Análise numérica de nanoestruturas de ouro na sensibilidade de redes de Bragg corroídas

Abstract

This work presents sensors based on etched fiber Bragg gratings (EFBGs) functionalized with gold nanostructures (AuNEs) and exposed to external media with refractive indices ranging from 1.00 to 1.44. Using COMSOL Multiphysics®, different configurations of AuNEs on the fiber core were evaluated, considering varying sizes and distances between them. Qualitative and quantitative results were obtained as a function of the effective refractive index, Bragg wavelength, sensor sensitivity, confinement loss, fiber guiding profile, electric field intensity in the sensor, modal field diameter, and other parameters. For a fiber without AuNEs coating, it was qualitatively observed that the modal diameter increases with the external refractive index, which in turn enhances the evanescent field, leading to increased sensor sensitivity to the external refractive index. When coated with AuNEs, a reduction of the electric field intensity in the core was also observed as the external refractive index increased, associated with the confinement loss phenomenon and the increased electric field intensity between the AuNEs. There is a clear correlation between the decrease in core electric field intensity and the increase of the electric field intensity between the nanostructures. Finally, laboratory results were presented to compare with computational simulations, highlighting the analysis of attenuation and sensitivity of EFBGs functionalized with gold nanoparticles (AuNPs) as a function of the external refractive index and AuNPs separation. A model was presented to predict a critical separation distance at which full attenuation occurs for all external refractive indices, along with a maximum sensitivity at this condition. These critical separation distances were found to be of the same order of magnitude as the AuNPs diameter, although not necessarily the same value. The model was experimentally verified in three EFBGs with different average AuNP separation distances. Despite inevitable differences between the model and experimental conditions, the attenuation and sensitivity profiles showed the same trends in both simulated and experimental results. This indicates that the model’s free parameters allow for the description of more complex conditions than those considered in the simulations. Therefore, the empirical models proposed in this work adequately describe attenuation and sensitivity as a function of the AuNP separation distance. Finally, these models may be useful for optimizing EFBGs functionalized with nanoparticles.