High Q-factor Fano resonances in a double D-type metasurface based on toroidal dipole bound states in the continuum

In this paper, a double D-type all-dielectric metasurface is proposed to achieve high Q-factor Fano resonances. This metasurface utilizes bound states in the continuum (BIC) and serves the dual detection function of refractive index (RI) and temperature. Using the time-domain finite-difference (FDTD) method, we analyze the near-field electromagnetic contribution at different resonance wavelengths. The study demonstrates that new resonances with a high quality factor (Q-factor) in the near-infrared wavelength are excited. This is achieved using the quasi-bound states in the continuum (quasi-BIC) resonance mode by changing the inner diameter of the half-hollow cylinders, highlighting the toroidal dipole (TD) resonance. Furthermore, we investigate the influence of the structural parameters on the Fano resonance. We also compute the spectral response of the RI of the medium, yielding an excellent Fano resonance with a Q-factor of 40,518. The sensing characteristics of the designed structure are evaluated. This reveals a RI sensitivity (S) of 194 nm/RIU at a figure of merit (FOM) of 7320, while achieving a temperature sensing sensitivity up to 47 pm/°C. The proposed structure demonstrates potential applications in the near-infrared region, which provides valuable insights for designing optical biosensors and switches based on quasi-BICs.