The dual-comb spectrometer, a powerful tool for broadband precision molecular spectroscopy, has been a state-of-the-art technology in the optical spectroscopy community for more than a decade. However, its implementation is largely limited to either laboratories capable of purchasing these instruments off-the-shelf, which can be prohibitively expensive, or to laboratories with specialized fabrication equipment. We will present our continued work on the development of a dual-comb spectrometer at Connecticut College, a small liberal-arts college. The spectrometer is based on a pair of fiber-based optical frequency combs constructed in-house by undergraduate students. We will report on the ongoing progress of this instrument and detail our efforts in stabilizing the combs thermally, mechanically, and optically in order to achieve sufficient resolution for rotationally-resolved measurements.

Acetylene is interrogated and averaged with an optical frequency comb generated by a single dual-drive Mach-Zehnder modulator (DDMZ). The compact architecture of the modulator enables coherent averaging of radiofrequency interferograms, which is typically impossible when using the traditional approach of an acousto-optic and electro-optic modulator. Acetylene center frequencies uncertainties are compared across averaging times. This low-cost approach enables precise spectroscopic measurements, is applicable to arbitrary wavelengths, and is amenable to chip-scale integration.

We present a UV spectroscopy system based on photon counting and differential chirped pulse down conversion Stroud, J. R., Simon, J. B., Wagner, G. A., and Plusquellic, D. F. Interleaved electro-optic dual comb generation to expand bandwidth and scan rate for molecular spectroscopy and dynamics studies near 1.6 μm. Optics Express, 29(21), 33155-33170 (2021). We use a single laser in the near-IR to generate two chirped pulse frequency combs. The near-IR combs are combined and sent into a nonlinear LBO crystal to double and generate UV photons. The UV photons are sent through a sample and detected by a photo multiplier tube (PMT) that accumulates individual photon counts to form an interference pattern D. F. Plusquellic, G. A. Wagner, A. J. Fleisher, D. A. Long, and J. T. Hodges, Multiheterodyne spectroscopy using multi-frequency combs, in Conference on Lasers and Electro-Optics, (OSA, 2017). We Fourier transform this interference pattern into a frequency comb that we use for precision spectroscopy. The differential chirp down conversion allows us to convert tens of GHz of optical bandwidth in the UV onto a MHz comb in the radio frequency domain with a PMT. We use this method to study a tropolone derivative that has an S₁ origin in our UV scan range.

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Footnotes:

Stroud, J. R., Simon, J. B., Wagner, G. A., and Plusquellic, D. F. Interleaved electro-optic dual comb generation to expand bandwidth and scan rate for molecular spectroscopy and dynamics studies near 1.6 μm. Optics Express, 29(21), 33155-33170 (2021).. D. F. Plusquellic, G. A. Wagner, A. J. Fleisher, D. A. Long, and J. T. Hodges, Multiheterodyne spectroscopy using multi-frequency combs, in Conference on Lasers and Electro-Optics, (OSA, 2017)..

There is a lack of experimental transition intensity data for many rovibrational modes of HCN. This work uses cross-dispersive frequency comb spectroscopy to provide broadband, rapid, and high-throughput spectral information on numerous HCN transitions. HCN is a prevalent molecule in space that has uses in modeling astrophysical processes which require large line lists.G. Ch. Mellau et al., J. Quant. Spectrosc. Radiat. Transfer 270, 107666 (2021)e use a spectrometer design that has comb teeth resolution for a mode-locked probe laser with 1 GHz repetition rate. The cross-dispersed frequency comb spectroscopy operates in the near-infrared region of 1.5 μm to 1.7 μm. The comb will be applied to investigate transition intensities and other spectral reference data parameters to benchmark ab initio line lists, and the results will aim to improve the potential energy surface and dipole moment surfaces required to calculate broadband line lists.

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Footnotes:

G. Ch. Mellau et al., J. Quant. Spectrosc. Radiat. Transfer 270, 107666 (2021)W

Cavity ringdown spectroscopy (CRDS) is a highly sensitive technique that allows for the detection and analysis of extremely dilute chemical species. This tool is particularly useful for the collection of high-resolution spectra of molecules of astrochemical interest, which can be used to assist molecular detection efforts and improve our understanding of the chemistry that can occur in space. We present three iterations of a near-IR (1.5μm) continuous-wave CRDS system which has been under development at the University of Maryland. HCN is chosen as a test molecule for each of these systems due to its importance in astrochemistry and its well-studied—experimentally and theoretically—spectra in the infrared. The rovibrational spectrum of the first overtone of the C-H stretch is analyzed on each system at room temperature, and in our newest system at cryogenic temperatures. The rotational temperature of HCN is calculated from this spectral data, and will be used in future work to quantify the rotational energy of neutral gaseous species for kinetics studies.

Cavity ringdown spectroscopy (CRDS) is a high-sensitivity, high-resolution spectroscopic technique used to study molecules in very low number densities. This tool, combined with buffer-gas cooling allows us to prepare and study astrochemically relevant species at cryogenic temperatures ( < 50K). We have developed a custom cryogenic buffer-gas cell with optical access for continuous-wave CRDS at the University of Maryland. Centered at 1.5 μm in the near-IR, we are able to observe the first vibrational overtone of the C-H stretch for astrophysically relevant cyanopolyynes and quantify the rotational cooling using their ro-vibrational spectra. We will present the design of this custom instrument, room temperature and cold spectra for molecules, as well as implications and future uses for this combined technique.

Polarization dependences of reflectance of cavity ringdown super mirrors have been investigated for commercial mirrors from LAYERTEC and Los Gatos. These mirrors have reflectivities of > 0.99995 and 0.99993, respectively, as catalog specs. Optical cavities consisting of pairs of these mirrors provide ringdown times of 45 and 27 μs at maximum in our measurement setup having a cavity length of 0.7 m. The polarization dependences were measured via ringdown times at 681.2 nm by rotation of a detector-side mirror. Reflectance depending on the mirror rotation showed periodic structures. The ringdown-time differences in the periodic structures were measured to be  ∼ 30% and < 5% at maximum in the cases of LAYERTEC and Los Gatos mirrors, respectively. We have formulated the polarization dependence of reflectance as a function of rotation angle, which can well explain the dependences observed. This large individual difference in the polarization dependence between the mirror pairs of LAYERTEC and Los Gatos is presumed to come from the reflectivity difference of the mirrors. This polarization dependence on reflectance has been explained so far by birefringence and/or absorption of mirrors [1-3]. However, the present measurements suggest absorption is a dominant reason for the dependence. This is because this dependence was measured even though the mirrors were aligned after every rotation of the mirror. If polarization dependence comes only from birefringence, the dependence cannot be measured. [1] Huang & Lehmann, App. Opt., 47, 3817, 2008. [2] Xiao et al., App. Opt., 59, A99, 2020. [3] Winkler et al., Optica, 8, 686, 2021.

Methanol is a six-atoms asymmetric top molecule presenting internal rotation. The size of this molecule and the presence of a large amplitude motion lead to a dense and complicated spectral signature. Complexity further increases when one goes up in vibrational excitations, i.e. the overtone range. Due to these features, this molecule remains poorly known, especially in the near infrared. To simplify its molecular signature, spectroscopy can be performed at cryogenic temperature so that only the states lying low in energy are populated, leading to an important simplification of the measured spectra along with a narrowing of the Doppler-broadened lines. In this talk, I will introduce the experimental development of a buffer gas cooling (BGC) setup combined with a cavity ring-down spectrometer with a detection limit α_\textmin=2×10⁻¹⁰ \text cm⁻¹ and spectral resolution of few 10⁻⁴ \text cm⁻¹. With this instrument, we measured the 2OH stretching band of methanol between 7165 \text cm⁻¹ and 7230 \text cm⁻¹ cooled down to 26±12 K. A total of 350 rovibrational transitions were assigned and 62 others were tentatively assigned. This assignment was performed by using and extending the pattern recognition method developed by Rakvoský et al. [PCCP, 2021, 23, 20193-20200]. I will explain how we used the relative intensities of the transitions to add one criterion to test the validity of our assignments. Improvements and future outcomes of this approach and of the BGC setup will be discussed.