Fig. 1. Geometry of a ridge on a (a) waveguide layer and of a (b) composite structure consisting of three ridges separated by phase-shift regions. Red arrows indicate the propagation directions of the incident wave I, reflected wave R, and transmitted wave T.

Fig. 2. Reflectance of an obliquely incident TE-polarized guided mode with free-space wavelength λ0=630  nm from the ridge versus ridge width w and angle of incidence θ. The white circle indicates the BIC position. White dashed lines indicate the ridge widths, at which the angular spectra in Fig. 3(a) are plotted.

Fig. 3. (a) Angular and (b) wavelength ridge reflectance spectra at w=355  nm (solid blue lines), w=360  nm (dashed red lines), and w=380  nm (dotted yellow lines). The angular spectra are calculated at the fixed free-space wavelength of 630 nm and are the cross sections of the reflectance distribution in Fig. 2 along the white dashed lines. The wavelength spectra are calculated at the angles of incidence θ1=53.28° (w=355  nm), θ2=53.37° (w=360  nm), and θ3=53.72° (w=380  nm), at which the resonant peaks are centered at λ=λ0=630  nm.

Fig. 4. Transmittance (black) and reflectance (red) spectra of composite structures consisting of (a) N=2, (b) N=4, and (c) N=6 ridges at the ridge width w=380  nm and the distance between the ridges l=948  nm (solid lines). Dashed lines show the spectra of a single ridge.

Fig. 5. Transmittance (black) and reflectance (red) spectra of the optimized composite structures consisting of (a) N=4 and (b) N=6 ridges at w=380  nm (solid lines). Dashed lines show the spectra of a single ridge.

Fig. 6. Transmittance (black) and reflectance (red) spectra of the optimized composite structures consisting of (a) N=2, (b) N=4, and (c) N=6 ridges at w=360  nm (solid lines). Dashed lines show the spectra of a single ridge.

Fig. 7. Reflectance RN(l,λ)=|rN(l,λ)|2 (top) and transmittance TN(l,λ)=|tN(l,λ)|2 (bottom) of the composite structures consisting of (a), (d) N=2, (b), (e) N=3, and (c), (f) N=4 ridges versus the distance between the ridges l and free-space wavelength. Vertical dashed lines show the distance lFP=970.2  nm corresponding to the Fabry–Perot resonance.

Fig. 8. Transmittance spectra of the composite structure consisting of N=3 ridges calculated at the width of the phase-shift region between the ridges l=lFP=970.2  nm corresponding to the BIC condition (solid blue line), and at the widths l=lFP+5  nm=975.2  nm (dashed red line) and l=lFP+10  nm=980.2  nm (dotted yellow line).