A multilayer dielectric coating would introduce an opposing phase anisotropy.

A method of compensation for the polarization-dependent phase anisotropy of a metal reflector has been proposed. The essence of the method is to coat the reflector with multiple thin alternating layers of two dielectrics that have different indices of refraction, so as to introduce an opposing polarization- dependent phase anisotropy.

The anisotropy in question is a phenomenon that occurs in reflection of light at other than normal incidence: For a given plane wave having components polarized parallel (p) and perpendicular (s) to the plane of incidence, the phase of s-polarized reflected light differs from the phase p-polarized light by an amount that depends on the angle of incidence and the complex index of refraction of the metal. The magnitude of the phase difference is zero at zero angle of incidence (normal incidence) and increases with the angle of incidence.

Figure 1. The Reflection-Induced Phase Difference between s- and p-polarized light was calculated using the complex index of refraction (1.44 + i5.23) of aluminum.

In the proposed method, one would coat a metal reflector with alternating dielectric layers having indices of refraction n₁ and n₂, and thicknesses d₁ and d₂, respectively. To obtain form birefringence, the thickness of each spatial period (d = d₁+d₂) must be much less than the shortest wavelength of light for which compensation is sought. For special case d₁ = d₂ = d/2 shown at the top of Figure 2, the resulting ordinary and extraordinary indices of refraction (n_o and n_e, respectively) would be given by

and
.