## 2 Conventional Approach to THz-TDS  
The conventional THz beamline, shown schematically in Figure 1, consists of the THz source, a beam-routing section composed of 90\({}^{\circ}\) off-axis parabolic mirrors, and the detection stage. The THz radiation is generated via rectification in an electro-optic medium such as GaP. In this experiment, a 1 mm thick antireflection-coated GaP crystal cut in the 110 plane was used for generation (UniversityWafer, Boston, MA, USA). The pump is injected into the GaP crystal and, after THz generation, is removed using filters [15; 23] or via a hole in the first parabolic mirror [22; 24]. This hole is commonly 3 mm in diameter as mirrors with these specifications are readily available from manufacturers; smaller diameter holes generally require custom fabrication. Due to the non-spherical geometry of these mirrors, it is difficult to manufacture parabolic mirrors with custom specifications unless carried out in large batch sizes. Stray pump light may be rejected using filters such as high-resistivity float zone silicon (FZSi) [25] placed at Brewster's angle, PTFE, or even paper. These materials are transparent to THz but reflect or absorb the pump light. However, the use of Silicon filters is not ideal in the case of high-power Yb-doped lasers due to the proximity of the silicon bandgap with the laser's photon energy (1.203 eV at 1030 nm vs. 1.124 eV at 300 K) [26].  
A second parabolic mirror refocuses the collimated THz radiation into a sampling spot; this is where a sample may be placed for spectroscopic measurements. Finally, a third and fourth parabolic mirror recollimate and refocus the THz radiation into the detection plane. Here, the THz radiation is focused onto an electro-optic crystal and overlaps spatially with the probe beam. A GaP wafer cut in the 110 plane with a thickness of 300 um (Pi-Kem, Tamworth, UK) was used in this case. The temporal overlap is controlled using a delay line stage. A standard polarimetry setup is employed to record the THz-induced polarisation rotation as a function of the delay between the THz pulse and the sampling (probe) ultrashort pulse--see [27] for a recent discussion, and a lock-in amplifier (synchronized to the amplitude modulation induced on the THz field via an optical chopper) is used to reduce background noise.  
To avoid THz losses and signal contamination due to water absorption, purging with nitrogen gas is employed to remove humidity from the atmosphere. This results in an increased signal strength (a 2-3 times greater peak signal has been observed in our case) and the suppression of the ringing signal originating from resonances in atmospheric water.  
To maximize the THz-TDS signal, the optical-to-THz conversion efficiency must be maximized. To this end, the pulse intensity on the generating crystal should be such that the conversion is close to nonlinear saturation at the highest possible energy per pulse. This defines an ideal beam size. When using a high-power pump beam, the ideal beam size may be larger than the rejection hole in the first parabolic mirror; this was indeed the case in our system. This can result in the clipping of the pump beam on the pump rejection hole and hence in scattered pump light that, at high powers, has the potential of warping or burning filters used to remove the residual scattered radiation, or even affecting the tested sample. Off-axis parabolic mirrors with larger holes are difficult to obtain and are undesirable as they result in a larger loss of THz radiation through the hole. With a pump energy at 150 uJ, after optimizing the pump beam size for maximal THz generation efficiency (3.5 mm \(1/e^{2}\)), we measured with a thermal detector \(\simeq\)22% of the residual pump in the beam path after reflection off a parabolic mirror with a 3 mm diameter hole. It is also to be noted that high pump powers result in thermal lensing in the generation crystal, leading to a degradation of the pump beam quality [28], which makes geometrical pump rejection more challenging, as shown in Section 5.