Theoretical Study On Plasmonic Sensing Of Metal Nanoparticle Dimers

Plasmonic structures and devices can realize the manipulation and control of photons at the nanometer scale,providing a reliable way for the development of all-optical integration and more efficient nanophotonic devices.They are also widely used in biological sensing,optical sensing,and nanophotonics,and show very good prospects in application fields such as electronic devices and high-density optical storage.Thus plasmonics has raised great interest in current scientists with diversified research fields.The antenna effect and local field enhancement effect produced by the interaction of light and metal nanostructures can be used as a basis for the design of a new generation of faster and more sensitive integrated photonics devices.This thesis mainly focuses on the sensing properties produced by the interaction between light and different metal nanoparticle dimers,and wishes to provide reliable numerical support for further exploration of the design of highly sensitive optical sensor devices.The main work of this paper is as follows:First,we use the finite difference time domain method to study the extinction spectrum and near-field distribution of single gold nanospheres.For a single nanosphere,the resonance wavelength has a nearly linear relationship with the refractive index,and the maximum field enhancement also increases with the increase in the refractive index.At the same time,the resonance wavelength is positively correlated with the extinction cross section and maximum field enhancement factor.However,for the double gold nanosphere structure,the extinction spectrum has different change features.At this time,there is one more variable that can be used to adjust the movement of the resonance peaks,that is,the distance between the two balls.By adjusting the nanogap spacing,we can control the movement of the resonant peak,and we have found that the movement of the resonant peak is no longer unidirectional.When the distance is greater than 5 nm,by increasing the distance,the resonance peak will blue shift;in contrast,when the distance is less than 5 nm,by increasing the distance,the resonance peak will red shift.At the same time,the influence of maximum field enhancement factor and extinction cross section no longer have equal correlation with the movement of the resonant peak.By discussing the relationship between the movement of the resonant peak and the maximum field enhancement factor and extinction cross section,the highly sensitive optical sensing characteristics can be well explained.Next,we study the spectral response of more complicated metal nanoparticle dimers.For the gold-silver nanosphere dimer,although the resonance peak moves in one direction,the coupling between the gold nanosphere and the silver nanosphere is very complicated and results in many large and small secondary peaks in the extinction spectrum.Moreover,as the background refractive index increases,the secondary peaks are red shifted while their intensity also increases,and they tend to become new resonant peaks.For the bow-tie gold nanoparticle dimer,we also study its extinction spectrum.We have found that the optical sensitivity of the bow-tie nanostructure is higher than that of the double-ball system.Due to the sharp apex of the bow-tie nanostructure,there is a strong electric field localized within the apex area.In the process of adjusting the background refractive index,the resonant peak presents a nearly linear red shift.At the same time,by adjusting the apex angle in the bow-tie nanostructure,when increasing the apex angle,the resonance peak moves in the direction of short wavelength.However,when the apex angle is greater than 50°,the blue shift of the resonance peak is almost zero,and when we continue to increase the apex angle,the resonance peak starts to blue shift again when the apex angle is greater than 60°,but the blue shift is very small.This means that the apex angle adjustment has a transition zone with a size of about 10°,within which optical characteristics such as resonant peak wavelength and extinction cross section remain basically unchanged.Plasmonics has been widely favored in various application fields due to its unique advantages,and the study of the interaction between light and metal nanostructures is very important.The electromagnetic field that is highly localized on the surface of metal nanostructure has a natural sensitivity to the characteristics of the electromagnetic medium on the surface of nanostructure.In addition,the highly enhanced electromagnetic field also enhances the interaction strength between the resonance mode and the surface-adsorbed object,which further enhances the sensitivity.It is expected that with further research in the future,we can have obtained a deeper understanding of its microscopic process and thereby promote the development of lower power consumption and high sensitivity optical sensing technology.