## 1 Introduction  
For a long time, the electromagnetic radiation in the terahertz (THz) spectral region from microwaves to the far infrared (100 GHz-30 THz) was known as the so-called THz gap due to the lack of suitable radiation sources and detectors in this spectral range. During the last three decades, however, this has changed as the related technology has advanced, and the emerging technology has started to leave the laboratory environment, leading to commercial applications. As a result, the field of THz technology has continued to develop and expand rapidly. Despite this progress, many challenges remain in the field of THz science and technology [1; 2; 3]. Among some of the most promising THz sources for high-resolution absorption spectroscopy are quantum cascade lasers (QCLs). The first QCL with an emission frequency in the THz range was reported in 2002 by [PERSON] et al. [4]. THz QCLs cover, at present, the emission frequency range of about 1 THz to 5.5 THz [5; 6; 7]. In this spectral region, QCLs are the only available powerful cw radiation sources, with optical output powers of up to a few mW and extremely narrow linewidths of a few MHzdown to kHz. The applicability of THz QCLs has already been demonstrated for numerous applications, such as THz real-time imaging [8; 9], THz sensing [10], and microscopy [11]. Another emerging application is the use of THz QCLs for high-resolution spectroscopy, since many transitions of atoms and molecules can be accessed, enabling selective and sensitive spectroscopic measurements of a large number of compounds. In particular, the detection of fine structure transitions in atoms is of high interest, as this could provide a means of developing absolute detection methods for atoms based on direct absorption spectroscopy. Recent examples of this are the detection of atomic oxygen by heterodyne spectroscopy in astronomy and atmospheric research [12; 13; 14] and the determination of the absolute densities of oxygen atoms in a plasma reactor [15; 16; 17]. There have been a few reports on the application of THz QCLs for absorption spectroscopy in the gas phase as well [18; 19; 20; 21]. However, the lack of suitable and miniaturised detectors has hampered the development of compact THz spectroscopy systems out of the laboratory environment as often helium-cooled bolometers or Ge:Ga photoconductive detectors are used as detectors. In addition to the requirement of cryogenic cooling, they typically have a limited bandwidth in the order of up to a few hundred Hz for the 3 dB roll-off frequency, which significantly reduces the laser tuning frequencies that can be applied and thus leads to longer measurement times when recording an absorption spectrum. For example, in previous work [15; 16], laser tuning frequencies of 10 Hz or 11 Hz had to be used to avoid any asymmetric deformation of the observed absorption features, which, in turn, also limited the spectral range that could be probed. Pyroelectrics are another type of thermal detector based on direct energy detection and have long been used to measure infrared and THz radiation. So far, however, it has been reported that pyroelectrics typically work at low speed and with relatively low sensitivity. In addition, microphonics is a problem that significantly increases the detector noise. This paper reports on the performance of a new high-speed pyroelectric receiver as a cryogen-free detector for THz absorption spectroscopy measurements. The novelty of the reported work is twofold. Firstly, it will be demonstrated that an absorption spectroscopy experiment in the THz range can be performed with an uncooled detector, for which the signal-to-noise ratio (SNR) of the recorded absorption spectra with the same measurement time is only a factor of 2 lower than measurements performed with a helium-cooled bolometer. Secondly, it is shown that this can be achieved by LiTaO\({}_{3}\)-based pyroelectric detector technology, whereas previous work was in favour of DLaTGS detectors (deuterated L-alanine-doped triglycene sulphate) [22].