Double-bowl state in photonic Dirac nodal line semimetal

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