Merging and separation of polarization singularities in complex lattices

Abstract Bound states in the continuum (BICs) and circularly polarized states (CPSs), as far-field polarization singularities, by controlling geometric parameters of photonic crystals, exhibit their evolution in momentum space that can be observed. This offers significant potential in optics and photonics. Here, we reveal that in complex lattices, far-field polarization singularities can be flexibly manipulated while preserving structural symmetry. The change of the topological charge for the at-Γ BIC can generate new BICs or CPSs. At an off-Γ point, a BIC can spawn from the collision of two CPSs. As the structure thickness increases, this BIC will meet the at-Γ BIC. The merging of BICs can induce the topological charge transition and yield a large wavevector space around Γ point with ultrahigh quality ( Q ) factors. Our findings provide a novel degree of freedom for manipulating polarization singularities, which holds great promise in radiation modulation and singular optics.