Exceptional points (EPs) have been widely studied in various non-Hermitian systems, exhibiting and underlying many unique topological properties. In photonic systems, multiple degrees of freedom of light enable the EPs with rich properties and high dimensional topology. In this Letter, we propose that the external magnetic field can serve as an additional parameter dimension to manipulate EPs with tunable evolutions and hidden topological structures in magneto-optical photonic crystal slabs. Continuous evolution of EPs can be derived by changing the external magnetic field, where paired EPs gradually approach each other as the Fermi arc shrinks, and eventually merge and annihilate. When considering the parameter space including the magnetic field dimension, these EPs form a closed ring, revealing novel topological structures. The discovered EP ring is further associated with more topological polarization properties, including the closed ring in Poincaré sphere and the transferred topological charge between the Fermi arc and the circularly polarized states in momentum space. Our Letter reveals complex topological properties in magneto-optical photonic crystal slabs, providing a framework to explore non-Hermitian topological systems with additional parameter dimensions.

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• Metamaterials
• Nanophotonics
• Photonic crystals
• Topological effects in photonic systems
• Non-Hermitian systems