## 1 Introduction  
The terahertz (THz) spectral window, generally considered to span from 1 to 10 THz, is nowadays crucial for plenty of normal day-to-day, as well as scientific applications, in areas that include things like homeland security [1], quality inspection of industrial products [2; 3], and biomedicine [4]. Among these applications, high-precision molecular spectroscopy and trace gas sensing, requiring powerful and widely tunable continuous wave (CW) sources, play a central role [5]. Indeed, in many cases, the chemical composition of gas mixtures of a very practical interest can be retrieved by observing only a limited part of their spectrum, as long as it contains a sufficient amount of intense transitions serving as a signature for the molecules of interest. For example, in measuring the concentration of specific chemical species on Earth and astrophysical environments, crucial information can be retrieved about global warming mechanisms [6], star and galaxies formation processes [7], and planetary atmospheres' composition [8]. Because of their strong permanent dipole moment, the rotational transitions of many light molecules, lying in the THz spectral range, are usually comparable in intensity with the strongest ro-vibrational transitions in the mid-IR fingerprint region [9]. Furthermore, the low sensitivity to diffusion losses by micrometric powders and the Doppler linewidths, being one order of magnitude smaller with respect to the mid-IR transitions, make the THz window particularly suitable for the identification of complex line patterns and for in situ measurements [10]. Anyway, despite its undoubted usefulness, the THz interval is still relatively unexploited as a \"fingerprint region\". This fact is mainly due to the lack of room-temperature and high-power source, able to combine the narrow-linewidth emission required for high-precision spectroscopy (typically in the order of a few kHz) with a spectral coverage and tunability extended to the whole 1-10 THz frequency range. Actually, emblematic and extreme examples are represented by tunable far-IR lasers (Tu-FIR) which date back to the mid-80s [11; 12; 13] and, on the other hand, by THz quantum cascade lasers (THz-QCLs) [14], whose specific features are, in some ways, complementary. Indeed, THz-QCLs are capable of providing THz radiation in the milliwatt range by a cascade of intra-sub-band laser transitions, but their spectral coverage is generally limited to a few percent of the emission frequency [15], which is fixed by the specific device design. Remarkably, at the state-of-the-art level, the maximum frequency achievable with CW THz-QCLs (requiring cryogenic cooling to operate) is limited to 4.75 THz [16; 17]. Conversely, an emission of up to 9 THz has been demonstrated with Tu-FIR lasers, taking advantage of a difference frequency generation (DFG) approach [18]. Despite the very low generated power (generally tens of nW, requiring liquid-He cooled detectors), such a wide-tunability approach has produced, in about fifteen years, plenty (several tens) of significant spectroscopic results (e.g., [6; 18; 19; 20; 21; 22; 23]). However, after a comparable time-span, THz QCLs have provided mostly proof-of-principle spectroscopic results [10; 24; 25; 26] to date, thus paying the price for insufficient tunability in regard to spectroscopy. Nonlinear generation of THz radiation by DFG in nonlinear ferroelectric materials, like Lithium Tantalite [27] and Lithium Niobate (LN), provides a suitable alternative with respect to the previous approaches. Indeed, DFG-based setups have been demonstrated to be capable of transferring some of the most interesting features of visible and near-IR sources directly in the far-IR part of the electromagnetic spectrum. For instance, generation of ultrashort THz pulses and comb-like THz emission has been demonstrated by optical rectification (OR) of femtosecond pulses provided by commercial mode-locked lasers [28; 29].