Bound states in the continuum (BICs) are nonradiative modes embedded in the radiation continuum, offering powerful opportunities for wave manipulation in open systems. Although BICs themselves cannot be assigned a well-defined polarization state in the far field, the surrounding polarization field encodes essential topological information. In conventional BIC systems, the polarization trajectory surrounding a BIC typically forms a loop near or parallel to the equator of the Poincaré sphere and encircles the $S_3$ axis, resulting in a nonzero topological charge. Here, we show that such $S_3$-axis winding is not a prerequisite for the existence of a BIC. By introducing structural shear deformation, the polarization trajectory evolves from an equatorial loop to a tilted orbit, undergoes pole crossings, and eventually becomes nearly perpendicular to the equator. During this process, an isolated BIC remains intact while its topological charge continuously evolves from $-1$ to 0 and $+1$ without conventional singularity creation, annihilation, or merging. Furthermore, we generalize the formation mechanism of BICs beyond conventional orthogonal polarization states and establish a semianalytical geometric approach that predicts the polarization winding directly from the nodal-line configuration. Our results reveal a new route for momentum-space polarization engineering and extend the topological understanding of BICs.

Close

PrevNext

View figure in article

• Metasurfaces
• Photonic crystals
• Topological effects in photonic systems
• Topological metamaterials