Fig. 2. Band diagram of silica (ε = 2.1) photonic crystal lattices uniform in the out-of-plane direction (z).

a Projected band diagram of the triangular photonic crystal, in which the nodal-line degeneracy is highlighted. b An extra air hole in the primitive-cell breaks the inversion symmetry, and the nodal-line is lifted into Weyl points. Insets: cross-section structures and in-plane band structures at k_za/2π = 2.02. Two different primitive-cell choices are drawn in a