It was recently demonstrated that the connectivities of bands emerging from zero frequency in dielectric photonic crystals are distinct from their electronic counterparts with the same space groups. We discover that in an AB-layer-stacked photonic crystal composed of anisotropic dielectrics, the unique photonic band connectivity leads to a new kind of symmetry-enforced triply degenerate points at the nexuses of two nodal rings and a Kramers-like nodal line. The emergence and intersection of the line nodes are guaranteed by a generalized 1/4-period screw rotation symmetry of Maxwell’s equations. The bands with a constant k_z and iso-frequency surfaces near a nexus point both disperse as a spin-1 Dirac-like cone, giving rise to exotic transport features of light at the nexus point. We show that spin-1 conical diffraction occurs at the nexus point, which can be used to manipulate the charges of optical vortices. Our work reveals that Maxwell’s equations can have hidden symmetries induced by the fractional periodicity of the material tensor components and hence paves the way to finding novel topological nodal structures unique to photonic systems.

最近证明，在介电光子晶体中从零频率出现的频带的连接性不同于具有相同空间组的电子对应物。我们发现，在由各向异性电介质组成的AB层堆叠的光子晶体中，独特的光子能带连通性导致在两个节点环和Kramers样节点线的交点处产生一种新的对称增强的三重简并点。麦克斯韦方程组的广义1/4周期螺旋旋转对称性保证了线节点的出现和交点。常数k _z的波段靠近交点的等频率表面都像自旋1的狄拉克锥一样分散，从而在交点处产生奇异的光传输特征。我们显示自旋1圆锥形衍射发生在联系点，可以用来操纵光学涡旋的电荷。我们的工作表明，麦克斯韦方程组可以具有由材料张量分量的分数周期性引起的隐藏对称性，因此为寻找光子系统独有的新型拓扑节点结构铺平了道路。