Coherent anti-Stokes Raman scattering (CARS) spectroscopy has been used to provide gas-phase quantitative scalar information (e.g. temperature, density, and species concentrations) for more than 5 decades. This technique is renowned for its ability to realize non-intrusive in-situ measurements in harsh environments with excellent spatial and temporal resolution and has become an important tool in multiple energy and combustion science applications, where high-fidelity data are needed. CARS is a non-linear optical process, where the signal originates from the coupling of multiple laser fields to the internal energy states of the probed molecules. This interaction results in excellent chemical specificity, while temperature information is obtained through the direct retrieval of the population distribution on the CARS signal spectrum. CARS thus represents the state-of-the-art in gas-phase thermometry, with unmatched accuracy and precision. The strong “laser-like” signal, which can be detected remotely from where it is generated, makes it also suited for extremely harsh and luminous environments such as flames and plasmas. The present chapter summarizes the fundamentals of gas-phase CARS and discusses a number of most recent advancements: i.e. single-shot CARS imaging, new light sources for ultrabroadband CARS, and simultaneous referencing of the femtosecond (impulsive) excitation efficiency. These recent developments open for interesting possibilities of using CARS in new type of experiments, with coverage of in principle all Raman active modes, obtained with space-time correlated resolution, and improved significance in the delivered data.

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This doctoral thesis presents the development and application of spatiotemporal coherent anti-Stokes Raman scattering (CARS) imaging for quantitative measurements of local structures in laminar and turbulent hydrogen flames. The study of spatial-thermochemical scalars in these flames is of paramount importance in advancing the next generation of sustainable combustion systems. The complexities of these environments require high-fidelity scalar measurements, demanding optical diagnostic tools with exceptional spatial and temporal resolution. Ideally, such instruments must be capable of delivering in-situ nonintrusive measurements of temperature, as well as species mole fraction in dynamical environments occurring across narrow spatial gradients of typically ~100’s μm. Recent developments in ultrafast —femtosecond (fs) and picosecond (ps)— laser technology have enabled the advancement of time-resolved CARS spectroscopy achieving independently, heightened repetition rates (from Hz to kHz), multidimensional imaging (1D and 2D), and multiplex spectroscopic measurements targeting major combustion species. Despite these advancements, there remains a gap in the availability of CARS instruments capable of seamlessly combining all these functionalities.
The primary goal of this research is to combine these advancements in a unique design, that allows for spatiotemporal 1D-CARS measurements of temperature and concentration of N2, H2, O2, and H2O across the flame front of hydrogen flames. Using a single ultrafast regenerative amplifier laser system (Astrella, coherent) combined with a Second harmonic bandwidth compressor (Light conversion), twobeam fs/ps pure-rotational CARS imaging is achieved across a one-dimensional field-of-view of ~1.5 mm at 1 kHz repetition rate in flames. To detect these signals, a novel polarization-sensitive coherent imaging spectrometer equipped with a highspeed sCMOS camera was designed, achieving an excellent spectral and imaging resolution (<20 μm). The performance of the instrument is demonstrated for cinematographic 1D-CARS gas-phase thermometry (300-2200K) across an unstable premixed methane/air flame-front, achieving a single-shot precision <1% and an accuracy of <3%.
Furthermore, the thesis introduces a groundbreaking strategy for deducing water vapor concentration from the pure-rotational N2 CARS signal in the time domain, overcoming the challenge posed by weak H2O pure-rotational Raman spectra. This is achieved by exploiting the sensitivity of the N2 rotational Raman coherence to energy transfer during inelastic collisions between N2-N2 and N2-H2O. The developed technique allows simultaneous measurements of temperature, as well as O2, H2, and water vapor concentrations in a laminar H2/air diffusion flame. These experimental findings are complemented by numerical investigations, underscoring the potential to extend the technique to measure water vapor in more complex ternary collisional systems found in hydrocarbon flames. The further application of these developments in the canonical H3 flame reveals the capability to directly measure molecular transport processes affecting flame structure using spatiotemporal 1D-CARS. A dual-probe CARS approach combined with a polarization-sensitive coherent imaging spectrometer enables simultaneous acquisition of molecular N2 coherence at short and long probe delays, providing one-dimensional thermometry and concentration measurements for H2, O2, and H2O in a single laser shot.@en

We present the first experimental application of coherent Raman spectroscopy (CRS) on the ro-vibrational ν2 mode spectrum of methane (CH4). Ultrabroadband femtosecond/picosecond (fs/ps) CRS is performed in the molecular fingerprint region from 1100 to 2000 cm-1, employing fs laser-induced filamentation as the supercontinuum generation mechanism to provide the ultrabroadband excitation pulses. We introduce a time-domain model of the CH4 ν2 CRS spectrum, including all five ro-vibrational branches allowed by the selection rules Δv = 1, ΔJ = 0, ±1, ±2; the model includes collisional linewidths, computed according to a modified exponential gap scaling law and validated experimentally. The use of ultrabroadband CRS for in situ monitoring of the CH4 chemistry is demonstrated in a laboratory CH4/air diffusion flame: CRS measurements in the fingerprint region, performed across the laminar flame front, allow the simultaneous detection of molecular oxygen (O2), carbon dioxide (CO2), and molecular hydrogen (H2), along with CH4. Fundamental physicochemical processes, such as H2 production via CH4 pyrolysis, are observed through the Raman spectra of these chemical species. In addition, we demonstrate ro-vibrational CH4 v2 CRS thermometry, and we validate it against CO2 CRS measurements. The present technique offers an interesting diagnostics approach to in situ measurement of CH4-rich environments, e.g., in plasma reactors for CH4 pyrolysis and H2 production.

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We present ultrabroadband two-beam femtosecond/picosecond coherent Raman spectroscopy on the ro-vibrational spectra of CO₂ and O₂, applied for multispecies thermometry and relative concentration measurements in a standard laminar premixed hydrocarbon flame. The experimental system employs fs-laser-induced filamentation to generate the compressed supercontinuum in-situ, resulting in a ∼24 fs full-width-at-half-maximum pump/Stokes pulse with sufficient bandwidth to excite all the ro-vibrational Raman transitions up to 1600 cm^-1. We report the simultaneous recording of the ro-vibrational CO₂ Q-branch and the ro-vibrational O₂ O-, Q- and S-branch coherent Stokes Raman spectra (CSRS) on the basis of a single-laser-shot. The use of filamentation as the supercontinuum generation mechanism has the advantage of greatly simplifying the experimental setup, as it avoids the use of hollow-core fibres and chirped mirrors to deliver a near-transform-limited ultrabroadband pulse at the measurement location. Time-domain models for the ro-vibrational Q-branch spectrum of CO₂ and the ro-vibrational O-, Q- and S-branch spectra of O₂ were developed. The modelling of the CO₂ Q-branch spectrum accounts for up to 180 vibrational bands and for their interaction in Fermi polyads, and is based on recently available, comprehensive calculations of the vibrational transition dipole moments of the CO₂ molecule: the availability of spectroscopic data for these many vibrational bands is crucial to model the high-temperature spectra acquired in the flue gases of hydrocarbon flames, where the temperature can exceed 2000 K. The numerical code was employed to evaluate the CSRS spectra acquired in the products of a laminar premixed methane/air flame provided on a Bunsen burner, for varying equivalence ratio in the range 0.6–1.05. The performance of the CO₂ spectral model is assessed by extracting temperatures from 40-laser-shots averaged spectra, resulting in thermometry accuracy and precision of ∼5% and ∼1%, respectively, at temperatures as high as 2220 K.

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We present a novel diagnostic technique to probe water vapor (H₂O) concentration in hydrogen (H₂) combustion environments via the time-resolved measurement of the collisional dephasing of the pure-rotational coherent anti-Stokes Raman scattering (CARS) signal of nitrogen (N₂). The rotational Raman coherence of the N₂ molecules, induced by the interaction with the pump and Stokes laser fields, dephases on a timescale of hundreds of picoseconds (ps), mostly due to inelastic collisions with other molecules in atmospheric flames. In the spatial region of H₂ flames where H₂O is present in appreciable amount, it introduces a faster dephasing of the N₂ coherence than the other major combustion species do: we use time-resolved femtosecond/picosecond (fs/ps) CARS to deduce the H₂O mole fraction from the dephasing effect of its inelastic collisions with N₂. The proof-of-principle is demonstrated in a laminar H₂/air diffusion flame, performing sequential measurements of the collisional dephasing of the N₂ CARS signal up to 360 ps. We measure the temperature and the relative O₂/N₂ and H₂/N₂ concentrations at a short probe delay, and input the results in the time-domain model to extract the H₂O mole fraction from the signal decay, thus measuring the whole scalar flow fields across the flame front. We furthermore present single-shot simultaneous thermometry and absolute concentration measurements in the turbulent TU Darmstadt/DLR Stuttgart canonical 'H3 flame' performed by dual-probe CARS measurements obtained with a polarization separation approach. This allows us to probe the molecular coherence simultaneously at -20 and -250 ps on the basis of a single-laser-shot, and record the resulting signals in two distinct detection channels of our unique polarization-sensitive coherent imaging spectrometer. The proposed technique allows for measuring the absolute concentrations of all the major species of H₂ flames, thus providing a full characterization of the flow composition, as well as of the temperature field.

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We report on the generation of coherent emission from femtosecond (fs) laser-induced filaments mediated by ultrabroadband coherent Raman scattering (CRS), and we investigate its application for high-resolution gas-phase thermometry. Broadband 35-fs, 800-nm pump pulses generate the filament through photoionization of the N₂ molecules, while narrowband picosecond (ps) pulses at 400 nm seed the fluorescent plasma medium via generation of an ultrabroadband CRS signal, resulting in a narrowband and highly spatiotemporally coherent emission at 428 nm. This emission satisfies the phase-matching for the crossed pump-probe beams geometry, and its polarization follows the CRS signal polarization. We perform spectroscopy on the coherent N₂⁺ signal to investigate the rotational energy distribution of the N₂⁺ ions in the excited B²Σ_u⁺ electronic state and demonstrate that the ionization mechanism of the N₂ molecules preserves the original Boltzmann distribution to within the experimental conditions tested.

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Time-resolved spectroscopy can provide valuable insights in hydrogen chemistry, with applications ranging from fundamental physics to the use of hydrogen as a commercial fuel. This work represents the first-ever demonstration of in-situ femtosecond laser-induced filamentation to generate a compressed supercontinuum behind a thick optical window, and its in-situ use to perform femtosecond/picosecond coherent Raman spectroscopy (CRS) on molecular hydrogen (H2). The ultrabroadband coherent excitation of Raman active molecules in measurement scenarios within an enclosed space has been hindered thus far by the window material imparting temporal stretch to the pulse. We overcome this challenge and present the simultaneous single-shot detection of the rotational H2 and the non-resonant CRS spectra in a laminar H2/air diffusion flame. Implementing an in-situ referencing protocol, the non-resonant spectrum measures the spectral phase of the supercontinuum pulse and maps the efficiency of the ultrabroadband coherent excitation achieved behind the window. This approach provides a straightforward path for the implementation of ultrabroadband H2 CRS in enclosed environment such as next-generation hydrogen combustors and reforming reactors.

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Cascaded coherent anti-Stokes Raman scattering (CARS) signals can be efficiently generated from CARS signals when propagating collinearly with the pump/Stokes and probe beams. This effect can be seen as the CARS beam acting as the probe beam and being inelastically scattered a ‘second time’ from the Raman coherence induced along the focus of the pump/Stokes beam axis. Although much weaker, this additionally scattered signal co-propagates with the CARS signal and may complicate the analysis of the CARS spectrum used for diagnostics in the gas phase. In particular, the occurrence of the cascaded CARS process needs to be taken into account analysing minor spectral signatures at relatively high number density of scattering molecules. Here we show how polarization control can be employed to generate CARS and cascaded CARS signals with orthogonal linear polarization and how, in this way, the cascaded CARS signals can be efficiently suppressed. However, instead of rejecting this signal, we collect both the generated CARS and cascaded CARS signals on the same detector frame, and we explore the use of these counterparts for absolute concentration measurements of the Raman-active species. The cascaded CARS signal has exponential-order higher sensitivity to the number density of the scattering molecules in the mixture. We demonstrate that the ratio of the CARS and the cascaded CARS signals is independent of the probe pulse energy in use, which can be a promising approach for wide-range absolute concentration measurements in gas-phase media.

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Simultaneous detection of resonant and non-resonant femtosecond/picosecond coherent anti-Stokes Raman spectroscopy (CARS) signals has been developed as a viable technique to provide in-situ referencing of the impulsive excitation efficiency for temperature assessments in flames. In the framework of CARS thermometry, the occurrence of both a resonant and a non-resonant contribution to the third-order susceptibility is well known. While the resonant part conceives the useful spectral information for deriving temperature and species concentrations in the probed volume, the non-resonant part is often disregarded. It nonetheless serves the CARS technique as an essential reference to map the finite bandwidth of the laser excitation fields and the transmission characteristics of the signal along the detection path. Hence, the standard protocols for CARS flame measurements include the time-averaged recording of the non-resonant signal, to be performed sequentially to the experiment. In the present work we present the successful single-shot recordings of both the resonant and non-resonant CARS signals, split on the same detector frame, realizing the in-situ referencing of the impulsive excitation efficiency. We demonstrate the use of this technique on one-dimensional CARS imaging spectra, acquired across the flame front of a laminar premixed methane/air flame. The effect of pulse dispersion on the laser excitation fields, while propagating in the participating medium, is proved to result, if not accounted for, in an ~1.3% systematic bias of the CARS-evaluated temperature in the oxidation region of the flame.

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We report spatiotemporal pure-rotational coherent anti-Stokes Raman spectroscopy (CARS) in a one-dimensional imaging arrangement obtained with a single ultrafast regenerative amplifier system. The femtosecond pump/Stokes photon pairs, used for impulsive excitation, are delivered by an external compressor operating on a ∼35% beam split of the uncompressed amplifier output (2.5 mJ/pulse). The picosecond 1.2 mJ probe pulse is produced via the second-harmonic bandwidth compression (SHBC) of the ∼65% remainder of the amplifier output (4.5 mJ/pulse), which originates from the internal compressor. The two pump/Stokes and probe pulses are spatially, temporally, and repetition-wise correlated at the measurement, and the signal generation plane is relayed by a wide-field coherent imaging spectrometer onto the detector plane, which is refreshed at the same repetition rate as the ultrafast regenerative amplifier system. We demonstrate 1 kHz cinematographic 1D-CARS gas-phase thermometry across an unstable premixed methane/air flame-front, achieved with a single-shot precision <1% and accuracy <3%, 1.4 mm field of view, and an excellent <20 µm line-spread function.

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