Fig. 4. Topological features of optical force around off-Γ BIC when the untwisted PhCS lattice constant is reduced to 517 nm.
(A) Quality factor of one eigenmode showing that four off-Γ BICs are obtained. (B) Topological distribution of optical force surrounding four BICs. (C) Absolute value of optical force in log scale shows force zeros occurring at four BICs and one additional at Γ point. (D) Map of the value ϕF(k) showing topological features of optical force around force zeros in momentum space. The black and white arrows have opposite topological charge. Map of sign of (E) Fx and (F) Fy in momentum space. (G) Quality factor of another eigenmode shows several BICs enclosing the Γ point. (H) Distribution of optical force around these BICs. Accumulation of these singular points results in nearly zero force within a diamond-like region around the Γ point. (I) Map of absolute value of optical force in log scale around the accumulating BICs.