## 2 Materials and Methods  
For this work, the uncooled pyroelectric receiver PR No1 THz (Laser Components) was used. While it is a state-of-the-art LiTaO\({}_{3}\)-based receiver, it incorporates a redesign of all core components: the detector element (see Figure 1a) and the amplifier electronics. The goal was to increase both speed and detectivity to enable undistorted broadband measurements of typical infrared signals down to 0.5 THz. Until now, pyroelectric detectors in spectroscopy, especially in Fourier-transform infrared (FTIR) spectroscopy, have mostly been operated in voltage mode. The PR No1 exploits a proprietary current mode configuration that is specifically tailored to the pyroelectric sensor elements. When radiation is absorbed by the black coating of the pyroelectric detector element, it causes a slight increase in its temperature. This, in turn, leads to a charge separation at the detector interfaces. The resulting current is amplified and converted into a voltage by a transimpedance amplifier. A simplified schematic circuit of this so-called current mode operation is shown in Figure 1b, where \(D_{\text{py}}\) indicates the pyroelectric detector element. The proprietary amplifier electronics of the PR No1 are optimised to provide a high gain at a very low noise level while still offering an electronic bandwidth of 8 kHz. In particular, the electronics are low-noise even for current sources with a high capacity, which is typical for thin and fast pyroelectric detector elements. The improved receiver technology enables high internal amplification and thus high responsivity of up to 70 kV/W at low noise density (down to 25 \(\upmu\)V/\(\sqrt{\text{Hz}}\)), resulting in a high detectivity of up to \(4\times 10^{8}\,\text{cm}\,\sqrt{\text{Hz}}/\text{W}\). At the same time, the receiver's 3 dB bandwidth of 8 kHz is much higher than the one of common pyroelectric detectors in voltage mode (typically 10-100 Hz). Up to the 3 dB bandwidth, the receiver is able to measure undistorted signals with a linearity of 1% or better over 4 decades in power. This characteristic is particularly important for this work. Furthermore, the spatial uniformity over the 2 \(\times\) 2 mm\({}^{2}\) active detector area is better than 10%. LiTaO\({}_{3}\) was chosen as basic detector material because of its ruggedness. No cooling is required, and all measurements in this work were performed at room temperature. The detector window was made from thin high-density polyethylene (HDPE) for good transmission in the far infrared. The absorption spectroscopy measurements were performed on a reference gas cell (RGC) filled with ammonia gas to a precisely known pressure. A tunable, continuous-wave (cw) THz QCL with an output power of approximately 4 mW was used as the THz source [7]. The same THz QCL has been used in previous work to detect the \({}^{3}\text{P}_{1}\leftarrow{}^{3}\text{P}_{2}\) fine structure transition of ground-state atomic oxygen at approximately 4.75 THz, which corresponds to approximately 158 cm\({}^{-1}\) and a wavelength of approximately 63.2 \(\upmu\)m [15; 16]. The QCL was operated in a Stirling cryocooler (Ricor K535) at a temperature of 43.3 K while  
Figure 1: (**a**) A photograph (30\(\times\) magnification) of the LiTaO\({}_{3}\) chip mounted on a header. A thin membrane (2.3 \(\times\) 2.3 mm\({}^{2}\), 6 \(\upmu\)m thickness) is etched into a frame (3.0 \(\times\) 3.0 mm\({}^{2}\), 20 \(\upmu\)m thickness) and then coated (2.0 \(\times\) 2.0 mm\({}^{2}\)). Vibration dampening is achieved by 4 flexible posts. All wire bonds have been made on the frame. The element is sealed by a TO-can assembly with an HDPE window (0.8 mm thickness). (**b**) A simplified schematic circuit of the current mode configuration. temperature fluctuations were minimised to be below 5 mK by an additional temperature controller (Stanford Research Systems, Sunnyvale, CA, USA, CTC100). Tuning of the laser output was performed by linearly ramping the input current.