Fig. 2: Merging split BICs at Γ.
From: Merging bound states in the continuum by harnessing higher-order topological charges
a Schematics of a PCS with a triangular lattice (lattice constant a = 336 nm) of elliptic cylindrical etched holes. The semi-major axis and semi-minor axis are r1 = 112 nm and r2 = 80 nm, respectively. The materials of the PCS and the background are the same as Fig. 1. The inset is the top-view of the unit cell. b Simulated TE-like band structure at t = 340 nm. The lowest TE-like band is highlighted in red. c Simulated polarization vectors around BICs with Q factors as background for the lowest TE-like band at different thicknesses. When t is tuned from 340 nm to 360 nm, the two split BICs with the same topological charge −1 bounce off each other. The transition (t = 346.7 nm) corresponds to a merging BIC with charge −2. d Simulated Q factors (dotted lines) and fitting curves (solid lines). The merging BIC case has considerably enhanced Q factors for nearby states compared with the isolated BICs cases
