## 3 Results and Discussion
### THz Absorption Spectroscopy Measurements  
First, etalon measurements were performed for different laser tuning frequencies to investigate the current-wavenumber relation and its possible dependence on the tuning frequency. Examples of etalon measurements for tuning frequencies of 11 Hz and 201 Hz are shown in Figure 4. The used etalon was made of silicon (refractive index: \(n=3.4175\pm 0.003\)[24]) and had a length \(L\) of 9 cm. The low transmission of this etalon led to very weak signals with fringes that were hardly discernable due to the poor SNR, even after averaging. Hence, the data were filtered with a Savitzky-Golay filter before determining the positions of the minima and maxima of the fringes. As can be seen from Figure 4, the minima and maxima occurred at the same wavenumber position independent of the laser tuning frequency. The distance in wavenumber between the fringes is given by \(1/(2 nL)\) and equal to 0.016 cm\({}^{-1}\), resulting in a tuning coefficient of \(7.8\times 10^{-4}\) cm\({}^{-1}\)/mA. This value is in good agreement with results of previous measurements performed with a bolometer as the THz detector [15; 16]. The same value for the tuning coefficient was obtained for various laser tuning frequencies up to 281 Hz. Therefore, it can be concluded that the tuning behaviour of the used THz QCL is not affected by the tuning frequency, at least not for tuning frequencies up to 281 Hz (for a temperature of 43.3 K and laser currents between 480 mA and 600 mA). This also confirms that the same tuning coefficient can be used when lowering the tuning speed by reducing the tuning range, which was a critical assumption for previous bolometer-based measurements as the tuning range needed for etalon measurements typically involved tuning speeds that proved too fast to accurately detect the shape of sharp absorption features [15]. Next, measurements were performed on an RGC with NH\({}_{3}\). The cell had a length \(L\) of 15 cm and silicon windows at both ends which were attached under an angle to avoid an etalon effect between the parallel window surfaces. It was filled with pure NH\({}_{3}\) to a precisely known pressure of (1.61 \(\pm\) 0.01) mbar, determined using frequency-comb-based Fourier-transform spectroscopy in the range from 3145 cm\({}^{-1}\) to 3390 cm\({}^{-1}\)[25]. Figure 5a shows the raw results of measurements on this gas cell for laser tuning frequencies of 11 Hz and 201 Hz using the same tuning range (480 mA to 600 mA) as for the etalon measurements. Two absorption features can be observed, of which the more prominent one located at approximately 580 mA was identified as the R(7,5)-transition of \({}^{15}\)NH\({}_{3}\) at 158.257314 cm\({}^{-1}\)[26,27]. The smaller absorption feature at approximately 530 mA was due to the aR(7,2)-transition of \({}^{14}\)NH\({}_{3}\) at 158.2958 cm\({}^{-1}\)[27,28; 15]. The assignment notation used here corresponds to \(\Delta J(J^{\prime\prime},K^{\prime\prime}_{\rm a})\), where \(J^{\prime\prime}\) and \(K^{\prime\prime}_{\rm a}\) are the lower-state quantum numbers of the \((J^{\prime},K^{\prime}_{\rm a})\leftarrow(J^{\prime\prime},K^{\prime\prime}_{ \rm a})\) transition, \(J\) is associated with the total angular momentum, and \(K_{\rm a}\) with its projection along the principle axis (C3) of the ammonia molecule. Notably, both absorption features were recorded without significant deformations, even for a tuning  
Figure 3: (**a**) Frequency-resolved responsivity, (**b**) noise spectral density, and (**c**) detectivity (D\({}^{\bullet}\)) of the PR No1 pyroelectric receiver. The responsivity and D\({}^{\bullet}\) values were measured with a 150 \({}^{\circ}\)C black-body emitter with 6.8 μm peak emission. The blue curves are raw measurement data while the green curves were calculated from these data to compensate for the transmission losses of HDPE at the wavelength of 6.8 μm. frequency of 201 Hz, whereas the same absorption features measured with a bolometer were already asymmetrically deformed for a tuning frequency of 10 Hz [15].