We analyze a plasmonic model on a honeycomb lattice of metallic nanodisks that hosts nodal lines enclosing the K points. An effective second-neighbor hopping between opposite synthetic spin states shifts the Dirac cones, leaving behind a nodal line at zero energy. Using both continuum and tight-binding models, we show that there is an ideal limit of the plasmonic crystal in which symmetries enforce the existence of nodal lines enclosing the K points. These nodal lines are not directly gapped even when these symmetries are weakly broken. The existence of the nodal lines in the weakly symmetry-broken case is verified using full-wave electromagnetic simulations. We also show that the nodal line degeneracies can be relieved by introducing a Kekulé distortion that acts to mix the nodal lines enclosing the K and ${\mathrm{K}}^{\prime }$ points. Our findings open pathways for designing unique plasmonic and photonic devices without reliance on complex symmetry engineering, presenting a convenient platform for studying nodal structures in two-dimensional systems.

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• Photonics
• Plasmonics
• Polaritons
• Symmetry protected topological states
• Honeycomb lattice