The past decade has seen a proliferation of topological materials for both insulators and semimetals in electronic systems and classical waves. Topological semimetals exhibit topologically protected band degeneracies, such as nodal points and nodal lines. Dirac nodal line semimetals (DNLS), which own four-fold line degeneracy, have drawn particular attention. DNLSs have been studied in electronic systems but there is no photonic DNLS. Here in this work, we provide a new mechanism, which is unique for photonic systems to investigate a stringent photonic DNLS. When truncated, the photonic DNLS exhibits double-bowl states (DBS), which comprise two sets of perpendicularly polarized surface states. In sharp contrast to nondegenerate surface states in other photonic systems, here the two sets of surface states are almost degenerate over the whole-spectrum range. The DBS and the bulk Dirac nodal ring (DNR) dispersion along the relevant directions, are experimentally resolved.

在过去的十年中，电子系统和经典波中绝缘体和半金属的拓扑材料激增。拓扑半金属表现出受拓扑保护的带简并性，例如节点和节点线。具有四重线简并性的狄拉克节点线半金属（DNLS）引起了特别关注。DNLS 已经在电子系统中进行了研究，但没有光子 DNLS。在这项工作中，我们提供了一种新机制，这是光子系统研究严格光子 DNLS 所独有的。当被截断时，光子 DNLS 表现出双碗态 (DBS)，其中包括两组垂直极化的表面态。与其他光子系统中的非简并表面态形成鲜明对比，这里两组表面态在整个光谱范围内几乎是退化的。沿相关方向的 DBS 和体狄拉克节环 (DNR) 色散通过实验得到解决。