Giant angle-insensitive photonic spin Hall effect empowered by Brillouin zone folding

The photonic spin Hall effect (PSHE) has emerged as a pivotal phenomenon in both fundamental photonics and precision metrology. In this study, we theoretically propose an angle-insensitive high-Q toroidal dipole (TD) mode supported by Brillouin zone folding. By exploiting the polarization-dependent guide mode resonance, we realize an elliptical phase singularity, which can be designed at any incident angle. Near the phase singularity, the enhanced PSHE is achieved, which response demonstrates strong robustness against variations in incident angle, underscoring the structural tolerance of the design. Furthermore, we design an ultrasensitive environmental sensor based on elliptical amplitude detection, achieving a sensitivity of 1200 rad/RIU. This enhanced sensitivity stems from the steep phase gradient near the singularity, which amplifies the response to minute environmental perturbations. Our work opens an avenue for polarization-engineered photonic devices and applications in high-precision sensing technologies.