Visible and near-IR optical frequency combs have become key metrological tools for atomic and molecular spectroscopy, in the last 20 years. Their extension to the mid-IR and THz spectral ranges required appropriate and non-trivial nonlinear techniques for down-conversion. Such developments significantly improved the accuracy of frequency measurements across vibrational and rotational spectra of molecules, thus enormously enlarging the quality and application range of spectroscopic measurements. Very recently, Quantum Cascade Lasers have shown a great potential for comb generation in spectral intervals chosen by design with all-in-one miniature devices. I will discuss progress in mid-IR and THZ combs for application to high resolution molecular spectroscopy and I will show very recent results for QCLs comb generation and control.

High-precision measurements with molecules may refine our knowledge of various fields of physics, from atmospheric and interstellar physics to the standard model or physics beyond it. Many of them can be cast as absorption frequency measurements, particularly in the mid-infrared molecular fingerprint region, creating the need for narrow-linewidth lasers of well-controlled frequency. We present a new technology for precision vibrational spectroscopy using quantum cascade lasers (QCLs)Argence et al., Quantum cascade laser frequency stabilization at the sub-Hz level, Nature Photon. 9, 456 (2015). Via an optical frequency comb, a QCL is stabilized at the sub-Hz level on an ultra-stable near infrared reference signal provided by the French metrology institute (SYRTE) to our laboratory and transferred through a 43-km long fiber cableW.-K. Lee et al., Hybrid fiber links for accurate optical frequency comparisons, Appl. Phys. B 123, 161 (2017). The stability of the reference is transfered to the QCL, which therefore exhibits a relative frequency stability lower than 2×10⁻¹⁵ between 1 and 100 s. Moreover its absolute frequency is known with an uncertainty below 10⁻¹⁴ thanks to the traceability to the primary standards of SYRTE. The setup allows the QCL to be widely scanned over  ∼ 1 GHz while maintaining the highest stabilities and accuracies. We report saturated absorption spectroscopy investigations conducted around 10 μm on osmium tetroxyde in a Fabry-Perot cavity and methanol in a multipass cell at low pressures ranging from 0.01 to 10 Pa, allowing central frequencies, pressure shifts and broadenings to be determined with record uncertainties. By combining wide tuneability, high sensitivity and resolution, this setup constitutes a key technology for precise spectroscopic measurements with molecules.

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Footnotes:

Argence et al., Quantum cascade laser frequency stabilization at the sub-Hz level, Nature Photon. 9, 456 (2015).. W.-K. Lee et al., Hybrid fiber links for accurate optical frequency comparisons, Appl. Phys. B 123, 161 (2017)..

Time-resolved vibrational spectroscopy is an important tool for understanding biological processes and chemical reaction pathways [1]. Today, all available methods to our knowledge require many repetitions of an experiment to acquire a microsecond time-resolved mid-infrared spectrum. We present the IRspectrometer, a quantum cascade laser dual frequency comb spectrometer [2-3]. It allows for parallel acquisition of hundreds of mid-infrared wavelengths with microsecond time resolution. The formation of the light-activated L, M and N-states in bacteriorhodopsin – which only have μs to ms lifetimes – has been recorded by analyzing the infrared response of bacteriorhodopsin to 10 ns visible light pulses with sub-microsecond time-resolution. The different wavelengths were all measured in parallel thanks to the dual-comb approach. The spectra as well as the kinetics show good agreement with those from step-scan FT-IR measurements. As a benchmark, the spectral signature of several intermediate states of the bacteriorhodopsin photocycle has been recorded in a single shot measurement. This approach greatly reduces the complexity of time-resolved spectroscopy measurements in the mid-infrared which currently require many repetitions. References [1] Ritter, E. et al. “Time-Resolved Infrared Spectroscopic Techniques as Applied to Channelrhodopsin.” Front. Mol. Biosci. 2:38. [2] Hugi, A. et al. “Mid-Infrared Frequency Comb Based on a Quantum Cascade Laser.” Nature 492: 229–33 [3] Villares, G. et al. “Dual-comb spectroscopy based on quantum-cascade-laser frequency combs.” Nat. Commun. 5:5192

Ultrafast optical spectroscopy methods, such as transient absorption spectroscopy and 2D spectroscopy, are typically restricted to optically thick samples, such as solids and liquid solutions. We have developed a technique, Cavity-Enhanced Ultrafast Spectroscopy, to study dynamics in a molecular beam with femtosecond temporal resolution. By coupling frequency combs into optical cavities, we previously demonstrated ultrafast transient absorption measurements with a detection limit of ∆OD = 2 ×10⁻¹⁰ (10⁻⁹ /√{Hz}).M. A. R. Reber, Y. Chen, and T. K. Allison, Optica 3, 311 (2016)n this talk, I will present a widely tunable version of this spectrometer operating at probe wavelengths between 450 and 700 nm (8000 cm⁻¹) using only one set of dispersion managed cavity mirrors. The tunable probe comb is generated using an intracavity doubled optical parametric oscillator. I will discuss the technical details of this spectrometer and its application to the dynamics of excited state intramolecular proton transfer (ESIPT) in jet-cooled molecules and clusters.

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Footnotes:

M. A. R. Reber, Y. Chen, and T. K. Allison, Optica 3, 311 (2016)I

Most spectroscopic data available in databases such as HITRAN are retrieved from FTIR measurements and suffer from uncertainties at the MHz level. Much more accurate data, by up to four orders of magnitude, can be achieved using an optical frequency comb to calibrate the frequency axis of a cw laser source. Actually, in the mid-infrared region, at least beyond 5 μm, the only available commercial solution for a widely tunable cw laser is represented by extended cavity quantum cascade lasers (EC-QCLs), whose locking to an optical frequency comb has been so far inhibited by a large amount of frequency noise, leading to linewidths of about 20 MHz Knabe K., Williams P. A., Giorgetta F., Armacost C. M., Crivello S., Radunsky M., and Newbury N., Opt. Express 20, 12432-12442 (2012) In this work we overcome this limitation and describe a spectrometer that relies on the frequency locking of an EC-QCL tunable in the 7.55-8.2 μm range to a 1.9 μm Tm fiber comb Lamperti M., Alsaif B., Gatti D., Fermann M., Farooq A., and Marangoni M., Sci. Rep. 8,1292 (2018) It is applied to the first comb-calibrated direct characterisation of the ν₁ fundamental band of N₂O, specifically of nearly 70 lines in the 1240 – 1310 cm⁻¹range, from P(40) to R(31). The spectroscopic constants of the upper state are derived from a fit of the line centers with an average rms uncertainty of 4.8x10⁻⁶ cm⁻¹(144 kHz). The coupling of the spectrometer to a high-finesse optical cavity to the purpose of enhancing its sensitivity and addressing weaker absorbers, is also discussed. Knabe K., Williams P. A., Giorgetta F., Armacost C. M., Crivello S., Radunsky M., and Newbury N., Opt. Express 20, 12432-12442 (2012). Lamperti M., Alsaif B., Gatti D., Fermann M., Farooq A., and Marangoni M., Sci. Rep. 8,1292 (2018).

Optical feedback frequency stabilized cavity ring-down spectroscopy is a very high resolution (sub kHz) and sensitive technique (2× 10⁻¹² cm⁻¹limit of detection). Both aspects will be emphasized through the two followingh applications: To illustrate the high resolution, Doppler-free saturated-absorption Lamb dips were measured at sub-Pa pressures on rovibrational lines of H₂O near 7180 cm⁻¹, The saturation of the considered lines was so high that at the early stage of the ring down, the cavity loss rate remained unaffected by the absorption. By referencing the laser source to an optical frequency comb, transition frequencies were determined down to 100 Hz precision and kHz accuracy. To highlight the high precision and stability of the instrument, we will present the first optical absorption measurements of O-17 anomalies in CO₂ with a precision better than 10 ppm, matching the requirements for paleo-environmental applications.

We applied a feed-forward frequency control scheme to establish a phase-coherent link from an optical frequency comb to a distributed feedback (DFB) diode laser: This allowed us to exploit the full laser tuning range (up to 1 THz) with the linewidth and frequency accuracy of the comb modes. The approach relies on the combination of an RF single-sideband modulator (SSM) and of an electro-optical SSM, providing a correction bandwidth in excess of 10 MHz and a comb-referenced RF-driven agile tuning over several GHz. As a demonstration, we obtain a 0.3 THz cavity ring-down scan of the low-pressure methane absorption spectrum. The spectral resolution is 100 kHz, limited by the self-referenced comb, starting from a DFB diode linewidth of 3 MHz. To illustrate the spectral resolution, we obtain saturation dips for the 2ν₃ R(6) methane multiplet at μbar pressure. Repeated measurements of the Lamb-dip positions provide a statistical uncertainty in the kHz range.

Fabry-Perot cavities provide high sensitivity to molecular absorption and dispersion since the mode position, width and amplitude are modified in the vicinity of molecular transitions. Moreover, the mode shift and broadening are directly proportional to the real and imaginary parts of the molecular complex index of refraction, but independent of cavity parameters, such as the cavity length and mirror reflectivity, which reduces the influence of systematic errors. Previous demonstrations of cavity enhanced complex refractive index spectroscopy were based on continuous wave lasers and limited to individual absorption linesCygan, A., et al., Opt. Express 21, 29744-29754 (2013).^,Cygan, A., et al.,Opt. Express 23, 14472−14486 (2015. Here we use an Er:fiber frequency−comb−based Fourier transform spectrometer with sub−nominal resolution Masłowski, P., et al., Phys. Rev. A 93, 021802 (2016).^,Rutkowski, L., et al., Opt. Express 25, 21711-21718 (2017).o measure a broadband transmission spectrum of a cavity filled with 1% of CO₂ in N₂ at 750 Torr. From Lorentzian fits to each cavity mode we retrieve mode positions and widths, which in turn yield high precision dispersion and absorption spectra of the entire 3ν₁+ν₃ absorption band of CO₂ at 1.6 μm. Fits to these spectra yield line intensities that agree to within 0.6%. Thus comb-based Fourier transform spectroscopy enables broadband cavity mode characterization and calibration-free determination of both the real and imaginary parts of entire molecular absorption bands with high accuracy and precision. Cygan, A., et al., Opt. Express 21, 29744-29754 (2013).\end Cygan, A., et al.,Opt. Express 23, 14472−14486 (2015..\end Masłowski, P., et al., Phys. Rev. A 93, 021802 (2016). Rutkowski, L., et al., Opt. Express 25, 21711-21718 (2017).t

Two relatively new spectroscopic techniques suitable for implementation in a broadband FTS are cavity mode-width spectroscopy and cavity mode dispersion spectroscopy [1,2]. They require scanning the cavity resonances to obtain information about their width and position, yielding information about molecular absorption and dispersion. Previously they used continuous wave lasers, showing signal-to-noise ratio and resolution similar to the well-established cavity ring-down spectroscopy. However in this implementation they shared the same limits of measurement range and relatively slow acquisition. Meanwhile the optical frequency comb-based cavity-enhanced FTS with sub-nominal resolution [3,4] is a perfect match for those methods, allowing for simultaneous measurement of thousands of cavity modes without the limit of cavity dispersion and the requirement of a reference measurement [5]. Here we present the measurements of 10 kHz HWHM cavity resonances, which are some of the narrowest features ever measured by the FTS, from which we derive the absorption and dispersion spectra of the 0-3 band of CO in Ar.

1. A. Cygan et al., Opt. Express 21-24, 29744-29754 (2013).
2. A. Cygan et al., J. Chem. Phys. 144, 214202-11 (2016).
3. P. Maslowski et al., Phys. Rev. A 93, 021802(R) (2016).
4. L. Rutkowski et al., Journal of Quantitative Spectroscopy and Radiative Transfer 204, 63-73 (2018).
5. L. Rutkowski et al., Opt. Express 25-18, 21711-21718 (2017).

Cavity mode-width spectroscopy (CMWS) [1] and cavity mode-dispersion spectroscopy (CMDS) [2] techniques provide a way to simultaneously determine absorption and dispersion of a sample in an optical cavity. It was shown recently that CMDS can also be efficiently combined with optical frequency combs (OFCs) [3] to perform dispersion measurements in a broad spectral range. In this work, we utilize a near-infrared frequency comb and a VIPA spectrometer to retrieve absorption and dispersion of an atmospheric pressure CO-N₂ sample in a high-finesse cavity, by measuring shapes and positions of 7-kHz-wide cavity modes at Hz-level precision. A Pound-Drever-Hall lock of a CW r0pt Figure laser to the cavity and a phase-lock of the OFC to the CW laser allow for arbitrary detuning between comb and cavity modes, while cavity length stabilization to a Rb frequency standard provides absolute frequency scale. The signal-to-noise ratios for CMWS and CMDS spectra were, respectively, 190 and 380. To the best of our knowledge, the 7-kHz-wide cavity resonances shown in this work are the narrowest spectral features measured directly with a frequency comb. The presented technique is capable of fast acquisition and ultrahigh resolution in a broad spectral range, which makes it particularly suitable for spectroscopy of cold molecules or monitoring of chemical kinetics.
[1] A. Cygan et al, Opt. Express 21, 29744 (2013).
[2] A. Cygan et al, Opt. Express 23, 14472 (2015).
[3] L. Rutkowski et al, Opt. Express 25, 21711–21718 (2017).