| With the continuous in-depth exploration of surface plasmons(SPPs),researchers found that surface plasmons can break the diffraction limitations and realize the light regulation in nano size.Metal dielectric metal(MDM)waveguide has a simple structure and is easy to excite SPPs.Combined with the continuous development of micro nano technology,it has a wide prospect in the high integration of photonic devices.In recent years,a large number of researchers at home and abroad have deeply explored the Fano resonance effect introduced into the MDM waveguide structure.The wide state is formed by adding branches on the straight waveguide or adding one or two metal baffles inside the straight waveguide,while the narrow state is generated by resonators with different shapes,such as disk-shaped,ring-shaped,square or defective resonators.The line shape of Fano resonance is sharp and extremely asymmetric.When the refractive index of the surrounding environment changes,the Fano resonance peak shifts significantly.Multiple Fano resonance has great advantages in the accuracy and fault tolerance of sensing parameters,so it has a good application in refractive index sensor.The main research contents of this paper are as follows:First of all,according to the propagation characteristics of the surface plasmon in the metal-dielectric-metal(MDM)waveguide,a square resonator connected with a rectangular groove is proposed to couple with the highly compact metal-dielectric-metal(MDM)waveguide containing double metal baffles.The transmission spectrums of the proposed waveguide structure have been calculated by the finite element method,which exhibits four sharp and asymmetric transmittance peaks and the magnetic field pattrrns.Based on the coupling interference between the broadband reflection mode generated by the double baffles and the narrow resonance modes occurred in the composite cavity,the generation mechanism of quadruple Fano resonance is explained.Moreover,the structural parameters of the square cavity and the rectangular groove are changed to study the tenability of the wavelengths of the quadruple transmittance peaks.When the composite cavity is filled with different dielectric media,the calculated sensitivity corresponding to four transmittance peaks are 960,1120,1320 and 1560 nm/RIU,respectively.and the quality factor value is also up to2.859×10~5.This waveguide structure provides a theoretical reference for the design of integrated multi-channel high-sensitivity nano optical refractive index sensor.Then,a structure of ring resonator waveguide with rectangular groove is proposed.Firstly,the coupling of square cavity with rectangular groove and MDM straight waveguide is studied.Four transmission minima are generated in the wavelength range of 850 nm～1800 nm.The transmission minima in the transmission spectrum are adjusted by adjusting the position and geometric parameters of rectangular groove.The continuous state generated by the bimetallic baffle is coupled with the discrete state generated by the square ring cavity waveguide structure containing RG to form Fano resonance.However,Fano resonance is only formed in the integer mode.The transmission minimum transmittance in the non integer mode is high,which cannot be coupled with the continuous state formed by the double baffle to form Fano resonance.In contrast,the continuous state produced by the single baffle structure is more likely to interfere with the discrete state,forming four Fano resonances.By changing the refractive index of the medium in the resonant cavity,the Fo M value can reach 1.53×10~4.The calculated sensitivity corresponding to four transmittance peaks are 1000,1200,1600 and 2000 nm/RIU,respectively. |
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