Topological charge abrupt switching of optical polarization singularity in momentum space driven by topological phase transition

Recent research on the far-field effects of topological phase transitions (TPTs) in photonic crystal slabs has demonstrated that the exchange of Γ-point bound states in the continuum (BICs) serves as a far-field hallmark of this band topology property. However, the systematic evolution of optical polarization singularities, a key signature of far-field topology, under TPTs still remains ambiguous. Here, we numerically reveal the intrinsic mechanism of the abrupt change in the topological charge at the Γ point under TPTs, and further clarify the evolutionary process governing the optical singularities. We demonstrate that the conservation of topological charge holds for each photonic band following the transition. The exchange of Γ-point BICs originates from the merging of off-Γ Dirac points into the Γ point. Furthermore, the Γ-point BICs transit across different photonic bands occurs as a continuous process for each individual band. Accordingly, the topological charge state during TPTs can be categorized into three stages. We further observe optical vortex switching driven by TPTs. These findings hold significant potential for applications in advanced optical devices and photonic technologies.