Time-resolved mid-infrared dual-comb spectroscopy

www.nature.com/scientificreports OPEN Time-resolved mid-infrared dualcomb spectroscopy Muhammad A. Abbas, Qing Pan, Julien Mandon, Simona M. Cristescu, Frans J. M. Harren & Amir Khodabakhsh* Dual-comb spectroscopy can provide broad spectral bandwidth and high spectral resolution in a short acquisition time, enabling time-resolved measurements. Specifically, spectroscopy in the mid-infrared wavelength range is of particular interest, since most of the molecules have their strongest rotationalvibrational transitions in this “fingerprint” region. Here we report time-resolved mid-infrared dual-comb spectroscopy, covering ~300 nm bandwidth around 3.3 μm with 6 GHz spectral resolution and 20 μs temporal resolution. As a demonstration, we study a CH4/He gas mixture in an electric discharge, while the discharge is modulated between dark and glow regimes. We simultaneously monitor the production of C2H6 and the vibrational excitation of CH4 molecules, observing the dynamics of both processes. This approach to broadband, high-resolution, and time-resolved mid-infrared spectroscopy provides a new tool for monitoring the kinetics of fast chemical reactions, with potential applications in various fields such as physical chemistry and plasma/combustion analysis. Time-domain monitoring of fast chemical reactions is of particular interest in several fundamental and applied scientific fields, including physical chemistry, plasma/combustion analysis, biology, and atmospheric studies1–4. Broadband, time-resolved absorption spectroscopy can provide the possibility to simultaneously monitor time-dependent parameters of the chemical reactions, such as concentrations of intermediate/final chemical products, transient free radicals and ions, as well as branching ratios, reaction rate coefficients, temperature and number densities of molecular excited-states. Generally, the main challenge is to obtain a broadband spectrum with high spectral resolution and high detection sensitivity in a short measurement time. Continuous-wave (cw) laser absorption spectroscopy can provide time-resolved measurements for a single chemical species with a high detection sensitivity. However, for a broad spectral coverage the laser source needs to be scanned over the spectral range, inevitably reducing the measurement speed. Alternatively, one can use broadband time-resolved absorption spectroscopy techniques, which are traditionally based on incoherent light sources. They can provide an ultra-broadband time-resolved spectrum, but they need a long averaging time to achieve a high signal-to-noise ratio (SNR) and detection sensitivity. Two widely used methods are step-scan mechanical Fourier transform spectroscopy (FTS)5–7 and dispersion-based detection8–11. The former exhibits very long measurement times due to the step-scanning, while the latter yields shorter measurement times, but usually has a coarse spectral resolution. In contrast to these traditional broadband methods, optical frequency comb spectroscopy (OFCS) simultaneously provides a broad spectral coverage and a high spectral resolution. It can also yield a high SNR within a short measurement time, due to the coherency and high spectral brightness of optical frequency comb sources. Specifically, OFCS in the mid-infrared (mid-IR) wavelength range (2–20 µm) has been of particular interest, since almost all molecules have their fundamental rotational-vibrational transitions in this region with distinct absorption patterns (i.e. fingerprints). Various OFCS techniques have been utilized in the mid-IR wavelength region; e.g. combining an optical frequency comb with a mechanical FTS12,13, dual-comb spectroscopy (DCS)14– 18 and dispersion-based methods19–22. A comprehensive review of these spectroscopic methods can be found elsewhere23. Monitoring of chemical reactions using OFCS in static or semi-static conditions can provide interesting results24–26, however the full potential of this technique would be exploited by time-resolved measurements. Time-domain/time-resolved spectroscopy using optical frequency combs with time resolutions well below second time scale has emerged strongly in the last decade. In a first demonstration, DCS was used for measuring molecular free induction decay in the near-infrared (near-IR) wavelength range using two Er:fiber mode-locked lasers27. A few other works have been reported in near-IR region using Ti:sapphire mode-locked lasers including dual frequency comb-based transient absorption (DFC-TA) spectroscopy for measurement of the relaxation Trace Gas Research Group, Department of Molecular and Laser Physics, Institute for Molecules and Materials, Radboud University, 6525 AJ, Nijmegen, The Netherlands. *email: Scientific Reports | (2019) 9:17247 | https://doi.org/10.1038/s41598-019-53825-8 1 www.nature.com/scientificreports/ www.nature.com/scientificreports Figure 1. Experimental setup. Two femtosecond Yb:fiber lasers with stabilized and slightly different frep (and free running fceo), synchronously pump two MgO:PPLN crystals in a single OPO cavity, providing two mid-IR idler beams. The two combined idler beams are sent through the sample (discharge) cell and a reference gas cell. The latter yields the dual-comb spectrum of a single well-defined absorption line, which is used to correct for the free running fceo, as well as absolute frequency calibration of the sample spectrum. M flat mirror, M1-4 concave dielectric mirror, CM flat chirped dielectric mirror, DM dichroic mirror, BD beam dump, BS beam splitter, λ⁄2 half-waveplate, L lens, PD photodetector, LPF low pass filter. processes of dye molecules in solution from femtosecond to nanosecond timescales28, and DCS for the study of laser-induced plasma from a solid sample, simultaneously measuring trace amounts of Rb and K in a laser ablation29. Er:fiber mode-locked lasers has also been used in time-resolved dual-comb spectroscopy (TRDCS) to monitor a fast, single shot reaction30 and also in continuous-filtering Vernier spectroscopy for combustion analysis31 both with milliseconds time scale resolution. In the visible range (~530 nm), cavity-enhanced transient absorption spectroscopy (CE-TAS) has been demonstrated for study of the ultrafast dynamics of I2 in a molecular beam32, and more recently, TRDCS has been reported for measurement of number density and temperature in a laser-induced plasma by monitoring three excited-state transitions of Fe33. In the mid-IR region, cavity-enhanced time-resolved frequency comb spectroscopy (TRFCS) is utilized for monitoring of transient free radicals and kinetics of the OD + CO → DOCO reaction by 2D cross-dispersion of the spectrum on a liquid N2 cooled camera using a virtually imaged phase array (VIPA) etalon in combination with a conventional grating3,34,35. Time-resolved dual-comb spectroscopy based on quantum cascade lasers (QCLs) has also been demonstrated in the mid-IR region. Generally, these spectrometers provide a shorter (...truncated)