Mini-Symposium: Precision Spectroscopy, Invited Lecture

Terahertz Frequency Metrology

L. Consolino1
1Istituto Nazionale di Ottica INO-CNR, Largo E. Fermi 6, Florence, Italy I-50125

   In the last few decades, the THz window of the electromagnetic spectrum has emerged as enabling breakthrough scientific and technological applications in such diverse fields as information and communications technology (ICT), biomedicine, homeland security, quality control of food and global environmental monitoring. Likewise, high precision THz spectroscopy of rotational and ro-vibrational molecular transitions promises to deliver many novel physical insights. In this framework we will report on two different approaches for THz metrological-grade radiation, the first related to difference frequency generation (DFG) based broadband continuous wave (CW) THz source, the second regarding the characterization and applications of Quantum Cascade Lasers (QCL) based THz frequency combs (FCs).

1.  DFG-based broadband CW THz source
   Regarding THz frequency metrology, a lot of work as been done on single mode QCL setups. However, in order to fully exploit the potential of this key spectral region, the challenge is to merge, in a single source, three crucial aspects: an even broader spectral coverage towards higher THz frequencies (covered neither by QCLs nor by other traditional THz sources), metrological-grade performances (i.e. high resolution and accuracy with referencing to the primary frequency standard), power levels sufficient for room-temperature detection. In our work we demonstrate room-temperature generation and detection of continuous-wave THz radiation spanning three octaves in the THz range, from 1 to 7.5 THz, and performing high-accuracy molecular spectroscopy. This unprecedented result makes use of a simple, reliable approach that combines, in a unique set-up, robust telecom laser components with difference-frequency nonlinear generation.
   In synthesis, the main contribution of our work is the combination of all the following aspects: i) room temperature CW generation, based on the fully-developed and commercial telecom fiber laser technology that grants a high level of compactness, stability and reliability to our new source; ii) a 3-octave spectral coverage, from 0.97 to 7.5 THz, that is obtained by the combined use of a Cherenkov emission scheme and strong light confinement in a surface nonlinear waveguide; iii) high power levels enabling both room temperature detection and high-precision THz spectroscopy, achieved thanks to a CW generation efficiency as high as 10−7 W−1 ; iv) frequency referencing to the primary frequency standard by means of a mode-locked femtosecond laser and a GPS disciplined Rubidium-Quartz oscillator; v) a state-of-the-art accuracy in the order of 10−9 obtained with a room temperature Golay cell detector. The proposed approach paves the way to a new class of metrological-grade sources spanning most of the THz range for countless demanding applications.

2. QCL-based THz frequency combs
   The most common sources for FCs are mode-locked lasers, whose high level of coherence enabled a myriad of scientific applications, providing a frequency ruler for any laser emitting within their spectral range. For this reason huge efforts have been spent to extend the characteristics of FCs to all spectral regions, and to extend the figure of merits of FCs to other non-conventional comb-like source. In this framework Quantum Cascade Lasers (QCLs) technology, both in the mid-infrared and THz region, is exploiting the extraordinary versatility of these devices for developing active regions with engineered optical dispersion that emit optical frequency combs. Thanks to four-wave-mixing non-linear processes happening inside the active medium, proper mode-locking is obtained. The Fourier modal phases, ultimately describing comb operation are retrieved thanks to the Fourier Analysis of Comb Emission (FACE) technique, which confirms the high level of coherence of these sources, and enables retrieval of the temporal emission profile.
   The metrological-grade performance of this class of devices is probed thanks to the full phase referencing to the primary frequency standard, achieving ~2 Hz in 1 s stability and ~6 Hz accuracy for the emitted modes. Independent and full control of the two comb degree of freedom is also demonstrated and characterized. Finally, application of a QCL-FC to high-accuracy molecular spectroscopy is shown in a hydrid dual comb spectroscopy (DCS) setup [7]. The most distinctive characteristic of this approach is the merging of two completely different comb in a dual comb spectrometer. This allows to merge the high power per mode emitted by the QCL device with the accuracy and frequency referencing allowed by an optically rectified free-standing THz FC.

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[7] Consolino, L., Nafa, M., De Regis, M., Cappelli, F., Garrasi, K., Mezzapesa, F.P., Li, L., Davies, A.G., Linfield, E.H., Vitiello, M.S., Bartalini, S., De Natale, P., Quantum cascade laser based hybrid dual comb spectrometer, Communications Physics, 2020, 3(1), 69.