  Three-dimensional (3D) topological nodal points, such as Weyl and Dirac nodes
have attracted wide-spread interest across multiple disciplines and diverse
material systems. Unlike nodal points that contain little structural
variations, nodal lines can have numerous topological configurations in the
momentum space, forming nodal rings, nodal chains and potentially nodal links
and nodal knots. However, nodal lines have much less development for the lack
of ideal material platforms. In condensed matter for example, nodal lines are
often fragile to spin-orbit-coupling, locating off the Fermi level, coexisting
with energy-degenerate trivial bands and dispersing strongly in energy of the
line degeneracy. Here, overcoming all above difficulties, we theoretically
predict and experimentally observe nodal chains in a metallic-mesh photonic
crystal having frequency-isolated linear bandtouching rings chained across the
entire Brillouin zone (BZ). These nodal chains are protected by mirror
symmetries and have a frequency variation less than 1%. We used angle-resolved
transmission (ART) to probe the projected bulk dispersions and performed
Fourier-transformed field scan (FTFS) to map out the surface dispersions, which
is a quadratic touching between two drumhead surface bands. Our results
established an ideal nodal-line material for further studies of topological
line-degeneracies with nontrivial connectivities, as well as the consequent
wave dynamics richer than 2D Dirac and 3D Weyl materials.
