Quarter-wave layers with 50% reflectance for obliquely incident unpolarized light

The conditions under which light interference in a transparent quarter-wave layer of refractive index n(1) on a transparent substrate of refractive index n(2) leads to 50% reflectance for incident unpolarized light at an angle phi are determined. Two distinct solution branches are obtained that correspond to light reflection above and below the polarizing angle, phi(p), of zero reflection for p polarization. The real p and s amplitude reflection coefficients have the same (negative) sign for the solution branch phi > phi(p) and have opposite signs for the solution branch phi < phi(p). Operation at phi < phi(p) is the basis of a 50%-50% beam splitter that divides an incident totally polarized light beam (with p and s components of equal intensity) into reflected and refracted beams of orthogonal polarizations [Opt. Lett. 31, 1525 (2006)] and requires a film refractive index n(1) >= (root 2 + 1)root n(2). A monochromatic design that uses a high-index TiO2 thin film on a low-index MgF2 substrate at 488 nm wavelength is presented as an example. (c) 2007 Optical Society of America.