An optically stabilized dual-comb system is used to drive a mid-infrared spectroscopy for parallel gas-phase detection in the functional group region from 2800-3600 cm⁻¹. The system comprises two amplified ultra-low noise fiber frequency combs (FC-1500-ULN from Menlo Systems), centered at 1550 nm wavelength, which are spectrally broadened to the mid-IR by two chip-based \textSi₃ \textN₄ waveguides. The probe comb passes through a gas-cell containing CH₄ (430 ppm), C₂H₂ (420 ppm), and N₂ as a buffer gas and is eventually superimposed with the reference comb on a photodetector. The dual-comb spectrum below reveals the corresponding absorption features, with sufficient resolution to even resolve isotopologues. The present absorption feature of H₂O is due to interaction with water vapour during free-space propagation. i0pt Figure

r0pt Figure We present a new approach to mid-infrared dual-comb spectroscopy. Strong fundamental ro-vibrational transitions are interrogated in the mid-infrared 3-μm region, while the detection is performed in the near-infrared telecommunication region where sensitive opto-electronic tools are available. Using difference-frequency generation, a near-infrared comb is converted to the range of 2700-3400 cm⁻¹, where it interacts with the sample before being converted back to the telecommunication region. There, it beats with a second comb of slightly different line spacing for multiheterodyne detection. The broadband spectra obtained within arbitrarily long averaging time show resolved comb lines, a frequency scale calibrated within the accuracy of an atomic clock and a negligible contribution of the instrument line shape, as in previous reports using our recent scheme of feed-forward stabilizationZ. Chen, M. Yan, T. W. Hänsch, and N. Picqué, A phase-stable dual-comb interferometer, Nat Commun 9, 3035 (2018).^,Z. Chen, T. W. Hänsch, and N. Picqué, Mid−infrared feed−forward dual−comb spectroscopy, Proc Natl Acad Sci USA 116, 3454−3459 (2019). A spectrum (Fig. a, expanded view on a single comb line on the radio−frequency scale in Fig.b) in the region of the Q −branch of the ν₃ band of ^12CH_4, is measured within 1000 s. The molecular profiles are sampled by the comb at a resolution of 3.3 10^-3 Z. Chen, T. W. Hnsch, and N. Picqu, Mid-infrared feed-forward dual-comb spectroscopy, Proc Natl Acad Sci USA 116, 3454-3459 (2019)..

Cavity ring-down spectroscopy (CRDS) is a widely used tool for trace gas sensing and molecular lineshape studies which involves the use of high-finesse optical cavities to provide long effective pathlengths and high spectral resolution. Here, we present a novel implementation of CRDS where the probe laser is phase locked to a self-referenced octave-spanning optical frequency comb referenced to a Cs clock, and in which the optical cavity is subsequently locked to the stabilized probe laser beam. This approach provides an absolute frequency axis and increased coupling efficiency. It allows for frequency steps of arbitrary size to be made, which can be as small as the order of the linewidth of the stabilized probe laser. The optical cavity follows tunable optical sidebands of the probe laser generated with an electro-optic modulator. This allows for up to 40 GHz of tuning in a single spectral scan, with spectral intervals as small as 200 kHz. The optical cavity is locked to the stabilized probe laser by inducing a slight axial dither (20 kHz modulation amplitude in the optical domain) on a piezo-driven cavity mirror. The resulting transmitted probe beam generates an error signal which can be used to center the piezo offset voltage with a low-bandwidth lock. The absolute frequency uncertainty of the locked probe laser is 1 kHz on a timescale of several hours, which is well in excess of the measurement time. We present a variety of measurements that highlight the power of this technique. Line positions and pressure shifting coefficients can be determined with nearly an order of magnitude smaller uncertainty by comparison to those obtained using conventional FS-CRDS measurements. We apply this technique to several H₂O and CO₂ transitions in the 1.6 μm wavelength region and report standard uncertainties in line positions as low as 20 kHz.

We have been studied the dynamics of polyaromatic hydrocarbons by high-resolution spectroscopy with a supersonic molecular beam. The target molecules are 9-methylanthracene, 1,2-benzanthracene, perylene, and so on. For the rotationally-resolved spectra of these molecules, the determination of the frequency axis is a crucial issue [1]. In this study, we developed a frequency control system of a CW Ti:Sapphire laser disciplined by an Er-doped fiber optical frequency comb. The frequency of the Ti:Sapphire laser is scanned over several GHz with the uncertainty of about 15 kHz. Or the Ti:Sapphire laser frequency is fixed to an arbitrary single value for a long time interval. Now we try to observe high-resolution spectra of polyaromatic hydrocarbons with this system. [1] A. Nishiyama, A. Matsuba, and M. Misono, Opt. Lett. 39, 4923 (2014).

Multidimensional spectroscopy has been shown to be a powerful tool to study dynamics of complex systems in the condensed phase. However, 2D spectroscopy in the gas phase, specifically of dilute species in molecular beams, has yet to be realized. There are many complex systems, such as small clusters or transient intermediates, for which the added information from 2D spectroscopy would aid in the understanding of structures and dynamics. We use the unique properties of frequency comb lasers to improve multidimensional spectroscopy with the goal of ultrafast, 2D-spectroscopy of dilute species in molecular beams. First, we are creating a novel 2D spectrometer utilizing a homebuilt Yb-fiber frequency comb laser and an electro-optic modulator-based frequency comb. Inspired by dual-comb spectroscopy, this converts the signal from optical to radio frequencies via heterodyne detection and eliminates the need for a traditional spectrometer. A second benefit of using frequency comb lasers is that the ultrafast pulses can be coupled into enhancement cavities, greatly increasing the sensitivity of the technique. By improving the sensitivity, ultrafast 2D spectroscopy of dilute species in molecular beams will be possible for the first time. Current progress towards cavity-enhanced 2D spectroscopy will be discussed.

The design and performance characteristics of an echelon-based single shot visible/near-infrared/far-infrared spectrometer are presented. The spectrometer can measure both the nonlinear optical and terahertz Kerr effects in neat molecular liquids. Tens of picoseconds of molecular information can be recorded at a time, with adequate sensitivity to measure molecular responses in just a few milliseconds of experimental time. The signal-to-noise performance was found to scale favorably with respect to the standard stage scan technique. These results demonstrate the viability of stage-free nonlinear Kerr effect measurements and provide a route for improvements to the speed of future multidimensional Kerr effect studies.

Strong-field coherence breaking (SFCB)A.O Hernandez-Castillo, Chamara Abeysekera, Brian M. Hays, Timothy S. Zwier, “Broadband Multi-Resonant Strong Field Coherence Breaking as a Tool for Single Isomer Microwave Spectroscopy.” J. Chem. Phys. 145, 114203 (2016).nd other multi-pulse techniques provide promise for greatly accelerating the rate at which complex spectra can be assigned. Last year, I presented on the use of SFCB for simplifying the spectra of allyl chloride.Riffe, Erika; Zwier, Timothy S.; Hernandez-Castillo, Alicia O.; Fritz, Sean; Shipman, Steven; Johnson, Erika. “The Conformer Specific Room-Temperature Rotational Spectrum of Allyl Chloride Utilizing Strong Field Coherence Breaking, International Symposium on Molecular Spectroscopy,” 73rd International Symposium on Molecular Spectroscopy, Talk TI11 (2018).ow, I present an update to the use of SFCB and other multi-pulse techniques for analysing rotational spectra collected at and near room temperature, using the molecules 2-chloroethyl methyl ether (2-CEME) and 1,2-epoxybutane as test cases. The use of these techniques and their significance in the simplification of complex spectra will be discussed.

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A.O Hernandez-Castillo, Chamara Abeysekera, Brian M. Hays, Timothy S. Zwier, “Broadband Multi-Resonant Strong Field Coherence Breaking as a Tool for Single Isomer Microwave Spectroscopy.” J. Chem. Phys. 145, 114203 (2016).a Riffe, Erika; Zwier, Timothy S.; Hernandez-Castillo, Alicia O.; Fritz, Sean; Shipman, Steven; Johnson, Erika. “The Conformer Specific Room-Temperature Rotational Spectrum of Allyl Chloride Utilizing Strong Field Coherence Breaking, International Symposium on Molecular Spectroscopy,” 73rd International Symposium on Molecular Spectroscopy, Talk TI11 (2018).N

Two-photon absorption (2PA) spectroscopy measures the simultaneous absorption of two photons, where the combined energy of the two photons corresponds to an allowed transition of a molecule. 2PA is a powerful spectroscopic tool that can probe the electronic structure of a molecule by accessing different electronic states, than in one-photon absorption (1PA) spectroscopy because 1PA and 2PA are governed by different selection rules. Many applications rely on nonlinear two-photon excitation, including 3D fluorescence imaging and photo-dynamic therapy; however, broadband 2PA spectra and accurate absolute cross-sections are rarely reported in the literature. Our broadband 2PA measurements use a femtosecond pump-probe technique that is uniquely able to fill this need by measuring continuous 2PA spectra and accurate absolute 2PA cross-sections using stimulated Raman scattering (SRS) as an internal standard. Our broadband 2PA technique also recovers additional information based on the relative polarization between the two photons, giving insight into the symmetry of the excited electronic states of a molecule.We report 2PA spectra for coumarin 153 in several solvents, for Mn and Re coordination compounds and for pure liquid benzene.

Our consortiumANR "HEROES: HEterodyne Receivers OptimizEd for Synchrotron sources", Grant number 16-CE30-0020-03s currently developing a new spectrometer on the AILES beamline of the SOLEIL synchrotron facility to achieve sub-MHz resolution in the THz and far-IR regions. This spectrometer is based on heterodyne mixing of the far-IR synchrotron radiation with various local oscillators (LOs). In past years, we used a frequency multiplication chain to provide LO frequencies which enabled both a deep characterization of the spectral composition of the synchrotron emissionTammaro, 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, 7733 (2015)nd the recording of its first Doppler limited absorption lines (of D₂O)talk WI02, ISMS 2018 We recently improved our set-up and measured absorption lines using a far-IR molecular laser pumped by a 10 μm QCL as the LO. The principle of the spectrometer, together with the first experimental results, will be presented in the talk.

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ANR "HEROES: HEterodyne Receivers OptimizEd for Synchrotron sources", Grant number 16-CE30-0020-03i 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, 7733 (2015)a talk WI02, ISMS 2018.

Our collaborative team is developing new experimental set-ups based on heterodyne mixing of synchrotron radiation (extracted by the AILES beamline of SOLEIL) with various Local Oscillators (LOs). In the sub-millimeter and THz regions (defined as 0.1-1 THz), LOs from electronic techniques are easy to implement through the use of multiplication chains. However, it is more challenging to produce fixed LOs in the far-IR domain (1-6 THz). The recent development of a new generation of molecular lasers pumped by 10 μm QCLsPagies et al., APL Photonics 1 (2016)llows us to generate many more far-IR frequencies than the previous approach which used CO₂ lasers as a pump source. The difficulty in this technique stems from selecting the proper far-IR transitions that both involve rotational states susceptible to lase and that are accessible with our 10 μm QCL source. We will present our program making use of HITRAN, JPL, CDMS, and ExoMol databases to produce lists of far-IR lasing frequencies.

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Pagies et al., APL Photonics 1 (2016)a