Fig. 1. The schematic diagram of single metal waveguide-cavity side-coupled structure. The MDM waveguide is consisted by silver-air-silver. The width of air layer is w_d. The length and width of metal microcavity are L and D, respectively. A slit is used to connect the waveguide and microcavity. The width and height of slit are denoted as C and S, respectively. The center-to-center distance between slit and cavity is denoted as d_sp.

Fig. 2. As the location offset of slit is fixed (d_sp = 120 nm), (a) the transmittance spectra of structure with the different width C, (b) the Q factor of structure versus the width C. The amplitude distribution of magnetic field at the resonant wavelength of structure with width C = 100 nm is also shown in the inset of Fig. 2(b). (c) The transmittance spectra of structure with the different location offset d_sp; (d) the Q factor of structure versus the d_sp. The length and width of microcavity are L = 650 nm and D = 200 nm, respectively. The thickness of waveguide w_d = 200 nm.

Fig. 3. The schematic diagram of metal waveguide-double microcavities side-coupled structure. A other microcavity is introduced into the structure shown in Fig.1. The two microcavities are numbered as ① and ②, respectively. The width of air layer is w_d.

Fig. 4. (a) The transmittance spectra of waveguide-microcavities structure. The red line and blue dotted line denote the results obtained from FDFD simulation and TCMT, respectively. For comparison, the transmittance spectra of the single waveguide-microcavity are shown with black dashed line; (b) for waveguide-microcavities structure, the phase of output wave versus the wavelength. In simulation, the parameters of structure are L₁ = 650 nm, D₁ = 200 nm, S₁ = 200 nm, C₁ = 100 nm, d_sp1 = 120 nm, L₂ = 625 nm, D₂ = 200 nm, S₂ = 250 nm, C₂ = 40 nm, d_sp2 = 80 nm and w_d = 200 nm.

Fig. 5. The peak value of transmission and the FWHM of EIT window versus the width C₂ of slit. In simulation, the geometrical parameters of structure are L₁ = 650 nm, D₁ = 200 nm, S₁ = 200 nm, C₁ = 100 nm, d_sp1 = 120 nm, L₂ = 625 nm, D₂ = 200 nm, S₂ = 250 nm, d_sp2 = 80 nm and w_d = 200 nm.