Diabolical point and conical-like diffraction in periodic plasmonic nanostructures.

We present the formation of a singular (diabolical) point in k-space from a periodic metal-dielectric waveguide array. The singularity originates from the balance between alternating normal and anomalous coupling. We numerically demonstrate a strong diffraction anomaly (conical-like diffraction) near the singular point. We also show the evolution of the diffraction pattern with band deformation. The resultant peculiar propagation dynamics of surface plasmon polaritons could provide a new toolset for manipulating light on the nano-scale.