Flexible standing-wave-node adjustment in square cavity to facilitate Fano resonance and nanosensing analyses

Abstract

The influencing factors and modulating effects of Fano resonance in a plasmonic metal–insulator–metal waveguide Fano system have been extensively studied in the past. While standing wave node of the electromagnetic field in the resonator in the Fano system, which can play a good and flexible regulating role in Fano resonance in its own way, has not been systematically studied. In view of the important and special influence of standing wave nodes on Fano resonance, it is necessary to be studied systematically. This study investigates a plasmonic Fano-resonance system comprising a metal–insulator–metal waveguide and stub resonator, which is side-coupled to a square-cavity resonator. The variations in the transmission properties of the system corresponding to changes in its different geometric parameters are investigated via finite-element analyses. Furthermore, the peak Fano-resonance wavelength equation deduced in this study is applied in combination with the standing-wave theory to explain the Fano-resonance phenomenon. The results obtained reveal that the effect of the nodes corresponding to the standing-wave modes (1, 0) and (1, 1) in square/rectangular cavities facilitates the remarkable and flexible regulation of Fano resonances corresponding to the said modes as the stub shifts from the horizontally self-symmetric to the self-asymmetric orientation about the vertical nodal lines. The formation mechanism of standing-wave-node adjustment in square cavity to facilitate Fano resonance is analyzed. When the stub remains horizontally self-asymmetric about the vertical nodal lines of the mode (0, 1), the first-order mode in the square cavities, which facilitates the realization of Fano resonance, is dual in nature and comprises the (0, 1) and (1, 0) modes. It is confirmed that more favorable relative positions of the stub and the square cavity result in a higher probability of realizing Fano resonance Six instances of Fano resonance of the structure comprising two single-side-coupled square cavities are realized in this study. The coupling arrangement is such that the Fano resonance remains unaffected by the standing-wave nodes in the square cavities. The proposed structure demonstrates a sensitivity of 1128 nm/RIU (RIU: refractive index unit). Furthermore, the geometric parameters are optimized considering a fixed environmental refractive index of 1.04. The resulting optimum figure of merit equals 2.76 × 104. The proposed plasmonic Fano system demonstrates great potential for use in micro–nano photonic integrated circuits and high-sensitivity sensors.