Research On Fano Resonance Effects In MIM Plasmonic Waveguides

Metal-insulator-metal(MIM)plasmonic waveguide can break the traditional diffraction limit,localizing light waves in the intermediate dielectric layer at a sub-wavelength scale,which makes it play an important role in the miniaturization of nanophotonic devices and sensing applications.Meanwhile,the Fano resonance effect,a research hotspot in recent years,has a sharp and asymmetric line shape,high spectral resolution,and is extremely sensitive to the changes of structure and dielectric environment,making it have excellent performance in high sensitivity sensing at the nanoscale.Due to the outstanding characteristics of the MIM plasmonic waveguide and Fano resonance effect,in recent years,the Fano resonance effects in MIM plasmonic waveguide structures have been widely studied,which will be the research focus of this dissertation.By drawing and studying the transmission spectra “nephogram” of the coupling of a ring-MIM(rectangle-MIM)plasmonic waveguide and a rectangular cavity resonator,the influencing factors and formation laws of the “direction” of Fano resonance line shape are revealed.Meanwhile,by analyzing the transmission spectrum “nephogram”,the locations and strengths of Fano resonances can be obtained intuitively and conveniently,and the optimal geometric parameters for realizing multiple Fano resonances are found.By applying the formation laws of Fano resonance shape “direction” obtained to other structures in this dissertation,the formation mechanisms of Fano resonance line shapes with different directions encountered in the study can be reasonably expounded.Combined with the standing wave theory,the peak Fano-resonance wavelength equation model of a stub-MIM plasmonic waveguide with a sidecoupled cavity resonator is established.The equations and finite element method(FEM)are simultaneously applied to the research of the above structure with the side-coupled resonator being a square cavity or rectangular cavity,achieving very consistent results.The general rule of the modulations of Fano resonances by the vertical standing wave nodes in the resonators is obtained,by studying the case of the change of horizontal relative positions between the stub resonators and the side-coupled resonators.Based on the principle of Fano resonance,the specific physical mechanism of a standing wave node modulating a Fano resonance is revealed.When the structure is unilaterally coupled to two adjacent square cavities,six Fano resonances are achieved.An optimal structure for sensing response with an optimal figure of merit(FOM)is obtained by optimizing several parameters of the structure.By studying the case of different relative horizontal coupling positions between a stub resonator in a MIM plasmonic waveguide and a triangular cavity,hexagonal cavity,circular cavity,or triangular ring cavity resonator,the general condition of magnetic field distributions in polygonal cavities is obtained when Fano resonances can be modulated by standing wave nodes in polygonal cavities.An optimal coupling position between the triangular ring cavity and the stub is found.By adding a funnel-shaped microcavity to the triangular ring cavity,the adjustment mechanisms of Fano resonances by the funnel-shaped microcavity are studied and revealed.A high FOM is obtained by studying the sensing performance of the structure,indicating that the Fano resonance system can be applied to high-performance nanosensors.In order to explore the formation mechanisms of Fano resonances in different structures,a new structure of a circular cavity resonator directly connected with the MIM plasmonic waveguide is designed and studied.By researching the changes of the position and size of the circular cavity in various ways,the double Fano resonances can be achieved when the symmetry of the circular cavity concerning the MIM plasmonic waveguide is broken,and the forming mechanisms of these Fano resonances are revealed.By comparing with the Fano resonances of other structures studied in this dissertation,it is confirmed that the Fano resonances can occur not only in the coupling of SPPs in different resonators in a MIM plasmonic waveguide system,but also in the coupling of different modes of SPPs in a single resonator which is symmetry breaking concerning a MIM plasmonic waveguide.