  The lack of coherent room-temperature sources in the whole terahertz spectral
window (0.3-10 THz) has significantly hampered the growth of scientific and
technological applications in this range. Among them, high-precision frequency
measurements of molecular transitions play a central role but remain an open
challenge. Here, room-temperature generation and detection of continuous-wave,
broadly tunable, narrow-linewidth THz radiation are presented, and their
application to high-resolution spectroscopy in the broad 1-7.5 THz spectral
range is demonstrated. This result has been achieved by implementing a
Cherenkov phase-matching scheme into a channel waveguide in a nonlinear
crystal. This simple approach, entirely based on robust telecom technology,
unprecedently merges in a single source an ultra-broad continuous-wave spectral
coverage and a state-of-the-art accuracy (approximately 10-9) in
molecular-transition-center determination.
