Quantum cascade laser THz metrology
Authors: De Natale P., Consolino L., Mazzotti D., Campa A., Ravaro M., Vitiello M. S., Bartalini S.
Autors Affiliation: Istituto Nazionale di Ottica, INO-CNR, Largo E. Fermi 6, Firenze, Italy; European Laboratory for Nonlinear Spectroscopy, LENS, Via N. Carrara 1, Sesto F. (FI), Italy; Istituto Nanoscienze, NEST-CNR, and Scuola Normale Superiore, Piazza S. Silvestro 12, Pisa, Italy
Abstract: The realization and control of radiation sources is the key for proper development of THz-based metrology. Quantum Cascade Lasers (QCLs) are crucial, towards this purpose, due to their compactness and flexibility and, even more important, to their narrow quantum-limited linewidth. We recently generated an air-propagating THz comb, referenced to an optical frequency comb by nonlinear optical rectification of a mode-locked femtosecond Ti:Sa laser and used it for phase-locking a 2.5 THz QCL. We have now demonstrated that this source can achieve a record low 10 parts per trillion absolute frequency stability (in tens of seconds), enabling high precision molecular spectroscopy. As a proof-of-principle, we measured the frequency of a rotational transition in a gas molecule (methanol) with an unprecedented precision (4 parts in one billion). A simple, though sensitive, direct absorption spectroscopy set-up could be used thanks to the mW-level power available from the QCL. The 10 kHz uncertainty level ranks this technique among the most precise ever developed in the THz range, challenging present theoretical molecular models. Hence, we expect that this new class of THz spectrometers opens new scenarios for metrological-grade molecular physics, including novel THz-based astronomy, high-precision trace-gas sensing, cold molecules physics, also helping to improve present theoretical models.
Conference title: Quantum Sensing and Nanophotonic Devices XII
KeyWords: Absorption spectroscopy; Locks (fasteners); Mode-locked fiber lasers; Molecular spectroscopy; Molecules; Nanophotonics; Quantum cascade lasers; Semiconductor lasers; Spectrometers; Terahertz waves; Units of measurement, Direct absorption spectroscopies; Frequency metrology; High-precision spectroscopy; Nonlinear optical rectification; Optical frequency combs; Parts per trillion; Quantum cascade lasers (QCLs); Rotational transition, Laser mode locking