The Investigation Of Refractive Index Sensing With Metafluids Nanostructures

Refractive index sensing based on localized surface plasmon resonances(LSPRs)with noble metallic nanoparticles has gained a lot of attentions in recent years,where strong near-field enhancements can be achieved with the excitation of LSPRs,and the resonances can be strongly tuned by manipulating the refractive index of the surrounding medium.Besides that,LSPRs are governed by the materials,the geometry and the size of the nanoparticles.These prosperities make LSPRs very useful for sensing applications,and LSPRs have been used for medical diagnosis,food security and the quantitative detection of biomolecules or heavy metallic ions.Compared with that of the conventional sensing method,refractive index sensing based on LSPRs has excellent properties.For example,it's very sensitive to the change of the refractive index of the surrounding medium,and the sensitivity can be strongly enhanced.In addition,the interactions between molecules and surface plasmon resonances can be very strong because of the small mode volumes and the enhanced optical density around the nanoparticles.However,due to the strong radiative andnonradiative losses of metallic nanoparticles,LSPRs often possess broad line widths,and it's hard to further enhance the sensing performance(figure of merit)with LSPRs.In order to solve the above problems,we propose a refractive index sensing method based on the transparency window of noble metal nanoparticles.In this paper,we use the seed growth method to prepare gold nanorods and Au core Ag shell nanorods with different aspect ratio.The transparent window can be designed according to the resonant positions of these metallic nanoparticles,we could adjust the position and line width of the transparency window by changing the concentration of nanoparticles colloidal solutions,which can be used to improve the refractive index sensitivity.The experimental results indicate that broadband absorption in the range of 300-1200 nm can be achieved,and a narrow(?150nm)transparency window appears at a specific wavelength,where the transparency window as well as the line width can be adjusted.The time-domain finite difference method has been used to calculate the optical responses,and the transparency window shows a strong red-shift when the refractive index of the surrounding medium is enlarged.Calculation results show that the line shift sensitivity is about 452 nm/ RIU,and the FOM is about 6.7.In addition,we designed a metal/dielectric hybridized nanostructure composed of a silver double rectangular nanoring and a silicon square plate.The coupling between the anapole mode of the Si nanoplate and the multipolemode of the Ag ring can generate double Fano resonances,and the radiative losses of the structure can be effectively suppressed,where the line width of the Fano resonances can be as narrow as 13 nm,and it would be useful to improve the sensing performance.