Study On Resonance Coupling Between Surface Plasmon-polarition And Waveguide

Surface plasmon-polarition?SPP?is an electromagnetic oscillation formed by the interaction of free electrons and photons in the surface area of a metal.Because SPP breaks through diffraction limit of light,electromagnetic wave can be conducted,split and focused on the dimensions of subwavelength,SPP has widely used in optical fiber communication,optical sensing,solar energy and other fields.Among all researches,structure based on the coupling mode is the most widely used sensor device due to its high sensitivity and stability.Currently,there have been a lot of reports on the SPP-coupled waveguide?WG?,but there is a lack of systematic research on the coupling behavior between SPP and WG.When the thickness of the coupling layer between metal and waveguide is large,coupling strength is weak,which exhibits as Fano resonance.When the thickness of spacer layer is below a certain critical value,strong coupling results in superposition hybrid behavior.Based on these,this paper proposes a sensor structure in which a bimetallic layer excited SPP and waveguide modes are coupled.The main research is as following:The common multilayer planar waveguide structure was studied and coupling coefficients between SPP and WG were defined.Influence of different structural parameters on coupling strength was studied by changing a series of structure parameters such as metal layer thicknesses,bimetallic layer ratio,and coupling layer thicknesses.The physical parameters such as coupling coefficient,resonance wavelength,half-peak width,and propagation constant difference were extracted.Based on these data,the parameters such as Au/Ag ratio and metal layer thickness were optimized.A new sensor structure model of WG coupled with SPP mode excited by bimetal layer was designed,and the reflectivity curves of the system under different waveguide thicknesses and Au/Ag ratios were calculated by the transmission matrix method.Fano model formula was used to fit reflectance curves,and Fano constants under different structures were obtained.Dependencies of Au/Ag ratio on Fano resonance peak position,coupling strength,half-peak width and Fano constant were systematically analyzed.It was found that the optimal Au/Ag ratio of the system is 5/45 nm.Furthermore,the sensitivity and stability of the structure under the change of ambient refractive index were discussed.It is found that the sensitivity of the system can reach S_max=1.5×10³ RIU^-1 and FOM=1.4×10⁵,which is about 2-3 times higher than reported results.The behavior of strong coupling hybrid mode between metal SPP and WG under single-mode and multi-mode conditions was further analyzed.Variations of physical quantities such as effective refractive index,loss,and propagation constant with the coupling distance were calculated.By comparing electric field inside the structure distribution at different distances,the evolution of the system under single-mode condition from Fano resonance to hybrid mode is discussed,and the boundary conditions of different resonance modes are finally defined.The evolution behavior of the hybrid mode between the SPP and the waveguide with the thickness of the coupling layer under multi-mode conditions is studied by the transfer matrix method and coupling mode theory.We use the parameter of mode loss to characterize the weight of SPP in hybrid mode.Three different stages are found and defined during the evolution process of hybrid plasmonic waveguide mode.