Coherent terahertz spectroscopy of molecular transients has so far relied on electronic sources whose bandwidth is limited to a fraction of their center frequency. Photoconductive devices offer a complementary approach, combining intrinsically broadband operation with coherent optoelectronic generation and detection. We report free-induction-decay spectroscopy of carbonyl sulfide at 0.29 THz in which, to our knowledge for the first time, both the pulsed emitter and the coherent heterodyne receiver are LT-GaAs photoconductors simultaneously pumped by a single free-running dual-frequency Ti:Sa laser. The receiver operates within 15 dB of the thermal noise limit, and the shared optical reference maintains phase coherence over more than 1000 averaged acquisitions. The measured transients agree with a time-domain Maxwell-Bloch model based on HITRAN parameters. These results demonstrate the feasibility of fully optoelectronic coherent THz spectroscopy and establish photoconductive optoelectronic mixing as a practical route toward frequency-agile, high-resolution coherent THz spectrometers over broad spectral ranges.

Atmospheric emissions from waste recovery centers are strictly regulated by industrial emission directives which stipulate that they should be continuously monitored. In this work, we demonstrate and evaluate the capacity of terahertz rotational spectroscopy for real-time diagnosis and monitoring of gases of environmental interest on industrial sites. A spectrometer based on the use of a frequency multiplication chain operating at 530-620 GHz has been constructed, enabling the detection of most of the regulated compounds in waste incineration. The detection limits obtained are compatible with regulatory emission limit values. The installation of the instrument at the waste recovery center in Dunkirk demonstrated its ability to perform real-time quantitative multi-component in situ monitoring in an industrial environment. Continuous measurements of SO2 and CO over more than 24 h are compared with the measurements performed by two certified infrared instruments. The agreement between the three sets of measurements is excellent, demonstrating that high-resolution terahertz spectroscopy is relevant for real-time in situ monitoring of regulated industrial emissions gases, constituting an interesting alternative to other techniques thanks to a higher degree of selectivity.

Cavity-Enhanced Absorption Spectroscopy (CEAS) and Cavity Ring-Down Spectroscopy (CRDS) are well established for sensitive infrared measurements of gas-phase compounds at trace levels using their rovibrational signatures. The recent successful development of a THz Fabry-Perot spectrometer by Hindle et al. (2019) shows that the adaptation of such techniques to submillimeter wavelengths allows to probe rotational transitions of light polar compounds. Here we report on the development of a new millimeter-wave resonator, covering the 150-215 GHz frequency range, and based on a low-loss corrugated waveguide with homemade highly reflective photonic mirrors obtaining a finesse above 3000 at around 164 GHz. With an effective path length of two kilometers, a significant sensitivity has been evaluated, and the detection of semi-volatile organic vapors at a trace level may be now envisaged at room temperature. We applied this technology to detect gas-phase explosive taggants and precursors, confirming a detection limit of 2 ppmv for nitromethane (NM). Leveraging the unique characteristics of the millimeter wave frequency band, we showcase highly selective detection in quasi-realistic environments of complex chemical mixtures involving explosive taggants with close chemical structures such as nitrotoluene isomers. Furthermore, we successfully address the challenge of detecting these nitro-derivative compounds vaporized from model matrices: KCl matrices, granular and plastic (NP91) explosives respectively in conventional and pyrotechnic laboratories. Our findings underscore this approach as a potent tool for practical explosive detection applications.

We demonstrate the advantages of THz frequency combs for high-resolution spectroscopy. This benefits from wide spectral coverage and the exact knowledge of the frequency position of each comb component. Heterodyne detection combined with a fast Fourier spectrometer enables rapid and simultaneous measurement of more than 80 frequency comb modes covering a 7.5 GHz bandwidth. A spectrum is obtained in under 20 minutes yielding a uniform resolution of 70 kHz. This new setup has been validated by recording more than 150 lines of methanol around 723 GHz, and represents a new solution to exploit THz frequency combs for high-resolution spectroscopy.

Terahertz spectroscopy leveraging frequency combs (FC) derived from femtosecond lasers enables high-resolution molecular analysis. Through precise calibration and innovative instrumentation, our setup demonstrates the capability to achieve detailed spectral resolution, showcasing the potential for advanced molecular spectroscopy. This method holds promise for diverse applications, offering insights into complex molecular structures

The correct interpretation of infrared (IR) observations of planetary atmospheres requires an accurate knowledge of temperature and partial and global pressures. Precise laboratory measurements of absorption intensities and line profiles, in the 200–350 K temperature range, are, therefore, critical. However, for gases only existing in complex chemical equilibria, such as nitrous or hypobromous acids, it is not possible to rely on absolute pressure measurements to measure absolute integrated optical absorption cross sections or IR line intensities. To overcome this difficulty, a novel dual-beam terahertz (THz)/mid-IR experimental setup has been developed, relying on the simultaneous use of two instruments. The setup involves a newly constructed temperature-controlled (200–350 K) cross-shaped absorption cell made of inert materials. The cell is traversed by the mid-IR beam from a high-resolution Fourier transform spectrometer using along a White-cell optical configuration providing absorption path lengths from 2.8 to 42 m and by a THz radiation beam (82.5 GHz to 1.1 THz), probing simultaneously the same gaseous sample. The THz channel records pure rotational lines of molecules for which the dipole moment was previously measured with high precision using Stark spectroscopy. This allows for a determination of the partial pressure in the gaseous mixture and enables absolute line intensities to be retrieved for the mid-IR range. This new instrument opens a new possibility for the retrieval of spectroscopic parameters for unstable molecules of atmospheric interest. The design and performance of the equipment are presented and illustrated by an example of simultaneous THz and mid-IR measurement on nitrous acid (HONO) equilibrium.

A novel high finesse cavity allowing THz CRDS has enabled accurate quantification of CF4. Yielding spectral parameters with unprecedented precision, these finding are critical to monitor and understand CF4's contribution to global radiative forcing.

A standard measuring gas cell used in absorption spectrometers is a cylinder enclosed by two transparent windows. The Fabry–Perot effects caused by multiple reflections of terahertz waves between these windows produce significant variations in the transmitted radiation intensity. Therefore, the Fabry–Perot effects should be taken into account to correctly measure absorption spectra in Bouguer law-based absorption spectroscopy. One approach to reducing the Fabry–Perot effects is based on inserting an additional external movable window with the standard measuring gas cell. This was proposed and numerically analyzed in our previous work. This paper is aimed at the experimental validation of this method when using amplitude modulation (AM) spectroscopy. Also, a comparison of the efficiency of reducing the Fabry–Perot effects using this method is experimentally compared to frequency modulation spectroscopy. The latter was shown to effectively reduce the Fabry–Perot effects compared to AM spectroscopy with the standard measuring gas cell, and the use of the external movable window was shown to further improve the elimination of Fabry–Perot effects.

The goal of this work was to update and significantly improve the line lists that have been generated recently for 11 µm bands of the trans- and cis- conformer forms of nitrous acid (HONO) [Armante R, Perrin A, Kwabia Tchana F, Manceron L. The ν4 bands at 11 μm: linelists for the trans- and cis- conformer forms of nitrous acid (HONO) in the 2019 version of the GEISA database. Molecular Physics 2021;120:e1951860]. That 2019 version of the 11 µm line list was generated using the spectroscopic parameters that were available, at that time, in the literature. During the present study, we used high-resolution Fourier transform spectra recorded at 11 µm to perform a large investigation of line positions and intensities for the ν4 bands of the trans- and cis-conformer of HONO. The resulting set of experimental ν4 (absolute) intensities and of 41 energy levels were used to determine, by least squares fit computations, improved position and intensity parameters for the ν4 bands of the trans- and cis-conformer of HONO. For trans-HONO, the ν4 band appeared not to be perturbed, while for cis-HONO a weak high order B-type Coriolis, coupling together the 41 and 61 energy levels was evidenced for the first time. This new list is of potential interest for the IASI-NG (Infrared Atmospheric Sounding Interferometer - New Generation) instrument which will be launched on board the METOP-SG satellite in 2025.

A high finesse cavity operating around 620 GHz has been constructed to improve the sensitivity limit for the measurement of weakly absorbing gases. An effective path length of 1 km is achieved significantly improving the available sensitivity. The measurement of the cavity decay time allows the quantification of the absorption coefficient.

This abstract presents a coherent THz spectroscopy experiment consisting in recording the free induction decay of a collection of carbonyde sulfide (OCS) molecules when polarized coherently by a similar to 0.29 THz incident pulse resonant with the J=24>23 OCS rotational line. This was accomplished by employing an optical-cavity-enhanced low-temperature-grown GaAs photoconductor, which was pumped by a free running dual-frequency Ti:Sa laser serving as both a THz pulsed emitter and an optoelectronics mixer. The success of this experiment can be attributed to the exceptional performance of our mixer, boasting a record-conversion efficiency of 30 dB, and the sustained long-term spectral purity of 100 kHz provided by the dual-frequency laser, lasting for over 10 seconds.

The development of cavity based infrared spectroscopy techniques has proved very successful, in particular cavity ring-down spectroscopy for sensitive measurements. The construction difficulty of high finesse cavities has hindered the application of this approach for THz frequencies. We have successfully demonstrated finesse values of 3500 by using a corrugated waveguide and photonic mirrors. This cavity which is 48 cm in length provides an interaction length of 1 km. A quantitative measurement of the absorption of a gas sample introduced into the cavity is made by THz Cavity Ring-Down Spectroscopy and applied to several gases and samples. Firstly, a pure gas that weakly absorbs the radiation at this frequency show that line strengths of 10(-27) cm(-1)/(molecule.cm(-2)) may be measured. A sub-ppm trace of HCN a particularly polar molecule with a strong THz signature has been examined and demonstrates a Limit Of Quantification (LOQ) of 3 ppb. Industrial samples extracted from a waste recovery facility have been measured and compared with a certified gas analyser. The results for SO2, NO2 and NO have been compared using the different techniques and show a good agreement.

Molecular lasers are powerful sources of continuous-wave THz radiation that can be used as local oscillators for heterodyne receivers. In this work we show that it is possible to measure precisely their emission frequency using a simple setup based on a THz frequency comb referenced to a microwave synthesizer. We show two examples: the first one is the measurement of the frequency of a NH 3 laser line around 2.029 THz and the second is the measurement of a D 2 O laser line around 2.450 THz.

The development of a high-finesse THz cavity has enabled the construction of a Fabry–Perot spectrometer. This approach can be applied to the THz frequencies (submillimeter wavelengths) allowing the rotational transitions to be probed. The intracavity power levels have permitted the sub-Doppler Lamb-dip profiles of ethanol to be observed. The quantification of a trace of H 2 S is demonstrated with a detection limit around 60 part per billion (ppb).

The first goal of this work is to improve the determination of the energy difference (& UDelta;ECis-Trans) between the ground vibrational state of the Cis- and Trans-HONO conformers of nitrous acid. For this, high resolution spectra were recorded in the 50-200 cm-1 spectral region at three different temperatures, 240, 270 and 296 K. The relative line intensities for the B-type transitions of pure rotational bands of Trans-HONO and Cis-HONO achieved from our measurements were combined in least squares fit computations to those measured previously by Sironneau et al. [Sironneau V, Flaud JM, Orphal J, Kleiner I, Chelin P. Absolute line intensities of HONO and DONO in the far-infrared and re-determination of the Energy Difference between the trans- and cis-species of nitrous acid. J Mol Spectrosc 2010;259:100-104]. In this way, we can significantly improve the accuracy on the HONO conformer energy difference, with a value for & UDelta;ECis-Trans = 95.8 & PLUSMN; 9.2 cm-1 compared to SIR & UDelta;ECis-Trans = 99 & PLUSMN; 25 cm-1 in the previous study of Sironneau et al. The second goal is to generate a line list with "absolute" line intensities for the pure rotational bands in the far infrared region of Trans-HONO and Cis-HONO, with both A- and B-type transitions. This new line list proved to be more robust for an improved detection of HONO in astrophysical objects [Coutens A, Ligterink NFW, Loison JC, Wakelam V, Calcutt H, Drozdovskaya MN, Jorgensen JK, Muller HSP, Van Dishoeck EF, Wampfler SF. The ALMA-PILS survey: First detection of nitrous acid (HONO) in the interstellar medium. Astronomy & Astrophysics 2019;623:L13].

By coating the inside of an overmoded, smooth wall metallic waveguide with a thin dielectric layer, one can obtain similar boundary conditions to corrugated waveguides and achieve extremely low transmission loss propagating the HE 11 mode. A theoretical model provides an analytic method for computing attenuation. The propagation mode of a prototype dielectric lined waveguide is characterized using low power transmission measurements to analyze the beam radiated from the waveguide aperture. Results indicate HE 11 mode purity of approximately 98%. Furthermore, we quantified attenuation with low power Fabry-Pérot measurements, verifying low loss performance of the experimental prototype. These results demonstrate that dielectric lined waveguides could have applications in many fields demanding low attenuation millimeter and terahertz transmission, such as radar, high-frequency communication systems, THz dynamic nuclear polarization, and electron cyclotron heating in magnetically confined fusion experiments.

Terahertz spectroscopy provides information on the motion of the charges in a sample at a picosecond scale. To recover this information from Terahertz time-domain spectroscopy (THz-TDS), one usually extracts the experimental refractive index then fits these curves. This approach suffers from several limitations, among them the difficulty to compare models of motions, provide the error bar associated with the extracted magnitude and a resolution limitation coming from the Fourier criteria of the fast Fourier transform. By adopting a Bayesian framework taking into account the experimental uncertainties and directly fitting the time-domain trace, we overcame these limitations. When correlated and epistemic uncertainties/noise are present, the algorithm considers its distribution as part of the data to fit and can mistake it for real physical features. Hence, it offers poor discrimination between good models and bad ones. After a thorough analysis of the experimental noise, we developed a preprocessing software removing epistemic noise on the time traces and providing an estimate of the noise correlation matrix (generalization of the standard deviation). It allows the proper weighting of the error function of the fit using these uncertainties and therefore the derivation of the Akaike information criteria, a metric enabling to calculate the most probable model from a set of models one wants to compare. In addition, by being in the time domain we avoid the Fourier criteria for the resolution and thus could get information on experimental lines down to 30 MHz with a commercial THz-TDS system.

Our team exploits the synchrotron radiation extracted by the AILES beamline of SOLEIL synchrotron facility to study different molecular physics problematics in the far-IR and THz spectral ranges. The existing experimental configuration of the beamline, with a high-resolution Bruker interferometer, allows the rotationally resolved spectra of a large number of molecules to be obtained. These absorption spectra cover a broad spectral range (typically between 1 and 30 THz, i.e., 30 and 900 cm -1 ) but are limited in terms of spectral resolution to 30 MHz (0.001 cm -1 ) by the ultimate optical path difference provided by the interferometer. To push the resolution limit further, we are developing a new spectrometer allowing to record sub-MHz resolution absorption spectra using heterodyne mixing of the synchrotron radiation with a THz molecular laser.