Design And Transmission Characteristic Analysis Of Optical Sensor Based On SPR

Surface plasmon polaritons（SPPs）are electromagnetic wave modes caused by the interaction of incident photons and free electrons on the metal surface to enhance the collective vibration of electrons.On the basis of breaking the conventional diffraction limit and controlling it within nanoscale domain,SPPs provide some potential advantages（high miniaturization and integration）for the development of nanophotonic devices.Metal-insulator-metal（MIM）waveguide,which is one of the most promising waveguide structures due to the advantages including:strong confinement of SPPs,low band loss and easiness to fabricate.As a special optical phenomenon,Fano resonance originates from the destructive interference between the narrow discrete state and the wide continent state,exhibiting a sharp and asymmetric line shape.Due to the merits of Fano resonance,such as steep dispersion and susceptibility to ambient refractive index（RI）changes,Fano resonance-based MIM waveguide devices（sensors,switches,filters and slow-light devices）have become a hot topic of incessant interest for researchers in recent years.One such particular example is the surface plasmon resonance（SPR）sensor,which utilizes the detection of the Fano resonance shift to obtain the medium-related parameters（temperature or concentration）.In this paper,the finite element method（FEM）is used to simulate and analyze the proposed novel nano-optical sensor structures based on SPPs,which are composed of MIM waveguide side-coupled the nano-resonator.The main research contents of this paper are as follows:（1）This paper combs the development history and research status of surface plasmons,and discusses the basic principle of the surface plasmons.Based on the Maxwell equations,the theory of surface plasmon polaritons excited on the interface between metal and dielectric and its basic characteristics are deduced in detail.Then the dispersion of the MIM waveguide is also introduced.（2）The MIM waveguide structure which comprises a circular resonator embedded with a defective metal nanocube（CRMN）and the MIM waveguide structure which comprises an eight-like resonant cavity are simulated and analyzed by FEM,correspondingly.For the first device structure,after analyzing the transmission and sensing characteristics of two different resonator structures,the CRMN resonator was selected.Then,the effects of the rotation angleθof the CRMN resonator,the length d and width b of the internal stub on the transmission and sensing performance of the device were investigated.The research shows that the introduction of stubs inside the resonator can lead to the splitting of the Fano resonance mode,and the change of the structural parameters will obviously affect the transmission and sensing characteristics of the device.For the MIM waveguide coupled eight-shaped resonator structure,after analyzing the transmission and sensing characteristics of the initial model,the parameters such as the side length S₁,stub width b,and coupling gap g of the eight-shaped resonator are mainly studied.Influence of sensory performance.The resonant cavity and the inner filling material of the MIM waveguide were replaced with polydimethylsiloxane（PDMS）.Based on the better sensing performance of the structure and the excellent temperature-sensitive properties of the PDMS material,the device achieved a-20.3nm/°C temperature sensitivity.Finally,the modeling parameters were optimized,and both sensor structures achieved good performance,with refractive index sensitivities of 10200nm?RIU^-1 and 6000nm?RIU^-1,respectively,and FOM values of 12.75RIU^-1 and 17RIU^-1.（3）In order to compare and analyze the performance difference between the axisymmetric structure and the non-axisymmetric structure,two non-axisymmetric structures,namely the MIM waveguide side-coupled Z-shaped nanocavity structure and the MIM waveguide side-coupled L-shaped nanocavity structure,were simulated and studied first.The influence of structural parameters such as the rotation angle of the cavity on the transmission and sensing characteristics of the device is analyzed.After parameter optimization,the sensitivity values were calculated to be 1500nm?RIU^-1 and 1986nm?RIU^-1,respectively.On this basis,the sensor structure based on the side-coupling Z and L-shaped cavity of the MIM waveguide structure is also studied.By optimizing the structural parameters such as the rotation angle of the resonant cavity and the distance between the two cavities,the refractive index sensitivity of the sensor reaches 3511 nm?RIU^-1.Finally,through the detailed comparative analysis of the performance of the axisymmetric structure and the non-axisymmetric structure,some theoretical support for the design of this type of sensor is provided.