Higher-order nodal ring photonic semimetal.

The intriguing discovery of higher-order topology has tremendously promoted the development of topological physics. Three-dimensional topological semimetals have emerged as an ideal platform for investigating novel topological phases. Consequently, new proposals have been theoretically revealed and experimentally realized. However, most existing schemes are implemented on the acoustic system, while similar concepts are rarely launched in photonic crystals due to the complicated optical manipulation and geometrical design. In this Letter, we propose a higher-order nodal ring semimetal protected by C symmetry originating from C symmetry. The higher-order nodal ring is predicted in three-dimensional momentum space with desired hinge arcs connected by two nodal rings. Fermi arcs and topological hinge modes generate significant marks in higher-order topological semimetals. Our work successfully proves the presence of a novel higher-order topological phase in photonic systems that we will strive to apply practically in high-performance photonic devices.