Study On Evanescent Field Optical Fiber Sensor Decorated With Micro/nano-structures

Biochemical sensors for the diagnosis of diseases, food, and environmentaldetection of biological agents have been increasingly in demand over the past fewdecades. Combined evanescent field optical fiber sensing with localized surfaceplasmon resonance technologies, this work presents an evanescent field enhancementoptical fiber sensor based on metal micro/nano-structures.This thesis is mainly carried out from these two parts:(1) as for the theoreticalsection, the sensing mechanism and the optical properties of the metalmicro/nano-structures have been both analyzed and optimized;(2) in the experimentalpart, a variety of analytes with fractional content have been tested by the evanescentfield optical fiber sensor decorated with metal nanostructures.Firstly, the sensing principle of the evanescent optical fiber is defined and theimpacts of several key parameters on the sensor sensitivity are also analyzed. Thesurface plasmon properties of the metal nanoparticles depend on the geometry,material, and dielectric environments, etc. and have been carefully studied. Besides,the interactions which occur between nanoparticle and the sensing optical fibersubstrate and among particles have been studied using the FDTD method. Secondly,combining both the sharp tips and the metal/dielectric/metal multilayer structurestogether, an Ag/SiO2/Ag sandwich nanocrescent has been proposed. The extinctionefficiency and LSPR property below different incident polarizations are studied byvarying the thickness of the SiO2layer. Furthermore, for better sensing ability, a novelsandwich delta nanostar that features an Ag/SiO2/Ag sandwich delta star nanoplatewith three sharp angles of30degree is introduced. The structure is shown to producea high local field enhancement as well as wide plasmon resonance tunabilities.Moreover, the influence of the vertex truncation on the refractive index sensitivity andthe extinction spectra has also been analyzed based on the existing fabricationcondition.In this work, the evanescent field optical fiber sensor modified with silvernanoparticles based on a MEMS micro-channel chip has been successfully fabricatedand experiment with different concentrations of the detecting analytes is performed.The concentration of the methylene blue solution varying per100nmol/mL can causethe absorbance change0.402dB, which is about three times higher than the same sensing without any decoration.Finally, the Ag-Au core-shell composite nanostructure is adopted for enhancingevanescent field energy. And by adjusting the size parameters can achieve the tuningof absorption properties, which is explained by the hybridization model theory for theplasmon response of complex nanostructures. Experimentally, the evanescent fieldenhanced optical fiber sensor based on the core-shell composite nanostructures isprepared and applied to the melamine detection. The sensing sensitivity is2.869dB/(ug·mL-1) with the detection limit of1ug/mL.From above studies, it shows that the evanescent field optical sensor decoratedwith metal nanostructure is of low cost, high sensitivity and convenience, which willpotentially operate on rapid, real-time and continuous detection in biochemical analytedetecting applications.