Fig. 1: Merging multiple accidental BICs with a higher-order topological charge at Γ.
From: Merging bound states in the continuum by harnessing higher-order topological charges
a Schematics of a PCS with a triangular lattice of cylindrical holes etched onto a Si3N4 slab and immersed in a liquid. The lattice constant is a = 336 nm and the diameter of the hole is D = 160 nm. 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. The inset shows the first Brillouin zone. c Simulated polarization vectors around the BICs with the Q factors as the background color for the lowest TE-like band at different thicknesses. When t is tuned from 340 nm to 300 nm, a chain of BICs with topological charge \(\pm 1\) merges into a symmetry-protected BIC with charge −2. d Simulated Q factors (dotted lines) and the corresponding fitting curves (solid lines). The right panel shows the scaling rules along the \(\Gamma {{{\mathrm{K}}}}\) direction, and the scaling rules are the same along the \(\Gamma {{{\mathrm{M}}}}\) direction. Merging BIC (t = 324.2 nm) has considerably enhanced the Q factors of nearby states compared with isolated BICs
