(a) Schematic of a THz polarimetry setup. (b) Projections of the electric field $E→$ leading to the measured signal $S(ϕ)$⁠. When a linearly-polarized THz radiation, with an electric-field amplitude $E→0$⁠, is sent through the DSO crystal oriented at 45° with respect to the x-axis, the transmitted electric field $E→$ first projects along the polarizer direction (gray, double-sided arrow P) and then along the analyzer direction (gray, double-sided arrow A). $ϕ$ is the rotation angle of the polarizer with respect to the x-axis. The resulting electric field amplitude is given by $S(ϕ)=|E→(ϕ)|$⁠. (c) Simulated amplitude, $S(ϕ)$⁠, for the circularly-polarized (CP) light, linearly-polarized (LP) light along y-axis and elliptically polarized light (⁠$b/a=0.6$⁠).