Toward metrological­ grade sub­-Doppler THz spectroscopy

High precision THz molecular spectroscopy promises amazing scientific and technological applications, provided that sources with suitable high power, wide tunability and narrow linewidth, such THz Quantum Cascade Lasers [1] are available. In this context, the first application of a QCL phase-locked to a free-standing THz Frequency Comb Synthesizer (FCS) [2] was recently demonstrated, achieving a state of-the-art accuracy of 4×109 in the determination of the absolute frequency of a THz transition [3]. In this experimental report the overall accuracy was limited by the resolution of the Doppler-broadened direct-absorption spectroscopy adopted therein. As a consequence, the the development of high-resolution THz spectroscopic techniques, such as saturatedabsorption spectroscopy (SAS), is a mandatory condition for further accuracy improvements. We therefore report on the evidence of saturation effects in a rotational transition of CH3OH around 2.55 THz, induced by a freerunning continuous-wave QCL [4], confirming the feasibility of QCL-based subDoppler spectroscopy, once provided that the QCL emission linewidth is comparable with, or narrower than the Lamb-dip width. The QCL emission is used for direct-absorption spectroscopy experiments, allowing to study the dependence of the absorption coefficient on gas pressure and laser intensity. A saturation intensity of 25 W/mm2, for a gas pressure of 17 bar, is measured.