Hidden-symmetry-enforced nexus points of nodal lines in layer-stacked dielectric photonic crystals

摘要

a Structure of the PhC, where the two insets display the iso-frequency surfaces in the xz plane of the biaxial dielectrics in layer A (orange) and layer B (green). b Band structure along high symmetry lines in the ky = 0 plane for the PhC with ε1=2,ε2=13,ε3=1, and rotation angle θ = π/5. The blue and magenta lines represent the bands with odd and even M^y-parities, respectively. The red dots correspond to the nodal lines along which two bands with opposite M^y-parities intersect. The orange and purple dots denote the threefold and fourfold degenerate nexus points, respectively. c Sketch map of two red nodal rings (corresponding to the two lowest red dots in (b)) crossing a Kramers-like nodal line (blue line) at two nexus points in the first (bulk) Brillouin zone of the PhC