Topological Transitions at Optical Interfaces Near Reflection Singularities.

Topological transitions of structured light at optical interfaces, particularly near singularities in reflection coefficients, hold significant potential for advanced light field manipulation. However, these phenomena remain largely unexplored beyond classical Brewster-angle configurations. In this work, we establish a unified theoretical framework to describe topological transitions-from an ideal vortex to a deformed off-axis vortex-near reflection singularities and provide experimental validation of the predictions. Through a full-wave model, we demonstrate that the transition is governed by three independent degrees of freedom: the incident angle, the incident polarization state, and the reflected polarization state. This triple-control mechanism enables precise manipulation of vortex deformation and lateral shifting. Furthermore, we extend this approach to multilayer systems by designing a PT-symmetric bilayer structure satisfying r s  = 0. This structure enables vortex generation and topological transition under a quasi-Brewster condition. Our findings pave the way for enhanced topological control of structured light and offer promising prospects for advanced photonic applications.