In this paper, a multichannel refractive index sensor based on a subwavelength metal–insulator–metal (MIM) waveguide coupled with tangent-ring resonators is proposed. When two tangent-ring resonators were placed above the MIM waveguide, Fano resonance with asymmetrical line shape appeared in the transmission spectrum due to the interference between the light–dark resonant modes. The sensitivity and figure of merit were as high as 880 nm/RIU and 964, respectively. Through adding more tangent-ring resonators, multiple Fano resonances with ultrasharp peaks/dips were achieved in the wavelength range of 800–2000 nm. Besides, negative group delays were also observed in the Fano resonant dips. Two-dimensional finite-difference time-domain (FDTD) method was used to simulate and analyze the performances of the proposed structures. These kinds of multiring structures can find important applications in the on-chip optical sensing and optical communication areas.

In this paper, an end-coupled hexagonal resonator inserted with dual parallel metallic blocks is proposed based on subwavelength metal–insulator–metal waveguides. When the blocks are vertically inserted into the resonator, more transmission channels (three peaks) with symmetrical spectral shapes than that (one peak) of the perfect hexagonal resonator are achieved in the same wavelength range. The transmission peaks all have high transmittances; thus, the structure can be performed as an on-chip optical filter. When the blocks are horizontally distributed in the resonator, the antinode and node of the magnetic field for the expected mode will arise inside and outside the blocks, leading to the mode interactions. Subsequently, Fano resonance with an asymmetrical peak is achieved in the structure. High index sensitivity and high figure of merit, which are significant factors for optical sensors, are investigated by using the finite-difference time-domain method. The proposed structure can highly support the development of integrated photonics and find wide applications in the on-chip optical filtering and sensing areas.

In this paper, multiple Fano resonances preferred in the refractive index sensing area are achieved based on sub-wavelength metal-insulator-metal (MIM) waveguides. Two slot cavities, which are placed between or above the MIM waveguides, can support the bright modes or the dark modes, respectively. Owing to the mode interferences, dual Fano resonances with obvious asymmetrical spectral responses are achieved. High sensitivity and high figure of merit are investigated by using the finite-difference time-domain (FDTD) method. In view of the development of chip-scale integrated photonics, two extra slot cavities are successively added to the structure, and consequently, three and four ultra-sharp Fano peaks with considerable performances are obtained, respectively. It is believed that this proposed structure can find important applications in the on-chip optical sensing and optical communication areas.

In this paper, multiple Fano resonances based on semi-ring-rectangular composite cavity (SRRCC) structure are numerically investigated and explained by FDTD method and MICMT method. High refractive-index sensitivities and high figure of merits (FOMs) are obtained in SRRCC system, and thus it may be preferred in the chip-scale optical sensing area. Besides, the phase response and the delay time within the Fano resonant windows are also investigated. Additionally, two more Fano resonances with high sensitivities and FOMs emerge through adding a new semi-ring cavity. These structures show widely applications in the sensing area and promote development of on-chip integrated photonics.