Topological semimetals, representing a new topological phase that lacks a full band gap in bulk states and exhibiting nontrivial topological orders, recently have been extended to photonic systems, predominantly in photonic crystals and to a lesser extent metamaterials. Photonic crystal realizations of Dirac degeneracies are protected by various space symmetries, where Bloch modes span the spin and orbital subspaces. Here, we theoretically show that Dirac points can also be realized in effective media through the intrinsic degrees of freedom in electromagnetism under electromagnetic duality. A pair of spin-polarized Fermi-arc-like surface states is observed at the interface between air and the Dirac metamaterials. Furthermore, eigenreflection fields show the decoupling process from a Dirac point to two Weyl points. We also find the topological correlation between a Dirac point and vortex or vector beams in classical photonics. The experimental feasibility of our scheme is demonstrated by designing a realistic metamaterial structure. The theoretical proposal of the photonic Dirac point lays the foundation for unveiling the connection between intrinsic physics and global topology in electromagnetism.

中文翻译：

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超材料中的三维光子狄拉克点

拓扑半金属代表了一个新的拓扑阶段，该状态在块态中没有完整的带隙并且显示出不平凡的拓扑顺序，最近已扩展到光子系统，主要是光子晶体，程度较小的是超材料。Dirac简并性的光子晶体实现受到各种空间对称性的保护，其中Bloch模式跨越自旋和轨道子空间。在这里，我们从理论上证明，狄拉克点也可以通过电磁对偶下电磁的固有自由度在有效介质中实现。在空气和狄拉克超材料之间的界面处观察到一对自旋极化的费米弧状表面状态。此外，本征反射场显示了从狄拉克点到两个Weyl点的解耦过程。我们还发现经典光子学中狄拉克点与涡旋或矢量束之间的拓扑相关性。通过设计一个现实的超材料结构，证明了我们方案的实验可行性。光子狄拉克点的理论建议为揭示电磁的本征物理和全局拓扑之间的联系奠定了基础。