Observation of Anomalous Charge-Free Bound States in the Continuum

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.