Conical diffraction from type-II Dirac point at metasurfaces

Abstract

Type-II Dirac point degeneracy can be realized at two-dimensional metasurfaces, where a pair of tilted cones is formed in the dispersion relation whose equal-frequency contour at the degeneracy frequency presents an X-shaped cross in k-space. Consequently, two group velocities are obtained at the Dirac point, due to the peculiar shape of the spatial dispersion, and point to the different directions with the intersection angle dependent on the cone's geometry. Therefore, a conical-diffraction phenomenon takes place. We observe the conical diffraction through launching into the metasurface a Gaussian-like beam at the Dirac frequency and mapping the propagation of the wave field. Both experimental and simulating results demonstrate the conical diffraction at the type-II Dirac point.