Type-III nodal loops and topological phase transition at metasurfaces.

Topological characteristics of Bloch energy bands have attracted a lot of attention in electronic systems as well as photonic systems. As an unique degeneracy configuration, nodal loop (NL)[1] is regarded as a twofold band degeneracy and forms a closed rings in the Brillouin zone. This kind of band structure has been studied in various periodic systems, but not yet been reported to realize in 2D microwave metasurfaces. Here, we experimentally demonstrate a NL degeneracy at a metasurface with bilayer subwavelength fractal pattern. Protected by mirror symmetry (MS) at z-direction, NL is realized in the metasurface and appears as a ring-like degeneracy between a nearly-flat band and a positive band. In addition, we experimentally show that the NL can transform into two Type-II Dirac points[2] along the Gama-X direction when Ms at z-direction is broken. After further symmetry breaking in fractal pattern, the DPs are gapped and the emergence of topological edge states is observed in the domain wall composed of the gapped metasurfaces.

[1] Gao, W., Yang, B., Tremain, B. et al. Experimental observation of photonic nodal line degeneracies in metacrystals. Nat Commun 9, 950 (2018).
[2] C. Hu, et al. Type-II Dirac Photons at Metasurfaces, Phys. Rev. Lett. 121, 024301 (2018).