Frequency-comb-assisted terahertz quantum cascade laser spectroscopy

We report a metrological-grade THz spectroscopy based on the combination of a THz frequency-comb synthesizer (FCS) and a THz quantum cascade laser (QCL). The QCL, emitting at 2.5 THz, is phase locked to the free-space THz FCS, and its frequency is swept across a methanol transition by tuning the comb-repetition rate, which is ultimately disciplined by the Cs primary frequency standard. The absolute frequency scale provides an uncertainty of a few parts in 10(-11) on the laser frequency and 10(-9) on the line-center determination, ranking this technique among the most precise ever developed in the THz range.