<p dir="ltr">The objective of this research is to advance the development and application of laser diagnostics in gas phase medium, which ranges from atmospheric non-reacting flows to turbulent reacting flows in high-pressure, high-temperature environments. Laser diagnostic techniques are powerful tools for non-intrusive and in-situ measurements of important chemical parameters, such as temperature, pressure, and species mole fractions, in harsh environments. These measurements significantly advance the knowledge across various research disciplines, such as combustion dynamics, chemical kinetics, and molecular spectroscopy. In this thesis, detailed theoretical models and experimental analysis are presented for three different techniques: 1. Chirped-probe-pulse femtosecond coherent anti-Stokes Raman scattering (CPP fs CARS); 2. Two-color polarization spectroscopy (TCPS); 3. Ultrafast-laser-absorption-spectroscopy (ULAS). The first chapter provides a brief survey of laser diagnostics, including both linear and nonlinear methods. The motivations behind the three studies covered in this dissertation are also discussed. </p><p dir="ltr">In the second chapter, single-shot CPP fs CARS thermometry is developed for the hydrogen molecule at 5 kHz. The results are divided into two parts. The first part concentrates on the development of H₂ CPP fs CARS thermometry for high-pressure and high-temperature conditions. The second part demonstrates the application of H₂ CPP fs CARS in a model rocket combustor at pressures up to 70 bar. In the first part, H₂ fs CARS thermometry was performed in Hencken burner flames up to 2300 K, as well as in a heated gas-cell at temperatures up to 1000 K. It was observed that the H₂ fs CARS spectra are highly sensitive to the pump and Stokes chirp. Chirp typically originates from optical components such as windows and polarizers. As a result, the pump delay is modeled to provide a shift to the Raman excitation efficiency curve. With the updated theoretical model, excellent agreement was found between the simulated and experimental spectra. The averaged error and precision are 2.8% and 2.3%, respectively. In addition, the spectral phase and pump delay determined from the experimental spectra closely align with the theoretical predictions. It is also found that pressure does not have significant effects on the H₂ fs CARS spectra up to 50 bar at 1000 K. The collision model provides excellent agreement with the experiment. This allows the use of low-pressure laser parameters for high-pressure thermometry measurements. In the second part, spatially resolved H₂ temperature was measured in a rocket chamber at pressures up to 70 bar. This is the first demonstration of fs CARS thermometry inside a high-pressure rocket combustor. These results highlight the potential of using H₂ CPP fs CARS thermometry to provide quantitative data in high-pressure experiments for the study of combustion dynamics and model validation efforts at application relevant operating conditions.</p><p dir="ltr">The third chapter presents the development of a TCPS system for the study of the NO (<i>A</i>²Σ⁺-<i>X</i>²Π) state-to-state collision dynamics with He, Ar, and N₂. Two sets of TCPS spectra for 1% NO, diluted in different buffer gases at 295 K and 1 atm, were obtained with the pump beam tuned to the R₁₁(11.5) and ^OP₁₂(1.5) transitions. The probe was scanned while the pump beam was tuned to the line center. Collision induced transitions were observed in the spectra as the probe scanned over transitions that were not coupled with the pump frequency. The strength and structure of the collision induced transitions in the TCPS spectra were compared between the three colliding partners. Theoretical TCPS spectra, calculated by solving the density matrix formulation of the time-dependent Schrödinger wave equation, were compared with the experimental spectra. A collision model based on the modified exponential-gap law was used to model the rotational level-to-rotational level collision dynamics. An unique aspect of this work is that the collisional transfer from an initial to a final Zeeman state was modeled based on the difference in the cosine of the rotational quantum number <i>J</i> projection angle with the z-axis for the two Zeeman states. Rotational energy transfer rates and Zeeman state collisional dynamics were varied to obtain good agreement between theory and experiment for the two different TCPS pump transitions and for the three different buffer gases. One key finding, in agreement with quasi-classical trajectory calculations, is that the spin-rotation changing transition rate in the <i>A</i>²Σ⁺ level of NO is almost zero for rotational quantum numbers ≥ 8. It was necessary to set this rate to near zero to obtain agreement with the TCPS spectra. </p><p dir="ltr">The fourth chapter presents the development and application of a broadband ULAS technique operating in the mid-infrared for simultaneous measurements of temperature, methane (CH₄), and propane (C₃H₈) mole fractions. Single-shot measurements targeting the C-H stretch fundamental vibration bands of CH₄ and C₃H₈ near 3.3 μm were acquired in both a heated gas cell up to ~650 K and laminar diffusion flames at 5 kHz. The average temperature error is 0.6%. The average species mole fraction error are 5.4% for CH₄, and 9.9% for C₃H₈. This demonstrates that ULAS is capable of providing high-fidelity hydrocarbon-based thermometry and simultaneous measurements of both large and small hydrocarbons in combustion gases. </p>

No description available.

<p dir="ltr">In recent years, nanosecond repetitively pulsed (NRP) plasma discharges have garnered significant interest due to their rapid generation of reactive excited-state species, reactive radicals, and localized heat release within nanosecond (ns) timescale. To effectively harness these plasmas for altering system-level thermal and chemical behavior, a thorough understanding of their governing physics is crucial. This knowledge enables the development of predictive plasma kinetic models for tailoring NRP plasmas to specific applications. However, achieving this requires high-fidelity experimental data to validate models and deepen our understanding of fundamental plasma physics. Advancing experimental spectroscopy and laser diagnostics methods is essential for probing such temporally highly dynamic and optically complex nonequilibrium environments. This includes developing novel <i>test platforms</i>, conducting <i>fundamental research</i> to address existing knowledge gaps, and constructing custom <i>ultrafast laser architectures</i> for probing plasma properties. </p><p dir="ltr">The pioneering development of Streak-based <i>test platform</i> in the diagnostics field of nanosecond pulsed plasmas and its successful application towards inferring the underlying ultrafast spatio-temporal evolution of nanosecond pulsed plasma discharges with an unprecedented time-resolution as short as ~25 ps is presented for the first time. Spectrally filtered, 1D line-imaging of nanosecond pulsed plasma discharges in a single-shot, jitter-free, continuously sweeping manner is obtained, and differences in discharge dynamics of air and N2 plasma environments are studied. Successive <i>test platform</i> advancement includes spectrally resolved Streak-spectroscopy measurements of thermal regime-transition evolution from early-nonequilibrium to local-thermal-equilibrium (LTE) to attain time-resolved quantitative insights into N2(C) state rotational/vibrational nonequilibrium temperatures, electron temperature/density, and spectral lifetime dynamics. </p><p dir="ltr">Ultrafast laser-based progression includes detailed <i>fundamental</i> investigation of higher-order optical nonlinearity perturbations of fs-EFISH by considering of – self-phase modulation induced spectral characteristic of fs-EFISH signal, calibration mapping during-below-and-beyond optical breakdown regime, optical Kerr effect consequences, impact of femtosecond (fs) laser seeding on the noninvasiveness of fs-EFISH, and spectral emission characteristics of fs laser filaments. To infer N2(X) state nonequilibrium of NRP pulsed plasmas, two hybrid fs/ps ro-vibrational coherent anti-Stokes Raman scattering (CARS) <i>ultrafast laser architectures</i> are developed. First architecture, single-laser-solution, reduces system’s energy budget by ~3 mJ/pulse for generating narrowband (~21 ps), high-energy (~420 μJ/pulse), 532 nm probe pulses through incorporation of custom built visible fs optical parametric amplifier (OPA) coupled with an Nd:YAG power amplifier module. The second architecture, two-laser-solution, improves system’s robustness through the development of a 1 kHz, 532 nm, high-energy (~600 μJ/pulse), low-jitter (<1 ps), narrowband (~27 ps), master-oscillator-power-amplification (MOPA) based picosecond probe pulse laser time-synchronized with fs master-oscillator. Single-shot, hybrid fs/ps narrowband ro-vibrational CARS demonstration in a combusting flame up to temperatures of ~2400 K is demonstrated. Experimental ro-vibrational CARS investigation includes polarization based nonresonant background suppression and demonstration of preferential Raman coherence excitation shift, a temperature sensitivity enhancing strategy for vibrationally hot mediums like nanosecond pulsed plasmas. Lastly, an ultrafast pulse-friendly optically accessible vacuum cell is designed and fabricated for controlled experiments of NRP fs/ps CARS. Special care is taken to prevent self-focusing and spectral-temporal chirp of fs CARS beams while maintaining Gaussian focusing beam caustic.</p>

Raman spectroscopy involves change in the polarizability of the molecular system on excitation and is based on scattering process. Spontaneous Raman scattering is a two photon process, in which the input light initiates the excitation, which then leads to an emission of another photon due to scattering. It is extensively used to understand molecular properties. As spontaneous Raman scattering is a weak process, the detection of these weak Raman photons are rather difficult. Alternatively, resonance Raman (RR) scattering is another technique where the excitation wavelength is chosen according to the material under study. The excitation wavelength is chosen to be within the absorption spectrum of the material under study. RR spectroscopy not only provides considerable improvement in the intensity of the Raman signal, but also provides mode specific information i.e. the modes which are Franck-Condon active in that transition can be observed. There are reports on RR studies of many systems using pulsed light as an excitation source. It is necessary to use at least two pulsed laser sources for carrying out the time resolved RR spectroscopy. A single pulse source for excitation would lead to compromise either with temporal or spectral resolution which is due to the uncertainty principle. If an excitation pulse has pulse width of ~100 femtoseconds then the spectral resolution will be ~ 150 cm-1. It is clear now that for improving the temporal and spectral resolution simultaneously, usage of single pulse for Raman experiments (spontaneous scattering) is not adequate. The usage of multiple laser pulses may provide the way out to improve the resolutions. Nonlinear spectroscopy in a broad view helps in understanding the structural and dynamical properties of the molecular systems in a deeper manner. There are a number of techniques as a part of nonlinear spectroscopy that have emerged in due course to meet different requirements and to overcome some difficulties while understanding the molecular properties. Stimulated Raman (SRS) gain, coherent anti-Stokes Raman scattering (CARS) and the inverse Raman spectroscopy are a few to mention as third order nonlinear spectroscopic techniques which give the similar kind of information about the molecular systems. Stimulated Raman scattering is a more general process involved in nonlinear Raman processes. SRS involves at least two laser pulses and the difference in their frequencies should match with the vibrational frequency of the molecule. The polarization has to be matched between the Raman pump and the Raman probe pulses. We have developed a new nonlinear Raman technique in our laboratory named as ultrafast Raman loss spectroscopy (URLS) using the principles of nonlinear Raman scattering. It involves the Raman pump (~ 1 picosecond (ps) or ~ 15 cm-1spectral resolution) and Raman probe as a white light continuum (100 fs) whose frequency components ranges from 400-900 nm. The laser system consists of Tsunami which is pumped by a Millennia laser and Spitfire-Pro, a regenerative amplifier which is pumped by an Empower laser. Tsunami provides a 100 fs, 780 nm centered, 80 MHz and ~6 nJ energy laser pulses. The Tsunami output is fed into Spitfire to amplify its energy and change the repetition rate to 1 KHz. The pulse length of the input pulse is preserved in amplification. The output of amplifier is split into two equal parts; one part is used to pump the Optical Parametric Amplifier (OPA) in order to generate wavelengths in the range 480-800 nm. The output of the OPA is utilized to generate Raman pump which has to be in ps in order to get the best spectral resolution. A small portion of the other part of amplifier output is utilized to generate white light source for the Raman probe. The remaining part of the amplifier output is used to pump TOPAS to generate wavelengths in the ultraviolet region. URLS has been applied to many molecular systems which range from non-fluorescent to highly fluorescent. URLS has been demonstrated to be very sensitive and useful while dealing with highly fluorescent systems. URLS is a unique technique due to its high sensitivity and the Raman loss signal intensity is at least 1.5-2 times higher as compared to the Raman gain signal intensities. Cresyl violet perchlorate (CVP) is a highly fluorescent system. URLS has been applied to study CVP even at resonance excitation. Rhodamine B has also been studied using URLS. Spontaneous Raman scattering is very difficult to observe experimentally in such high quantum yield fluorescent systems. The variation in the lineshapes of the Raman bands for different RP excitation wavelengths in URLS spectra shows the mode dependent behavior of the absorption spectrum. The experimental observation of variation in the lineshape has been accounted using theoretical formalism. The thesis is focused on discussing the development of the new nonlinear Raman spectroscopic technique URLS in detail and its applicability to molecular systems for better understanding. A theoretical formalism for accounting the uniqueness of URLS among the other nonlinear Raman techniques is developed and discussed in various pictorial representations i.e. ladder, Feynman and closed loop diagrams. A brief overview of nonlinear spectroscopy and nonlinear Raman spectroscopy is presented for demonstrating the difference between the URLS and the other nonlinear Raman techniques.

<p>Ultrafast laser-based diagnostic techniques are powerful tools for the detailed understanding of highly dynamic combustion chemistry and physics. The ultrashort pulses provide unprecedented temporal resolution along with high peak power for broad spectral range−ideal for nonlinear signal generation at high repetition rate−with applications including next-generation combustors for gas turbines, plasma-assisted combustion, hypersonic flows and rotating detonation engines. The current work focuses on advancing (i) femtosecond (fs) two-photon laser-induced fluorescence, and (ii) hybrid femtosecond/picosecond vibrational and rotational coherent anti-Stokes Raman scattering (fs/ps RCARS and VCARS) to higher pressures for the first time. </p><p>Quantitative single-laser-shot kHz-rate concentration measurements of key atomic (O-atom) and molecular (CO) species is presented using femtosecond two-photon laser-induced fluorescence (TP-LIF) for a range of equivalence ratios and pressures in diffusion flames. A multitude of signal-interfering sources and loss mechanisms−relevant to high-pressure fs TP-LIF applications−are also quantified up to 20 atm to ensure high accuracy. The pressure scaling of interferences take into account degradation, attenuation and wave-front distortion of the excitation laser pulse; collisional quenching and pressure dependent transition line-broadening and shifting; photolytic interferences; multi-photon ionization; stimulated emission; and radiation trapping. </p><p>Hybrid fs/ps VCARS of N₂ is reported for interference-free temperature measurement at 1300-2300 K in high-pressure, laminar diffusion flames up to 10 atm. A time asymmetric probe pulse allowed for detection of spectrally resolved CARS signals at probe delays as early as ~200-300 fs while being independent of collisions for the full range of pressures and temperatures. Limits of collisional independence, accuracy and precision of the measurement is explored at various probe-pulse delays, pressures and temperatures. </p><p> </p><p>Additionally, a novel all diode-pumped Nd:YAG amplifier design is presented for generation of time-synchronized ps-probe pulses for hybrid fs/ps RCARS of N₂. High-energy, nearly transform-limited, single-mode, chirp-free ps probe-pulses are generated at variable pulsewidths. The detailed architecture and characterization of the laser is presented. kHz-rate RCARS thermometry is presented up to 2400 K. Excellent spatial, spectral, and temporal beam quality allowed for fitting the theoretical spectra with a simple Gaussian model for the probe pulse with temperature accuracies of 1-2%. </p> <p><br></p>