  Metamaterials, artificially engineered materials consisting of subwavelength
unit cell, have shown potentials in light manipulation with their extraordinary
optical properties. Especially, topological metamaterials possessing
topologically protected surface states enable extremely robust control of
light. Here, we demonstrate extremely broadband topological phase in a photonic
topological metamaterials with double helix structure. In particular
topological surface states are observed for all the frequencies below a certain
cut-off, originating from a double Weyl point at zero frequency. The extreme
bandwidth and robustness of the photonic topological metamaterial are
beneficial for practical applications such as one-way waveguide and photonic
integrated systems but also advantageous in design and fabrication since the
only necessary condition is to satisfy the effective hyperbolic and chiral
properties, without entailing strict periodic arrangement.
