Polar map of the normalized amplitude $S(ϕ)$ at several THz frequencies: (a) 0.51, 0.60, and 0.68 THz and (b) 0.43 and 0.77 THz for the (110)-cut DSO. The filled circles are experimental data, while the solid curves are theoretical fits. (c) The corresponding fitted ellipticity parameters are plotted as a function of frequency. The black curve represents the amplitude ratio between b and a, while the red curve shows the phase retardation $δ/π$⁠. The blue-dashed lines mark the frequency range where the phase retardation is within a tolerance of ±3% and the amplitude ratio is within a tolerance of ±10%. (d)–(f) Polar maps and the fitted ellipticity parameters for the (001)-cut DSO.