Fig. 4

For the structure presented in Fig. 1b the above plots shows a zoom-in of the variation of transmittance (a) and reflectance (b) (with color scale) versus the reduced wave vector \(k L/2 \pi\) and the length difference parameter \(\delta\). The considered parameters are \(L_1=(100+\delta )\) nm, \(L_2=(100-\delta )\) nm. In this plot we are focusing on the BIC associated to the pair \((p=3, q=1)\) and \(n=2\), i.e., at \((k L/2 \pi , \delta )=(5,10)\). One can notice the narrowing of the resonance of the quasi-BIC, then its transformation into a BIC (vanishing of linewidth) at \((k L/2 \pi , \delta )=(5,10)\). A transparency window between two zeros (induced by the SIBICs) appears when we deviate slightly from the BIC condition, giving rise to symmetric Fano resonance. (c) and (d) shows a three dimensional maps of the transmittance and reflectance given in (a) and (b) respectively.