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.
Toward metrological grade sub-Doppler THz spectroscopy
Ravaro M;Consolino L;Campa A;Cancio P;Mazzotti D;Vitiello M S;Bartalini S;De Natale P
2015
Abstract
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.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.