Quantum cascade laser (QCL) technology has made a breakthrough in THz radiation sources, opening a new window into variety of applications. QCLs are compact, low-consumption and narrow linewidth lasers with relatively high output powers, capable to emit radiation across the whole super-THz (2-6 THz) range. These are all demanded for spectroscopy, which is so interesting in THz due to the fingerprint molecular rotational absorption/emission spectrums. Among many other potential applications of spectroscopy including chemical identification, security checking and biomedical diagnosis, high-resolution probing of celestial fine structured lines is prominent for addressing the human permanently staying curiosity about the origins of stars, galaxies and the Universe...