Nonlinear Optical Response and Plasma Generation in Noble Gases and Molecular Nitrogen: Development of the Metastable Electronic State Approach for Optical Filamentation

Abstract

This dissertation reports on the development of the quantum-theory-based Metastable Electronic State Approach (MESA) for modeling light-matter interactions in the near- IR to long-IR wavelength regions. MESA is a first-principle description for the non- linear optical response of the medium, making use of the metastable solutions of the stationary Schro ̈dinger equation including the quasi-static homogeneous electric field. Its primary purpose is to function as a computationally efficient model of the medium with minimal parameters. The theoretical underpinnings and preliminary numerical tests of MESA have been demonstrated in recent publications. This dissertation develops upon previous work by finding numerical solutions for metastable resonances to bring to fruition the single-state Metastable Electronic State Approach (ssMESA). To create the numerical toolkit to practically implement ssMESA, we utilize the Single-Active-Electron (SAE) approximation, design a parameter-free model, and present a quantitative assessment of the theory, initially for noble-gas atoms. Additionally, with the use of data from the multi-electron hybrid-antisymmetrized Coupled Channel Approach, the ssMESA has been extended to molecular Nitrogen. The extension to Nitrogen demonstrates the utility of the ssMESA beyond the SAE description and noble gases, moving towards a full model of the atmosphere. The core of this dissertation deals with the extension beyond ssMESA, first realized in the form of “Post-Adiabatic” corrections. The post-adiabatic (paMESA) model is used to demonstrate the wavelength-dependent enhancement of strong-field ionization. We further show that the paMESA is efficient in computationally capturing the light-matter dynamics in two-color optical pulses. An important outcome of this dissertation is a first of its kind experiment-theory comparison of the transient nonlinear response for a light-matter interaction model applicable to optical filamentation. The quantitative agreement has been demonstrated for peak intensities with complex dynamics due to competition between self-focusing and plasma-induced defocusing where the optical field acts in the intermediary region between perturbatively and a strong field. The comparison encompasses more than forty experiments on several gas species and highlights the robustness of the MESA. Finally, in looking forward the MESA is developed beyond the paMESA model by explicitly including the first excited state to describe non-adiabatic e↵ects in a two-state system. The inclusion of the excited state eliminates the need for the sole fit parameter in the paMESA model as the initial exploratory step toward building a multi-state MESA. In summary, the work described in this dissertation elevates the MESA from the proof-of-principle stage and transforms this framework into a practical tool with applications throughout Nonlinear Optics.