Diabolical point and conical diffraction in periodic plasmonic nanostructures

We present formation of a singular (diabolical) point in k-space of a periodic metal-dielectric waveguide array. The singularity originates from the balance in the 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 peculiar propagation dynamics of surface plasmon polaritons could provide a new toolset for manipulating light on the nano-scale.