## 2 Experimental Setup  
A schematic and a photograph of the designed terahertz time-domain ellipsometer are shown in Figure 1. The ellipsometer uses a TeraFlash pro system from TOPTICA Photonics (TOPTICA Photonics AG, Munich, Germany). The system employs a new generation of fiber-coupled photoconductive antennas consisting of InGaAs:Rh [35] emitters in combination with photoconductive membrane receivers made of a 1.2 \(\upmu\)m thin-film of InGaAs:Fe on a silicon substrate [36]. While the new emitter allows for THz output powers of \(>\)100 \(\upmu\)W, the photoconductive membrane receiver increases the dynamic range of the system by more than 10 dB compared to the current state-of-the-art, especially for frequencies above 2 THz. With these components, the THz ellipsometer enables measurements with an unprecedented dynamic range from 0.1 THz to more than 5 THz. The THz radiation emitted from the photoconductive antenna is collimated using a protected aluminum coated off-axis parabolic mirror with an effective focal length of 50.8 mm. Although the THz field emitted by the photoconductive antenna is already polarized linearly, the collimated beam is filtered once again with a tungsten wire grid polarizer (WGP) with high extinction ratio. The diameter of the wires measures 5 \(\upmu\)m, the wire spacing is 12.5 \(\upmu\)m and the aperture of the WGP is 34 mm. The extinction ratio of the WGP is of the order of \(10^{-4}\) for 0.5 THz and increases to \(10^{-2}\) for 5 THz. This procedure guarantees a high degree of polarization. For the ellipsometric measurements, the WGP is rotated by 45\({}^{\circ}\) relative to the plane of incidence using a motorized rotary stage. The THz antenna is rotated around its optical axis to the identical azimuthal angle to obtain the maximum intensity. Four apertures are used to minimize stray light and to control the beam diameter. This results in a spot size of about 50 \(\times\) 16 mm\({}^{2}\) on the sample (beam diameter about 16 mm). The maximum sample size is roughly 250 \(\times\) 140 mm\({}^{2}\). The samples are measured at 70\({}^{\circ}\) angle of incidence. This is a compromise between the different samples, as the angle of incidence with highest sensitivity is near the Brewster angle and therefore it is material dependent. The reflected THz radiation from the sample passes a second WGP. This analyzer is also mounted on a motorized rotary stage, which can be set to 0\({}^{\circ}\) or 90\({}^{\circ}\) with respect to the plane of incidence. An off-axis parabolic mirror focusses the beam on the fiber-coupled detector, whose preferred polarization axis is rotated by 45\({}^{\circ}\) with respect to the plane of incidence. This enables nearly equal detection sensitivity for the two orthogonal analyzer settings of 0\({}^{\circ}\) and 90\({}^{\circ}\). For precise measurements, the sample must be accurately aligned with the optical beam path. Therefore, an auto collimating telescope (ACT) in combination with a microscope objective was used for correctly aligning height and tilt of the samples. Badly aligned samples will impact the measurement accuracy. A wrong height and/or a wrong tilt of the sample will reduce the signal quality and decrease the signal-to-noise ratio. In addition, a wrong tilt leads to errors in the angle of incidence and in the azimuthal angles of the polarizers relative to the plane of incidence. Similar effects could occur for curved samples. In contrast to ellipsometric measurements in the visible or near infrared spectral range, THz TDS measurements allow the determination of the amplitude and phase of the detected radiation. Therefore, only two measurements at analyzer settings of 0\({}^{\circ}\) and 90\({}^{\circ}\) relative to the plane of incidence are needed to calculate the basic ellipsometric parameters  
Figure 1: Experimental setup of the ellipsometer: (**a**) Schematic diagram.