title: Accidental degeneracy in photonic bands and topological phase transitions in two-dimensional core-shell dielectric photonic crystals.
abstract: A simple core-shell two-dimensional photonic crystal is studied where the triangular lattice symmetry and the C6point group symmetry give rich physics in accidental touching points of photonic bands. We systematically evaluate different types of accidental nodal points at the Brillouin zone center for transverse-magnetic harmonic modes when the geometry and permittivity of the core-shell material are continuously tuned. The accidental nodal points can have different dispersions and topological properties (i.e., Berry phases). These accidental nodal points can be the critical states lying between a topological phase and a normal phase of the photonic crystal. They are thus very important for the study of topological photonic states. We show that, without breaking time-reversal symmetry, by tuning the geometry of the core-shell material, a phase transition into the photonic quantum spin Hall insulator can be achieved. Here the "spin" is defined as the orbital angular momentum of a photon. We study the topological phase transition as well as the properties of the edge and bulk states and their application potentials in optics.
authors: Lin
authors: L
authors: Xu
authors: Hai-Xiao
authors: HX
authors: Wang
authors: Ya-Dong
authors: YD
authors: Xu
authors: Huan-Yang
authors: HY
authors: Chen
authors: Jian-Hua
authors: JH
authors: Jiang