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Terahertz sources based on intra-cavity difference-frequency generation inmid-infrared quantum cascade lasers (THz DFG-QCLs) have recently emerged as thefirst monolithic electrically-pumped semiconductor sources capable of operatingat room-temperature (RT) across the 1-6 THz range. Despite tremendous progressin power output, that now exceeds 1mW in pulsed and 10 {\mu}W incontinuous-wave regime at room-temperature, knowledge of the major figure ofmerits of these devices for high precision spectroscopy, such as spectralpurity and absolute frequency tunability, is still lacking. Here, by exploitinga metrological grade system comprising a terahertz frequency comb synthesizer,we measure, for the first time, the free-running emission linewidth (LW), thetuning characteristics, and the absolute frequency of individual emission linesof these sources with an uncertainty of 4 x 10-10. The unveiled emission LW(400 kHz at 1ms integration time) indicates that DFG-QCLs are well suited tooperate as local oscillators and to be used for a variety of metrological,spectroscopic, communication, and imaging applications requiringnarrow-linewidth THz sources.

Unveiling spectral purity and tunability of terahertz quantum cascade laser sources based on intra-cavity difference frequency generation