Metamaterials, benefitting from the flexibility in engineering the geometries of their artificial atoms and correspondingly the effective electromagnetic properties, provide a unique platform for investigating the topological properties of photons. During the past few years, various topological photonic phases have been demonstrated with metamaterials, such as Weyl degeneracies, Dirac points and 3D topological insulators. Dirac point is a four-fold band crossing defined in 3D momentum space, away from which energy band exhibits linear dispersion along arbitrary directions. As a central gapless topological phase, Dirac semimetal bridges conventional insulator, topological insulators and Weyl semimetals. In this talk I will talk about how to use metamaterials to realize Dirac points in a four band system by exploiting both the electric and magnetic bulk plasmons, based on the effective medium approach. I will show that the Dirac points can be promoted to Yang monopoles in a suitably chosen 5D synthetic space. The Yang monopoles can be further transformed into linked Weyl surfaces by introducing certain perturbations to the system. An important signature of the second order nontrivial topology - the presence of Weyl arcs and Fermi hypersurfaces at the boundary of the 5D system, will be discussed.
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