A design procedure is presented for a near-optimal, single-layer-coated prism beam splitter that serves as the key optical element of the division-of-amplitude photopolarimeter (DOAP). For given film and substrate refractive indices, the angle of incidence and film thickness are selected such that the ellipsometric differential phase shifts in reflection and transmission Delta(r) and Delta(t) differ by +/-pi/2, and the normalized determinant of the instrument matrix is maximized. The best results are obtained by using high-index films on low-index substrates. This is illustrated by examples of ZnS and Gal? films on silica prisms in the visible and Si, Ge, and PbTe films on Irtran 1 substrates in the infrared. A 16degrees Si-prism DOAP beam splitter at the 1.55-mum lightwave-communications wavelength is also presented. It uses a 163-nm SiO2 coating on the entrance face to satisfy the optimum delta condition at 73degrees incidence, and the determinant of the instrument matrix is 78.23% of its theoretical maximum. The exit face of the Si prism is antireflection. coated with a 208-nm Si3N4 film. (C) 2005 Optical Society of America.
