Quasi index matching for minimum reflectance at a dielectric-conductor interface for obliquely incident p- and s-polarized light.

Conditions for reducing the reflectance of a dielectric-conductor interface for p- and s-polarized light to a minimum at any angle of incidence phi are determined. The refractive indices of a transparent immersion medium (liquid) that achieve minimum reflectance at normal incidence, phi=0, and at phi=45 degrees 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 phi=45 degrees and that the associated p-reflection phase shift is also minimum at that angle. For phi> or =65 degrees 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.