Research On Fano Resonance Mechanism In Subwavelength MIM Waveguide With Multiple Cavities

Photonic devices have gradually become the main components of integrated circuit chips,because of their superior information capacity,response speed and anti-cross interference ability,but there is a bottleneck of classical diffraction limit in micro/nano dimension.Surface plasmon polaritons(SPPs),as one of the ways to modulate light in the subwavelength range,have the ability to break through the classical diffraction limit,which brings a promising prospect for the integration and chip of photonic devices.Metal-insulator-metal(MIM)waveguide has become a hot plasmonic structure in recent years,due to its advantages such as easy fabrication,simple structure,and tens of microns propagation distance for SPPs.Fano resonance is an interference effect between resonant modes generated in the plasmonic structure.Its special sharp and asymmetric spectral line shape contains the properties of extremely sensitivity to refractive index variation and strong dispersion,which makes Fano resonance has important applications in biosensors,optical switches and slow light devices.In this paper,by employing numerical simulation calculations of finite difference time domain(FDTD)method and theoretical analyses of multimode interference coupled mode theory(MICMT),we mainly studied the Fano resonance mechanism and its sensing performance and slow light characteristic in the composite MIM waveguide structures with multiple resonators.The main research contents and results are as follows:(1)We designed a MIM waveguide structure end-coupled with cross-shape cavities,and new Fano resonance could be generated by breaking the symmetry of the structure.Then,we discussed the propagation characteristics of SPPs and the generation mechanism of Fano resonance in this asymmetric structure,followed by analyzing the sensing performance and slow light characteristic of the structure.The proposed structure finally generated 7 Fano resonances using the asymmetrical structure and the coupling effect of multiple resonators.(2)A metal baffle was introduced into a MIM waveguide with a side-coupled semi-ring cavity.The influence of the metal baffle has been studied in particular,conclusion of which was that the design of the metal baffle could generate extra Fano resonances compared to other methods with the condition of equal device area.The proposed structure had 6 spectral responses of Fano resonance.The corresponding maximum positive and negative delay time were 0.21 ps and-0.31 ps,respectively,and the highest attainable refractive index sensitivity was 1405 nm/RIU.Therefore,this proposed structure could be used to construct refractive index sensors or slow light devices.(3)A tooth-shape cavity,which was embedded in an end-coupled MIM waveguide structure,could be used to capture part of SPPs energy to form new resonant modes,so that new Fano resonances would be generated without increasing the device area cost.Based on this approach,the proposed structure formed up to 8 Fano resonant windows,within which the maximum positive and negative delay time were 0.16 ps and-0.39 ps,respectively,and the sensitivity was high to 1477 nm/RIU.(4)According to the biosensing conditions for refractive index sensors,a MIM waveguide structure with water-based dielectric was proposed.We emphatically studied the propagation characteristics of SPPs in aqueous dielectric environment and the generation mechanisms of Fano resonances.Through the numerical analyses,it was found that the proposed structure had good sensing performances in aqueous dielectric environment of an ultra-high sensitivity of 1813 nm/RIU and a figure of merit of 5.98×10~6.We believed the proposed structure may have excellent sensing performance in biochemical sensing.