l0pt Figure We present a multi-channel harmonic frequency comb covering the mid-infrared spectral range between 15 and 85 THz (or 3.5 - 20 μm, or 500 to 2860 cm⁻¹) with a record 1-mW/THz-level power spectral density. An Er-fiber-based oscillator is wavelength-shifted to a central wavelength of 1960 nm and a chirped-pulse Tm-fiber amplifier provides a 50-MHz-repetition-rate train of 250-fs pulses with 120 W of average power. Nonlinear self-compression in two fused-silica fibers results in two channels, yielding 11-fs pulses with 4.5 W (Channel 1) and 25-fs pulses with 25 W (Channel 2). Subsequent intrapulse difference-frequency generation (DFG) in 1-mm-thin GaSe crystals results in a coverage of the entire molecular fingerprint region with only two phase matching angles for each channel (see Figure). DFG inherently provides phase-stable pulses, leading to a harmonic frequency comb. The 120-W average power of the near-infrared frontend suffices for the parallel implementation of multiple channels, facilitating broadband spectroscopy.

Using frequency combs and optical cavities, we have previously demonstrated ultrafast transient absorption measurements with a detection limit of ∆OD = 1 ×10⁻⁹ /√{Hz}, enabling work in dilute molecular beams.M. A. R. Reber, Y. Chen, and T. K. Allison, Optica 3, 311 (2016).imilar methods can be applied to multidimensional spectroscopy as well.T. K. Allison, J. Phys. B: At. Mol. Opt. Phys. 50, 044004 (2017).ince molecules undergoing ultrafast dynamics have broad spectral features, cavity-enhanced ultrafast spectroscopy then demands broadband and widely tunable frequency combs. Here we present a frequency conversion setup for the generation of high power mid infrared frequency combs in the 3-10 μm region. The initial comb is generated using an Er:fiber oscillator with 100 MHz repetition rate. After nonlinear amplification, the comb is shifted in a highly nonlinear fiber (HNLF) to 1 μm and amplified to 10 W in a home built, multi-stage Yb:fiber amplifier. We have measured the output comb tooth linewidth to be less than 10 kHz and the pulse duration is 120 fs. This laser is then used as a pump for several nonlinear difference frequency generation stages seeded by additional HNLF-shifted combs. Cavity-enhanced mid-infrared combs in the 3-5 μm region will be applied to studying ultrafast dynamics of hydrogen-bonded clusters.

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Footnotes:

M. A. R. Reber, Y. Chen, and T. K. Allison, Optica 3, 311 (2016).S T. K. Allison, J. Phys. B: At. Mol. Opt. Phys. 50, 044004 (2017).S

We have studied the electronic excited states of aromatic hydrocarbons such as, benzene or naphthalene by high-resolution spectroscopy.^a In the excited electronic states of these molecules, there are various interesting interactions such as intramolecular vibrational energy redistribution (IVR), intersystem crossing (ISC), and internal conversion (IC). In the present study, we observe high-resolution spectra of larger polycyclic hydrocarbons such as perylene. In our experiment, we use a frequency-doubled single mode Ti:Sapphire laser as a light source. Sub-Doppler spectra are obtained with a supersonic molecular beam, which crosses the laser light at right angles. A GPS-disciplined Er-doped fiber optical frequency comb is used as a frequency ruler to decide transition frequencies at the uncertainty of 10 kHz. ^a A. Nishiyama, K. Nakashima, A. Matsuba, and M. Misono, J. Mol. Spectrosc. 318, 40 (2015).

We provide the most accurate absolute temperature measurement ever performed on an atomic or molecular sample with a Doppler-Broadening-Thermometry approach. Specifically, the absorption profile of the P_e(12) line of the (30012) - (00001) band of a CO₂ sample at thermodynamic equilibrium is accurately measured at 1.578 μm by a comb-assisted cavity-ring-down spectrometer that combines an extremely dense frequency axis (3000 points over 4.2 GHz) with an acquisition time as low as a few seconds. The Doppler width is extracted from a refined multi-spectrum fitting procedure accounting for the speed dependence of the relaxation rates, which were found to play a role even at the very low pressures explored, from 1 to 7 Pa. The thermodynamic gas temperature is retrieved with relative uncertainties of 8·10⁻⁶ (type A) and 11 ·10⁻⁶ (type B), which rank the system at the first place among optical methods. Thanks to a measurement time of only  5 h, the technique represents a promising pathway towards the optical determination of the thermodynamic temperature with a global uncertainty at the 10⁻⁶ level Gotti R., Moretti L., Gatti D., Galzerano G., Castrillo A., Laporta P., Gianfrani L., and Marangoni M., Phys. Rev. A 97, 12512 (2018) An additional element of interest derives from the forthcoming redefinition of the unit Kelvin J. Fischer, Phil. Trans. R. Soc. A 374, 20150038 (2016) in 2018, which calls for primary thermometers that are capable to operate over a large part of the temperature scale with very high accuracy.

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Footnotes:

Gotti R., Moretti L., Gatti D., Galzerano G., Castrillo A., Laporta P., Gianfrani L., and Marangoni M., Phys. Rev. A 97, 12512 (2018). J. Fischer, Phil. Trans. R. Soc. A 374, 20150038 (2016),

In this talk, we report on progress toward high resolution infrared frequency comb spectroscopy of buckminsterfullerene, C₆₀. A rotationally resolved spectrum of C₆₀ has to date remained elusive, despite the very intense research into this molecule's chemical and physical properties since its discovery in 1985. Our approach utilizes cyrogenic buffer gas cooling of the output of a 1000 K effusive oven to prepare cold gas phase C₆₀ molecules. We subsequently probe these with a difference frequency generation-based frequency comb tuned to the 8.5 μm IR active fundamental. The combination of a high finesse absorption enhancement cavity and Fourier transform interferometry read-out provide sensitive, broadband detection, while retaining the high spectral resolution of the frequency comb light. We will discuss our preliminary results, which tentatively suggest successful ground state vibrational cooling and observation of resolved rotational fine structure, as well as experimental modifications that we expect to improve the C₆₀ number density and internal state cooling efficiency.

Mechanical Fourier transform spectrometers (FTS) based on optical frequency combs (OFC) allow precise measurement of comb line intensities when the nominal resolution of the FTS is precisely matched to the repetition rate of the comb Masłowski, P., et al., Phys. Rev. A 93, 021802 (2016).^, Rutkowski, L., et al., J. Quant. Spectrosc. Radiat. Transf. 204, 63−73 (2018). Under this condition, the resolution and frequency scale accuracy are given by the narrow comb lines rather than the spectrometer, and the measured spectra are not influenced by the instrumental lineshape. Here we use an FTS based on an Er:fiber frequency comb to perform high−precision direct absorption measurements of the 3_1+_3 absorption band of pure CO_2 at 1.57 m. The sample is held inside a multipass cell, and a spectrum of the entire band with signal to noise ratio (SNR) up to 1000 is acquired in Rutkowski, L., et al., J. Quant. Spectrosc. Radiat. Transf. 204, 63-73 (2018)..Larcher, G., et al., J. Quant. Spectrosc. Radiat. Transf. 164, 82-88 (2015)..Gordon, I. E., et al., J. Quant. Spectrosc. Radiat. Transf. 203, 3-69 (2017).aLong, D.A., et al., J. Quant. Spectrosc. Radiat. Transf. 161, 35-40 (2015)..

We present a combined theoretical and experimental study of the hyperfine-resolved spectrum of ammonia and its deuterated isotopologues. The calculations have been performed using the variational approach TROVE, a new spectroscopically determined potential energy surface, and ab initio quadrupole, spin-spin, and spin-rotation coupling surfaces. The computed spectroscopic line lists cover transitions between levels with rotational excitations J = 0...20 and vibrational band centers with up to 8000 cm⁻¹above the zero-point-energy level. For the spectroscopic observation of the para-ortho interconversion we use mid-infrared frequency comb spectroscopy in both ammonia vapour and a cold molecular beam. Furthermore, its modulation by external electric field is discussed. Our theoretical model, i.e., the underlying potential energy surface will be refined using the experimentally observed transitions.

Inversion splitting of phosphine molecules has been one of open questions in molecular spectroscopy. A recent calculation predicted that the splitting is 300 kHz and 3 MHz in the v₂ = 3 and 4 states [1], where v₂ is the vibrational quantum number of the ν₂ mode. We have observed three Q-branch transitions in the 3ν₂ band of phosphine using a comb-referenced sub-Doppler resolution spectrometer [2]. The spectrometer consists of a difference-frequency-generation source and a cavity-enhanced absorption cell with large beam spot size at beam waist to reduce transit-time broadening. The observed spectral linewidths are 150 kHz, but no inversion splitting has been observed. We now try to observe the 4ν₂ − ν₂ hot band. [1] C. Sousa-Silva, J. Tennyson, S. N. Yurchenko, J. Chem. Phys. 145, 091102 (2016). [2] S. Okuda, H. Sasada, J. Mol. Spectrosc., in press (2018).

A dual-frequency comb-based spectrometer for the mid-infrared (3-5 micrometer) wavelength region will open many opportunities for spectroscopic applications. Non-linear conversion provides a wide spectral coverage in the mid-infrared using Optical Parametric Oscillators, keeping the optical properties of the well-established near infrared frequency combs with good frequency accuracy, high spectral resolution at seconds time scale. A number of challenges remain when OPOs are used for optical conversion, because the OPO cavity generate frequency and intensity fluctuation in the combs. Here, we present how these variations are measured and used, in real-time, to correct the recorded broadband spectrum. By monitoring the frequency and amplitude variations of a single absorption line in a reference gas cell, each individual spectrum is normalized in amplitude and corrected by an offset-frequency. As such, real-time averaging is achieved over minutes with minor losses in spectral resolution or degradation, leading to an improvement in spectral resolution. A high-to-noise ratio of about 2400 is achieved with such spectral resolution, demonstrating the efficiency of the proposed method.