Time-Resolved Dual-Comb Spectroscopy (DCS)

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 studies [1-4]. 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 detection [8-11]. 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 FTS [12,13], dual-comb spectroscopy (DCS) [14-18] and dispersion-based methods [19-22]. A comprehensive review of these spectroscopic methods can be found elsewhere [23].

Monitoring of chemical reactions using OFCS in static or semi-static conditions can provide interesting results [24-26], 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 lasers [27]. 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 processes of dye molecules in solution from femtosecond to nanosecond timescales [28], and DCS for the study of laser-induced plasma from a solid sample, simultaneously measuring trace amounts of Rb and K in a laser ablation [29]. Er:fiber mode-locked lasers has also been used in time-resolved dual-comb spectroscopy (TRDCS) to monitor a fast, single shot reaction [30] and also in continuous-filtering Vernier spectroscopy for combustion analysis [31] 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 beam [32], 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 Fe [33]. 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 grating [3,34,35]. 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 spectral bandwidth and coarser spectral resolution compared to mode-locked-laser-based spectrometers; however, they can provide a better time resolution. A single-shot sub-microsecond demonstration is reported for monitoring of protein reactions in the liquid phase [36] and the same system is also used for monitoring of high-temperature reaction between propyne and oxygen in the gas phase [37].

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