<p>Constructive elastic microwave scattering, or, historically, coherent microwave scattering (CMS), refers to the inference of small plasma object characteristics via in-phase electromagnetic scattering – and has become a valuable technique in applications ranging from photoionization and electron-loss rate measurements to trace species detection, gaseous mixture and reaction characterization, molecular spectroscopy, and standoff measurement of local vector magnetic fields in gases through magnetically-induced depolarization. Notable advantages of the technique include a high sensitivity, good temporal resolution, low shot noise, non-intrusive probing, species-selectivity when coupled with resonance-enhanced multiphoton ionization (REMPI), single-shot acquisition, and the capability of time gating due to continuous scanning.</p> <p>Originally, the diagnostic was used for the measurement of electron total populations and number densities in collisional, weakly-ionized, and unmagnetized small plasma objects – so called collisional scattering. However, despite increased interest in recent years, the technique’s applicability to collisionless plasmas has remained relatively unexplored. This dissertation intends to expand upon the theoretical, mathematical, and experimental basis for CMS and demonstrate the constructive Thomson & Rayleigh scattering regimes for the first time. Furthermore, this work seeks to explore other novel and relevant capabilities of CMS including electron momentum-transfer collision frequency measurements via scattered phase information and spatially-resolved electron number characterizations of elongated plasma filament structures.</p> <p>This dissertation additionally leverages the technique to diagnose microplasmas and situations of particular interest. Primarily, photoionization (PI) – including UV resonance-enhanced multiphoton ionization, non-resonant visible PI, and mid-IR tunneling ionization in gaseous media. Such processes bear importance to studies on nonequilibrium plasmas, soft ionization in mass spectrometry, the development of compact particle accelerators, X-ray and deep UV radiation sources, laser-assisted combustion, laser-induced breakdown spectroscopy, species detection, mixture characterization and spectroscopy, studies on nonlinear beam propagation (filamentation, self-trapping and pulse splitting, dispersion, modulation instabilities), and so on. Finally, the application of CMS to ion thrusters is demonstrated.</p>

<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>