Suppression of frequency splitting induced by weak anisotropy via construction of anti-parity-time symmetry in polarization space

Abstract From optical sensors to precision measurement, modern photonic systems are demonstrating heightened sensitivity to even slight frequency splitting between polarization eigenmodes. Nevertheless, the inherent weak anisotropy of the crystal inevitably induces such splitting. Here, we demonstrate that such frequency splitting can be suppressed by constructing an anti-parity-time (PT) symmetry system in polarization space. In a laser system with weak anisotropy, we observe the transition from splitting to degeneracy in the frequency domain as the system enters the exact anti-PT-symmetric phase, consistent with a non-Hermitian two-component system. Our results reveal a physical mechanism by which anti-PT symmetry counteracts weak anisotropies without requiring complex control schemes. This approach offers a compact and intrinsically self-balancing pathway toward polarization stabilization, with potential applications in integrated optical sensors, topological photonic systems, and quantum photonic systems.