer of a first anisotropic material having a first surface, the first layer having a first optical index of refraction corresponding to a first internal axis and a second optical index of refraction corresponding to a second internal axis, the first and second indices being different; a second layer of a material different from the first anisotropic material and having a second surface in intimate contact with the first surface, the second layer having a third optical index of refraction in a region adjacent to the second surface that is a geometric mean of the first and second optical indices of refraction, the second layer having a non-zero index gradient from the region adjacent to the second surface, the second layer including a third surface separated from the second surface; and a third layer of a third material having a fourth surface in intimate contact with the third surface, wherein the third material is oriented relative to the second material an angle at which the second material principal axis and third material principal axis are related according to the equation: β 2 ⁢ cos 2 ⁢ ϕ 2 + 1 β 2 ⁢ sin 2 ⁢ ϕ 2 = β 3 ⁢ cos 2 ⁢ ϕ 3 + 1 β 3 ⁢ sin 2 ⁢ ϕ 3 where: β 2 is a ratio of dielectric constants of the second material relative to its principal material axis and another material axis; β 3 is a ratio of dielectric constants of the third material relative to its principal material axis and another material axis; and φ 2 is an orientation angle of the principal material axis of the second material and φ 3 is an orientation angle of the principal material axis of the third material in a region adjoining the second surface.