Optical nonlinearities in microstructures produced by ultrashort laser pulses

• Ano de defesa: 2021
• Autor(a) principal: Henrique, Franciele Renata
• Tipo de documento: Tese
• Tipo de acesso: Acesso aberto
• Idioma: eng
• Instituição de defesa: Biblioteca Digitais de Teses e Dissertações da USP
• Palavras-chave em Português: Direct laser writing; Eagle Glass; Escrita direta a laser; Glass waveguides; Gorilla Glass; Guias de onda em vidros; Índice de refração não linear; Nonlinear refractive index
• Link de acesso: https://www.teses.usp.br/teses/disponiveis/76/76131/tde-09092021-114656/

Direct laser writing (DLW) techniques with ultrashort laser pulses have been extensively used for materials processing. The nonlinear nature of the interaction between ultrashort pulses and matter confers high spatial localization to these techniques and allows the fabrication of compact devices in a single step with resolution in the micro/nanoscale. In this context, we can highlight the fabrication of microstructures in transparent materials, like polymers and glasses, since they allow for the realization of three-dimensional devices. For instance, high-quality direct laser-written waveguide circuits have been fabricated and applied to different fields, such as telecommunications, quantum optics, sensors, etc. As photonic circuits often guide high light intensity, the characterization of the nonlinear optical properties of these structures becomes essential to direct their technological application. In this work, we realized the third-order nonlinear optical characterization of fs-laser written waveguides, which were fabricated in special glasses: Corning® Gorilla® and Eagle XG® Glass. The nonlinear characterization was performed through the Dispersive-scan (D-scan) technique, the temporal analog of Z-scan that can be applied to waveguides. The results obtained for the Gorilla® Glass waveguides indicate that the fs-laser writing process is responsible for reducing the magnitude of the nonlinear refractive index, when compared to the bulk material. This effect depends on the writing parameters, and the reduction is stronger when higher energy pulses are employed. Raman spectroscopy measurements revealed that this reduction could be related to the generation of non-bridging oxygen hole centers (NBOHCs) upon fs-laser irradiation. The characterization of Eagle XG® Glass waveguides, fabricated in different writing regimes (repetitive and cumulative), confirms the reduction of the nonlinear refractive index in the repetitive regime. However, it also revealed that little to no modification occurs when the cumulative regime is used, meaning that heat accumulation can be responsible for erasing NBOHCs and restoring the optical nonlinearities. Our results bring new light to what is currently known about the light-matter interaction when ultrashort pulses are involved. Furthermore, they can help tailor the application of fs-laser written glass waveguides, according to the desire for high or low optical nonlinearities.