Conditions for reducing the reflectance of a dielectric-conductor interface for p- and s-polarized light to a minimum at any angle of incidence ϕ are determined. The refractive indices of a transparent immersion medium (liquid) that achieve minimum reflectance at normal incidence, ϕ=0, and at ϕ=45° are independent of polarization. These indices provide sufficient data to determine the real and imaginary parts of the complex refractive index of an absorbing substrate. Reflection at a dielectric-Au interface at 500 nm wavelength is considered as an example. It is shown that the lowest possible reflectance is attained for p-polarized light at ϕ=45° and that the associated p-reflection phase shift is also minimum at that angle. For ϕ≥65° the lowest reflectance of p-polarized light occurs when the ambient is vacuum or air. However, this lowest reflectance at the air-Au interface is not a true minimum in a mathematical sense.
R. M. A. Azzam and A. Alsamman, "Quasi index matching for minimum reflectance at a dielectric-conductor interface for obliquely incident p- and s-polarized light," Appl. Opt. 47, 3211-3215 (2008).