Analytical study of Fano phenomenon in plasmonic hexamer and heptamer using characteristic mode theory

In this paper, the optical properties of several Plasmonic structures including single nanodisk, Hexamer and Heptamer, using the Characteristic Mode Theory (CMT) are investigated. Single nanodisk and Hexamer have shown a bright mode with Lorentzian response. In Heptamer, a strong interaction is created between the nanoparticles and thus the Fano resonance occurs with a bright and a dark mode. Based on the CMT analysis and the definition of the Fano response, it has been observed that the collision of the modes in the modal significance diagrams is where the Fano response occurs. Due to the characteristic current distributions, bright and dark modes have been observed that are necessary to create a Fano response. So, using CMT, it can be determined whether the structure is capable of generating a Fano response or not. Also, the frequency of bright and dark modes and the Fano minimum can be obtained using CMT. Moreover, according to the characteristic currents, the structure can be modified to achieve a sharper Fano response. We have selected such structures to investigate this method in these types of Heptamers and it has been shown that the proposed method is applicable for such structures.