Due to the lack of high-power, tunable sources and fast, sensitive detectors, the terahertz (THz) region of the electromagnetic spectrum has been lagging behind from a technological point of view. The lack of suitable technologies led to the THz band being called the “THz gap”. This technological gap has been rapidly diminishing during the last three decades, giving rise to scientific opportunities that could not be considered in the past.
THz radiation has got plenty of attractive applications, among which THz high-precision molecular spectroscopy has a central role. In this window, in fact, linestrengths of molecular transitions are generally larger than in the microwave region and comparable with the strongest fundamental ro-vibrational transitions in the mid-IR. Moreover, Hz-linewidth transitions represent key molecular signatures and the THz range can well represent a novel molecular fingerprint region. For this reason, astronomy and space science have recently moved to THz technology. Other spectroscopic applications include plasma fusion diagnostics or identification of different crystalline polymorphic states of a drug. In this spectral range, natural transition linewidths can be as narrow as a few hertz; therefore, it is crucial to probe them with narrow-emission tunable sources, which have been studied throughout the years.
Our research activity focuses on table-top sources, based on direct THz lasing action and frequency down-conversion of visible/infrared light in non-linear media.
1. Direct THz lasing action
THz-emitting Quantum Cascade Lasers (QCLs) are proving to be good candidates to fill the THz gap, and have attracted considerable attention thanks to the high
output power (>100 mW), spectral purity, stability, compactness, and reliability, and have now a realistic chance of making a deep impact on technological applications. Among the features that a THz QCL has to show to become a useful radiation source for many applications is a reliable, repeatable, and tunable single-frequency emission. Many quantum-design related approaches, technological solutions, and/or optical configurations have been indeed recently tested to tune the emission frequency of THz QC sources over quite a large bandwidth.
In the framework of high-resolution spectroscopy, knowledge of the intrinsic linewidth, ultimately related to the uncertainty principle of quantum mechanics, plays a key role, determining the maximum achievable spectral resolution and coherence length in a free-running laser. The spectral purity of a THz QCL has been investigated via the measurement of its frequency–noise power spectral density (FNPSD), providing an experimental evaluation and a theoretical assessment of its intrinsic LW. Intensity measurements were performed to retrieve information in the frequency domain by converting the laser frequency fluctuations into detectable intensity (amplitude) variations [1]. These measurements led to a measured FWHM δυ = 90 ± 30 Hz.
In order to exploit this high spectral purity frequency and phase stabilization of THz QCL is needed. Our approach is based on the beat-note detection between the device and a free-standing and air-propagating THz FCS [2].
The THz comb has been successfully used as local oscillator to narrow and measure the frequency of a THz QCL down to the 10−11 level, only requiring a small fraction of the radiation emitted by the laser. With this setup, high-resolution spectroscopy has been performed on single methanol transitions,
achieving an accuracy of 4 × 10^−9 [3]. As a matter of fact, the main limitation to the accuracy achieved in is the Doppler-limited spectroscopy setup used in the experiment. A way to improve the accuracy of the retrieved frequencies has to exploit sub-Doppler resolution, which can be obtained by means of the Lamb-dip technique in case of rotational spectroscopy.
The recent advancements of solid state QCL-based comb emission in the THz region has also enabled the possibility to develope setups for this novel devices characterization [4], controlling [5], full phase stabilization [CONSOLINONATCOMM] and employment in innovative hybrid dual comb spectrometers [6].
2. Down-conversion of visible/infrared light in non-linear media
QCLs are intrinsically limited by the semiconductor material at high frequencies, with a present-day limit set at about 4.7 THz in cw operation, while the widest tunability range is restricted to 330 GHz. These limits, added to the need for cryogenic cooling, have hampered the range of precise spectroscopic measurements in the THz window. For these reasons we developed a novel spectrometer setup, where we preserve the simplicity of a DFG scheme, combining it with rugged, reliable, compact, and high-power telecom lasers. Our THz source uniquely combines, in a single source, an ultra-broad spectral coverage, spanning from 0.97 to 7.5 THz, a μW-level emission power, and an absolute frequency reference, enabling measurements of molecular transition frequencies at an accuracy of approximately 10^−9 [7]. The achieved level of accuracy, if systematically reproduced over a large set of transitions of selected gases, could help to significantly improve molecular models.
The broad spectral coverage of the presented source is enabled by the simultaneous use of a Cherenkov emission scheme and strong light confinement in a surface non-linear waveguide [8], leading to a significant broadening of the phase-matching bandwidth. Moreover, because of the guided-wave approach, our setup reaches cw generation efficiencies as high as 1.5 × 10^−7 W^−1 , and grants power levels high enough for both room-temperature detection and high-accuracy THz spectroscopy.
[1] Vitiello, M. S. et al. Quantum-limited frequency fluctuations in a terahertz laser. Nat. Photon. 6, 525–528 (2012).
[2] L. Consolino, A. Taschin, P. Bartolini, S. Bartalini, P. Cancio, A. Tredicucci, H. E. Beere, D. A. Ritchie, R. Torre, M. S. Vitiello, and P. De Natale, Phase-Locking to a Free-Space Terahertz Comb for Metrological-Grade Terahertz Lasers, Nat. Commun. 3, 1040 (2012).
[3] S. Bartalini, L. Consolino, P. Cancio, P. De Natale, P. Bartolini, A. Taschin, M. De Pas, H. Beere, D. Ritchie, M. S. Vitiello, and R. Torre, Frequency–Comb–Assisted Terahertz Quantum Cascade Laser Spectroscopy Phys. Rev. X 4, 021006 (2014)
[4] F. Cappelli, L. Consolino, G. Campo, I. Galli, D. Mazzotti, A. Campa, M. Siciliani de Cumis, P. Cancio Pastor, R. Eramo, M. Rösch, M. Beck, G. Scalari, J. Faist, P. De Natale, S. Bartalini, Nat. Photonics 2019, 13, 562.
[5] L. Consolino, M. Nafa, F. Cappelli, K. Garrasi, F. P. Mezzapesa, L. Li, A. G. Davies, E. H. Linfield, M. S. Vitiello, P. De Natale, S. Bartalini, Nat. Commun. 2019, 10, 2938.
[6] L. Consolino, M. Nafa, M. De Regis, F. Cappelli, K. Garrasi, F. P. Mezzapesa, L. Li, A. G. Davies, E. H. Linfield, M. S. Vitiello, S. Bartalini, P. De Natale, Commun. Phys. 2020, 3, 69.
[7] De Regis, M. et al. Room-temperature continuous-wave frequency-referenced spectrometer up to 7.5 THz. Phys. Rev. Appl 10, 064041 (2018).
[8] De Regis, M., Consolino, L., Bartalini, S. & De Natale, P. Waveguided approach for difference frequency generation of broadly-tunable continuous-wave terahertz radiation. Appl. Sci. 8, 2374 (2018).