  We study a polarizer-analyzer mounting for the terahertz regime with
perfectly conducting metallic polarizers made of a periodic subwavelength
pattern. We analytically investigate the influence on the transmission response
of the multiple reflections which occur between polarizer and analyzer with a
renewed Jones formalism. We demonstrate that this interaction leads to a
modified transmission response: the extended Malus' Law. In addition, we show
that the transmission response can be controlled by the distance between
polarizer and analyzer. For particular set-ups, the mounting exhibits extremely
sensitive transmission responses. This interesting feature can be employed for
high precision sensing and characterization applications. We specifically
propose a general design for measuring electro-optical response of materials in
the terahertz domain allowing detection of refractive index variations as small
as $10^{-5}$.
