Abstract
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.
Highlights
Discovering new topological phases of matter is of significant importance for both fundamental physics and materials science[1,2,3,4,5,6,7]
This structure is perceived as a photonic Dirac nodal line semimetals (DNLS)
In conclusion, we have experimentally demonstrated a new mechanism to realize a photonic type-II DNLS, which has no counterpart in existing electronic DNLSs where electron spin plays the role of polarization
Summary
Discovering new topological phases of matter is of significant importance for both fundamental physics and materials science[1,2,3,4,5,6,7]. Theory of symmetry indicators is successful in identifying electronic topological materials[8]. Extensive efforts have been taken to diagnose topological characters of electronic materials in the crystal structure database exhaustively[9,10,11]. The topological classification of the photonic systems was originally thought to be a trivial extension of the electronic counterpart and described by spinless space groups. Detailed analyses reveal that photonic systems are distinct from the electronic counterparts, and connectivity at zero frequency in dielectric materials and hidden symmetry enforced nexus points are latter found to be unique to photonic systems[12,13]. We provide a stringent photonic realization of Dirac nodal
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