On the AILES beamline of the SOLEIL synchrotron, the HEROES consortium is currently developing new spectrometers based on heterodyne mixing of the THz synchrotron radiation with dedicated laser sources. We report here the first results on one of these spectrometers that aims at exploiting the discrete nature of coherent synchrotron radiation (CSR) in the 100-1000 GHz region, revealed a few years ago by our teamTammaro, S., Pirali, O., Roy, P., Lampin, J.F., Ducournau, G., Cuisset, A., Hindle, F., Mouret, G. "High density terahertz frequency comb produced by coherent synchrotron radiation" Nature Communications., 6: art. 7733. (2015) to perform dual-comb THz spectroscopy. CSR generated by the so called low-α mode of the SOLEIL machine produces a relatively intense, offset-free, high density frequency-comb in the THz range (THz-FC). We will present the details of our preliminary experimental set-up mixing the THz-FC from SOLEIL with an optical comb from Menlo C-fiber femtosecond laser. Pure rotation absorption transitions of acetonitrile in the frequency domain (covering the 100-500 GHz range) as well as time-domain free induction decays (FIDs) were observed allowing to establish the performances of this new instrument. Tammaro, S., Pirali, O., Roy, P., Lampin, J.F., Ducournau, G., Cuisset, A., Hindle, F., Mouret, G. "High density terahertz frequency comb produced by coherent synchrotron radiation" Nature Communications., 6: art. 7733. (2015),

We demonstrated high-resolution microsecond-time-resolved dual-comb spectroscopy of an important short-lived molecule, the simplest Criegee intermediate (CH₂OO). A dual-comb spectrometer with central wavelength near 8 μm was built by means of single-pass difference frequency generation between a near-infrared dual-comb source and a continuous-wave Tm-doped laser in an OP-GaP crystalRef: Opt. Lett., 45, 6791 (2020) By coupling the 8-μm dual-comb source into a multipass cell, the time-dependent absorption signals of CH₂OO and its reaction product (formaldehyde) can be simultaneously obtained upon 248-nm photolysis of the flowing mixtures of CH₂I₂/O₂. Furthermore, the pressure broadened spectra of CH₂OO were measured by employing spectrally interleaved dual-comb spectroscopy to derive the collisional broadening coefficient.

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Ref: Opt. Lett., 45, 6791 (2020).

Optical frequency combs provide broadband coverage while still being able to take full advantage of cavity-enhanced methods. The mid-infrared is highly desirable for molecular spectroscopy, where many strong fundamental vibrational bands exist. Optical frequency comb development in this region has lagged behind that of the near-IR. Interband Cascade Lasers are monolithic devices which can provide mid-infrared frequency combs in the C-H stretching region (3 μm) with high power per comb tooth and large repetition rates ( ∼ 9 GHz). Although they lack the octave-spanning coverage of fiber-based mode locked lasers, they can still provide sensitive multi-species detection. We have investigated different cavity enhanced schemes with these devices, including Vernier spectroscopy where we were able to resolve the individual comb teeth without the use of any diffraction optics. Here we discuss the development of these devices and their application towards cavity enhanced detection of trace molecules.

Optical frequency comb spectroscopy has proven a very useful tool for high resolution molecular spectroscopy. Frequency combs based on quantum cascade lasers (QCL) offer the possibility to easily explore the mid-infrared spectral range (4-12 μm), but suffer from very large repetition frequencies ( ∼ 10 GHz) which make them seemingly unsuitable for high resolution spectroscopy. Here, we present techniques to overcome this limitation. We have employed the combined advantages of high temporal ( < 4 μs) and high spectral resolution to measure the IR spectra of CF₄ and CHCl₂F in pulsed, skimmed supersonic beams. The low rotational temperature of the beams and the narrow expansion cone after the skimmer enabled the recording of spectra of cold samples with high resolution. The spectra cover the range from 1200 to 1290 cm⁻¹ and the narrowest lines have a full width at half maximum of 15 MHz, limited by the Doppler effect. The results demonstrate the potential of QCL dual-comb spectroscopy for broadband ( > 60 cm⁻¹) acquisition of spectra at high resolution ( < 15 MHz) and high sensitivity in the mid-infrared range. The power of the new technology is demonstrated by comparison with previous results on these molecules obtained by FTIR and diode laser spectroscopy of seeded cw and pulsed supersonic jets.^a a) M. Snels and M. Quack, J. Chem. Phys.1991,95,6355, M.Snels, V. Horka-Zelenkova, H. Hollenstein, M. Quack ,‘High resolution FTIR and diode laser spectroscopy of supersonic jets’, in Handbook of High-resolution Spectroscopy, Eds.; M. Quack and F.Merkt , Vol. 2, pp. 1022-1067, Wiley, Chichester, 2011; M. Caviezel, V. Horka, Z. Guennoun , G. Seyfang, M. Quack, unpublished manuscript (in preparation for Mol Phys.)

We use Fourier transform spectrometers based on mid-infrared frequency combs to measure and analyze broadband spectra of molecules of environmental importance in the two water windows around 3.3 μm and 7.8 μm. Both comb sources are based on difference frequency generation (DFG) using signal and pump waves derived from a single femtosecond laser, and are therefore inherently carrier-envelope-offset-free, which simplifies their frequency stabilization [1,2]. We use the sub-nominal sampling-interleaving method to measure spectra with resolution limited by the comb-mode linewidth [3,4]. At 3.3 μm, we measured high-resolution spectra of multiple absorption bands of two halogenated volatile organic compounds, methyl iodide, CH₃I [5], and dibromomethane, CH₂Br₂. We assigned the ν₄ band of CH₃I with improved accuracy compared to previous work based on FTIR measurements [6]. At 7.8 μm, we measured the spectrum of the ν₁ band of ¹⁴N₂¹⁶O and retrieved line center frequencies with precision of the order of 100 kHz [7], in excellent agreement with previous high-accuracy measurement using a comb-referenced continuous wave quantum cascade laser [8]. [1] G. Sobon et al., Opt. Lett. 42, 1748 (2017). [2] K. Krzempek et al., Opt. Express 27, 37435 (2019). [3] P. Maslowski et al., Phys. Rev. A 93, 021802(R) (2016). [4] L. Rutkowski et al., J. Quant. Spectrosc. Radiat. Transf. 204, 63 (2018). [5] I. Sadiek et al., J. Quant. Spectrosc. Radiat. Transf. 255, 107263, 107263 (2020). [6] R. Anttila et al., J. Mol. Spectrosc. 119, 190 (1986). [7] A. Hjältén et al., in preparation. [8] B. AlSaif, et al., J. Quant. Spectrosc. Radiat. Transf. 211, 172 (2018).

There is an increasing demand for precise molecular spectroscopy, in particular in the mid-IR fingerprint window, whether it be for modelling our atmosphere, interpreting astrophysical spectra or testing fundamental physics. Here, we present a new technology for ultra-high-resolution vibrational spectroscopy using quantum cascade lasers (QCLs) traceable to primary standards Santagata, R., et al., Optica 6, no. 4 (2019): 411-423. Using an optical frequency comb, a 10 μm QCL is stabilized at the sub-Hz level to an ultra-stable near infrared reference signal operated at the French metrology institute. This signal is calibrated there to some of the best atomic clocks and transferred through a noise-compensated 43-km long fiber cable. This results in a record  0.1 Hz QCL linewidth. We have also developed a method to continuously scan the stabilized QCL over 1.5 GHz at the precision of the frequency reference. We have used the apparatus to carry out saturated absorption spectroscopy in a compact multipass cell. We have measured absolute frequencies (as well as frequency shifts and pressure broadening) of osmium tetroxyde, methanol, ammonia and trioxane, with record uncertainties, from 10 Hz to 10 kHz (depending on the species and the details of the apparatus). We were able to resolve yet unreported subtle structures in methanol (doublets and triplets induced by the combined effects of tunnelling and asymmetry). We have recently added to our spectrometer a 3 meter-long Fabry-Perot resonator to perform cavity enhanced measurements in order to improve our resolution and extend saturated absorption spectroscopy to more complex molecules. First results will be presented.

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Santagata, R., et al., Optica 6, no. 4 (2019): 411-423..

Dual-comb spectroscopy (DCS) with quantum cascade lasers (QCL) offers direct access to the mid-infrared region and, thus, to the strong and characteristic ro-vibrational absorption bands of many molecules of interest. In DCS, two frequency comb lasers with unequal repetition frequency produce an array of beat notes which carry the information about magnitude and phase of the optical frequency components that generate them. A large beat note spacing is a requisite if the two sources are free-running, since the beat notes will exhibit a width determined by the (uncorrelated) phase noise of the two lasers. This results in a relatively large acquisition bandwidth, and thus requires a correspondingly large sampling rate. Furthermore, since co-averaging of interferograms in free-running systems is not possible, data processing is computationally intensive, leading to a low duty cycle (time spent acquiring data versus total time) around 1%. We have developed data processing routines that extract all the relevant spectroscopic information from the measured interferograms in less than the duration of the interferogram itself, paving the way for continuous acquisition with on-the-fly processing (100% duty cycle). The algorithm can equally handle static measurements, where all the comb line frequencies are constant over time, step sweep measurements, where the frequencies of the combs are shifted step-wise, and rapid sweep measurements, where the frequencies of the combs are shifted in a continuous and periodic fashion. In the last two approaches, the various offset spectra are interleaved to produce a composite spectrum with smaller point spacing. This algorithm represents an important advance towards high-sensitivity, high-resolution DCS measurements with QCLs, especially when combined with continuous scans and spectral interleaving, which produce large amounts of raw data.

We report a proof-of-principle demonstration of intracavity pump-probe, cavity ringdown (CRD) detection in a three-mirror, traveling-wave cavity. With both cavity-enhanced pump power and probe absorption pathlength, the technique is a generalized, high-sensitivity, high-selectivity trace detection method. In our experiments, the pump radiation is switched off during every other probe ringdown, which allows uncorrelated measurements of analyte and background cavity decay rates. The net, two-color signal from the difference between the pump-on and pump-off decay rates is immune to empty cavity ringdown fluctuations and spectral overlaps from non-target molecular transitions. The immunity to the ringdown fluctuations allows longer signal averaging, and thus higher detection sensitivity. The ability to compensate for the unwanted, background molecular absorption greatly enhances the detection selectivity in spectrally congested regions. While its application is not limited to a specific spectral range, the technique is well-suited for trace detection in the mid-IR region, where pump-probe schemes based on strong ro-vibrational transitions can be applied. In this work, our two-color CRD detection is implemented on a ladder-type, three-level system based on the N₂O, ν₃ = 1←0, P(19) (pump) and ν₃ = 2←1, R(18) (probe) ro-vibrational transitions. By frequency-locking two quantum cascade lasers to the p-polarization (pump, Finesse = 5280) and s-polarization (probe, Finesse = 67700) modes of the ring-cavity, we achieve high intracavity pump power (36 W) and high probe ringdown rates ( > 2 kHz). The observed two-color CRD spectra are simulated based on a density-matrix, three-level-system model which is solved under the constraints of the cavity resonance conditions. In addition to its background compensation capability, experimental flexibilities in the selection of pump-probe schemes and signal insensitivity to intracavity laser power are further features which enhance the general utility of our technique for mid-IR trace detection.

Measuring the rotational spectra of a molecule in excited vibrational modes can be challenging due to the low thermal population of these rotational levels and the weakness of the underlying absorption lines. We recently demonstrated a quantum cascade laser (QCL)-pumped molecular laser (QPML) concept in which virtually any rotational transition of any excited vibrational mode of virtually any molecule with a permanent dipole moment and a vapor pressure can be made to lase by pumping its corresponding ro-vibrational transition with a frequency-tunable QCL. Here, we show how the frequency of those QCL-pumped rotational transitions in excited vibrational modes may be measured with accuracy comparable to transitions in ground vibrational levels using high precision modulation spectroscopy techniques. We applied this concept to nitrous oxide (N₂O), and measured the individual frequencies of 20 lines in the v3 vibrational mode (2225 cm⁻¹ ≈ 10 kT) by means of heterodyne spectroscopy with an experimental uncertainty of 200 kHz when lasing and without modulation, improving to 50 kHz with QCL-enhanced absorption or amplification with modulation. We report the measured spectra and the fitted rotational constants B₃ and DJ₃ corresponding to this vibrational mode.

We report progress on the construction and development of a cryogenic buffer gas beam (CBGB) instrument for studying the IR spectroscopy of atomic and molecular clusters. Clusters are produced via laser ablation of a solid target inside the CBGB cell, and the nascent clusters are quickly cooled to the buffer gas temperature before exiting the CBGB cell. Upon exiting the cell, the clusters are probed with  ∼ 5 μm tunable light from a quantum cascade laser (QCL). Light from signal and reference beams each hit detectors, and the difference signal is collected using an autobalanced subtractor circuit to achieve shot-noise limited measurements. Recent developments in the construction of the instrument and spectra measured will be presented.

The SMAUG apparatus (Spectroscopy of Molecules Accelerated in Uniform Gas flows) was developed to produce hot spectra of various molecules of interest for hot astrophysical atmospheres, like the one surrounding hot Jupiters, reaching up to 2500 K. High-temperature IR spectroscopic data is needed to retrieve temperature and concentration profiles from astronomical spectra. A small dimension Laval nozzle connected to a compact high enthalpy source equipped with cavity ringdown spectroscopy (CRDS) is used to produce high-resolution infrared spectra of polyatomic molecules in the 1.67 μm region.^a. The experimental setup can operate according to two complementary working regimes: non-local thermodynamic equilibrium (non-LTE) (vibrationally hot and rotationally cold) and LTE conditions, to interpret the complex pattern of highly-excited vibrational states. Two different gases, carbon monoxide (CO) and methane (CH₄) were used as test molecules. Using non-LTE conditions vibrational (T_vib) and rotational (T_rot) temperatures were extracted from the recorded infrared spectrum leading to T_vib = 1346(52) K and T_rot = 12(1) K for CO. A rotational temperature of 39(3) K was measured for CH4, while two vibrational temperatures were necessary to reproduce the observed intensities. The population distribution between vibrational polyads was correctly described with T_vib,I = 825(50) K" , while the population distribution within a given polyad was modelled correctly by T_vib,II = 39(5) K" , testifying to a more rapid vibrational relaxation between the vibrational energy levels constituting a polyad. Using a “post-shock CRDS” technique CO and CH₄ LTE spectra were recorded at 950 K and 1400 K.
^aE.Dudás et al. High-temperature hypersonic Laval nozzle for non-LTE cavity ringdown spectroscopy. J Chem Phys 2020;152:134201. https://doi.org/10.1063/5.0003886.

Cavity enhancement of molecular absorption through laser action has made intracavity laser spectroscopy (ILS) a powerful tool for the detection of trace species and weakly absorbing molecules. The effective pathlength for ILS measurements is proportional to the speed of light, leading to a high degree of sensitivity for spectroscopic measurements, and effective pathlengths of up to 70,000 km have been demonstrated with this technique. Fourier-transform spectroscopy (FTS) is a powerful technique for detection of spectroscopic signals, benefitting both from Fellgett’s advantage and inherent calibration derived from the interference zero crossings of a single frequency light source. The traditional dispersive ILS system at the University of Missouri – St. Louis has been integrated with a Bruker IFS-125M FTS spectrometer. The two time-based techniques are synchronized using a National Instruments field-programmable gate array (FPGA). The maximum instrumental resolution for the combined technique is improved by nearly an order of magnitude, from 0.02 cm⁻¹resolution for 2 m monochromator with 9th order diffraction to 0.0035 cm⁻¹with the Bruker FTS. Similarly, the detection bandwidth (7 cm⁻¹per dispersed ILS spectrum) also has improved by an order of magnitude,l0ptFigure enabling the collection of the entire broadband laser profile (50-100 cm⁻¹) in a single measurement. In addition, a 3-fold improvement in absolute wavenumber accuracy is achieved due to the internal calibration of the FTS detection. The improved resolving power of the integrated spectroscopic system enables Doppler-width limited detection of 5d-metal diatomic molecules. These species are of fundamental interest for the quantitation of the relativistic effects that dominate their electronic energy structure, and newly observed transitions of platinum sulfide and tungsten sulfide have already been recorded and analyzed. Details of the instrumentation, integration method, and improved capabilities will be presented.

Owing to their narrow linewidth and broad tunability, continuous wave (CW) quantum cascade lasers (QCLs) have become ubiquitous for mid-infrared rovibrational molecular spectroscopy. However, while the 4-11 μm region is fairly well covered by available room-temperature QCLs, the extension to longer wavelengths is a challenge of great interest for molecular spectroscopy. Here we present spectroscopy measurements using a newly fabricated room temperature CW QCL working at 17 μm, a new spectral region for QCLs H. Nguyen Van, et al., “Long Wavelength (λ > 17 μm) Distributed Feedback Quantum Cascade Lasers Operating in a Continuous Wave at Room Temperature”, Photonics 6, 1, pp. 31 (2019). and delivering a few mWs of output power. We will describe our characterization of this laser, including measurements of its spectral range, frequency noise and linewidth. To do this, we park the laser on the side of a strong N₂O absorption line which we use as a frequency-to-intensity converter. The frequency noise spectrum of the source is deduced from the measured intensity fluctuations. We have also demonstrated the QCL's capabilities by performing linear rovibrational absorption spectroscopy of the ν₂ fundamental mode of N₂O in a single-pass cell, covering the 580 to 582.5 cm⁻¹ spectral region. Doppler-limited and pressure-broadened lines are measured with various N₂O pressures in the range from 1 to 500 Pa leading to a precise investigation of pressure broadening in this mode. We will finally discuss the potential of this source for precise measurements in ultra-cold diatomic species and for probing low energy vibrations in polyatomic molecules. H. Nguyen Van, et al., “Long Wavelength (λ > 17 μm) Distributed Feedback Quantum Cascade Lasers Operating in a Continuous Wave at Room Temperature”, Photonics 6, 1, pp. 31 (2019).,