Characterization of gases nonlinear refractive index and energetic deep UV generation in hollow core fiber

• Autor(a) principal: Souza, Tiago Gualberto Bezerra de
• Data de Publicação: 2019
• Tipo de documento: Tese
• Idioma: eng
• Título da fonte: Biblioteca Digital de Teses e Dissertações da USP
• Texto Completo: https://www.teses.usp.br/teses/disponiveis/76/76131/tde-18052020-142901/
• topic: Atmospheric gases; Gases atmosféricos; Geração de UV; Índice de refração não linear; Nonlinear ellipse rotation; Nonlinear refractive index; Óptica ultrarrápida; Rotação não linear da polarização elíptica; Ultrafast optics; UV generation

We have measured, for the first time, the magnitude and temporal pulse width dependency of the nonlinear refractive index of gaseous samples using the nonlinear ellipse rotation effect. Due to their low density, gases nonlinear refractive index are hardly accessed through available techniques commonly used for solid and liquid samples. There are techniques based on spectral broadening, interferometry, four wave-mixing, and filamentation which probe gases nonlinear refractive index, though each one present some sort of limitation or constrain. In order to provide an alternative to the previous techniques, we present a novel experimental setup to measure gases nonlinear refractive index for several excitation pulse width. The weak gases nonlinearity was overcome by extending the gas-light interaction length using a hollow core fiber, also known as capillary. By this way, we measured the nonlinear refractive index of helium, argon, nitrogen, and oxygen, along with their mixture into the atmospheric gas. Molecular gases nonlinear refractive index contribution results from its intra-pulse molecular alignment, which is dependent on the pulse width. On the other hand, we have profited from noble gases instantaneous electronic contribution, as they are monoatomic gases, to obtain a temporal pulse calibration based on argon nonlinear ellipse rotation signal. In order to unveil the transition and the amount of each contribution in gases, we have studied and characterized the effective nonlinear refractive index of the studied samples varying the pulse width from 40 fs up to 3 ps in a continuous manner. An empirical model has been proposed in order to distinguish the electronic nonresonant from the molecular orientation contribution for the effective nonlinear refractive index. In the second part of this work, we have explored infrared pulses spectrum broadening in hollow core fiver filled with noble gases, followed by proper temporal compression to reach sub-10 fs IR pulse, which was used to excite a soliton breaking phenomena in order to study the generation of energetic and ultrashort tunable deep ultraviolet light source. The dispersion wave emission in gas filled hollow core fibers have been recently explored for this very end. New sources of light, with enough energy and ultrashort pulse width, capable to excite nonlinearities in the materials, has been extremely desired for spectroscopy in the deep UV region.