  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 comprises 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.
