In this paper, three Fano resonances based on three different physical mechanisms are theoretically and numerically investigated in a plasmonic resonator system, comprised of two circular cavities. And the multimode interference coupled mode theory (MICMT) including coupling phases is proposed to explain the Fano resonances in plasmonic resonator system. According to MICMT, one of the Fano resonances originates from the interference between different resonant modes of one resonator, the other is induced by the interference between the resonant modes of different resonators. Mode degeneracy is removed when the symmetry of the system is broken, thereby emerging the third kind of Fano resonance which is called degenerate interference Fano resonance, and the degenerate interference coupled mode theory (DICMT) is proposed to explain this kind of Fano resonance. The sensitivity and FOM* (figure of merit) of these Fano resonances can be as high as 840 nm/RIU and 100, respectively. These are useful for fundamental study and applications in sensors, splitters and slow-light devices.

In this paper, an asymmetric plasmonic structure composed of two MIM (metal-insulator-metal) waveguides and two rectangular cavities is reported, which can support triple Fano resonances originating from three different mechanisms. And the multimode interference coupled mode theory (MICMT) including coupling phases is proposed based on single mode coupled mode theory (CMT), which is used for describing and explaining the multiple Fano resonance phenomenon in coupled plasmonic resonator systems. Just because the triple Fano resonances originate from three different mechanisms, each Fano resonance can be tuned independently or semi-independently by changing the parameters of the two rectangular cavities. Such, a narrow ‘M’ type of double Lorentzian-like line-shape transmission windows with the position and the full width at half maximum (FWHM) can be tuned freely is constructed by changing the parameters of the two cavities appropriately, which can find widely applications in sensors, nonlinear and slow-light devices.

A coupling system is proposed to active control of strong exciton–plasmon–exciton coupling, which consists of a silver nanoprism separated from a monolayer WS₂ by J-aggregates. The scattering spectrum of the hybrid system calculated by the finite-difference time-domain (FDTD) method is well reproduced by the coupled oscillator model theory. The calculation results show that strong couplings among WS₂ excitons, J-aggregate excitons, and localized surface plasmon resonances (LSPRs) are achieved in the hybrid nanostructure, and result in three plexciton branches. We further analyze the exciton–plasmon–exciton coupling behaviors and obtain the weighting efficiencies of the original modes in three plexciton branches. The strong couplings between two different excitons and LSPRs can be active manipulated by tuning the temperature or the concentration of J-aggregates. The proposed systems make up a simple platform for the dynamic control of exciton–plasmon–exciton couplings and have potential applications in optical modulators at the nanoscale.

A plasmonic resonator system composed of a metal-insulator-metal waveguide coupled with a disk and a ring cavity was proposed to produce double Fano resonances, which resulted from the coupling between the narrow discrete resonances of the ring cavity and the broad spectrum of the disk resonator. Based on the proposed structure, end-coupled cavities are added to form new coupled plasmonic resonator systems, which can support triple and quadruple Fano resonances, and the multiple Fano resonances can be tuned semi-independently by changing the parameters of the end-coupled cavities. These characteristics offer flexibility in the design of the highly integrated circuits. Also, the proposed structure can work as a highly efficient plasmonic nanosensor, which yields a sensitivity of ~1100 nm/RIU and figure of merit (FOM) of ~2.73 × 10⁴. The proposed structure may have potential applications for nanosensing, slow light, and nonlinear devices in highly integrated devices.